GNU Coreutils 1 Introduction 2 Common options 3 Output of entire files 4 Formatting file contents 5 Output of parts of files 6 Summarizing files 7 Operating on sorted files 8 Operating on fields 9 Operating on characters 10 Directory listing 11 Basic operations 12 Special file types 13 Changing file attributes 14 File space usage 15 Printing text 16 Conditions 17 Redirection 18 File name manipulation 19 Working context 20 User information 21 System context 22 SELinux context 23 Modified command invocation 24 Process control 25 Delaying 26 Numeric operations 27 File permissions 28 File timestamps 29 Date input formats 30 Version sort ordering 31 Opening the Software Toolbox Appendix A GNU Free Documentation License Index GNU Coreutils 1 Introduction 2 Common options 2.1 Backup options 2.2 Block size 2.3 Signal specifications 2.4 chown, chgrp, chroot, id: Disambiguating user names and IDs 2.5 Sources of random data 2.6 Target directory 2.7 Trailing slashes 2.8 Traversing symlinks 2.9 Treating ‘/’ specially 2.10 Special built-in utilities 2.11 Exit status 2.12 Floating point numbers 2.13 Standards conformance 2.14 ‘coreutils’: Multi-call program 3 Output of entire files 3.1 ‘cat’: Concatenate and write files 3.2 ‘tac’: Concatenate and write files in reverse 3.3 ‘nl’: Number lines and write files 3.4 ‘od’: Write files in octal or other formats 3.5 ‘base32’: Transform data into printable data 3.6 ‘base64’: Transform data into printable data 3.7 ‘basenc’: Transform data into printable data 4 Formatting file contents 4.1 ‘fmt’: Reformat paragraph text 4.2 ‘pr’: Paginate or columnate files for printing 4.3 ‘fold’: Wrap input lines to fit in specified width 5 Output of parts of files 5.1 ‘head’: Output the first part of files 5.2 ‘tail’: Output the last part of files 5.3 ‘split’: Split a file into pieces. 5.4 ‘csplit’: Split a file into context-determined pieces 6 Summarizing files 6.1 ‘wc’: Print newline, word, and byte counts 6.2 ‘sum’: Print checksum and block counts 6.3 ‘cksum’: Print and verify file checksums 6.3.1 cksum output modes 6.3.2 cksum general options 6.3.3 cksum common options 6.4 ‘md5sum’: Print or check MD5 digests 6.5 ‘b2sum’: Print or check BLAKE2 digests 6.6 ‘sha1sum’: Print or check SHA-1 digests 6.7 sha2 utilities: Print or check SHA-2 digests 7 Operating on sorted files 7.1 ‘sort’: Sort text files 7.2 ‘shuf’: Shuffling text 7.3 ‘uniq’: Uniquify files 7.4 ‘comm’: Compare two sorted files line by line 7.5 ‘ptx’: Produce permuted indexes 7.5.1 General options 7.5.2 Charset selection 7.5.3 Word selection and input processing 7.5.4 Output formatting 7.5.5 The GNU extensions to ‘ptx’ 7.6 ‘tsort’: Topological sort 7.6.1 ‘tsort’: Background 8 Operating on fields 8.1 ‘cut’: Print selected parts of lines 8.2 ‘paste’: Merge lines of files 8.3 ‘join’: Join lines on a common field 8.3.1 General options 8.3.2 Pre-sorting 8.3.3 Working with fields 8.3.4 Controlling ‘join’’s field matching 8.3.5 Header lines 8.3.6 Union, Intersection and Difference of files 9 Operating on characters 9.1 ‘tr’: Translate, squeeze, and/or delete characters 9.1.1 Specifying arrays of characters 9.1.2 Translating 9.1.3 Squeezing repeats and deleting 9.2 ‘expand’: Convert tabs to spaces 9.3 ‘unexpand’: Convert spaces to tabs 10 Directory listing 10.1 ‘ls’: List directory contents 10.1.1 Which files are listed 10.1.2 What information is listed 10.1.3 Sorting the output 10.1.4 General output formatting 10.1.5 Formatting file timestamps 10.1.6 Formatting the file names 10.2 ‘dir’: Briefly list directory contents 10.3 ‘vdir’: Verbosely list directory contents 10.4 ‘dircolors’: Color setup for ‘ls’ 11 Basic operations 11.1 ‘cp’: Copy files and directories 11.2 ‘dd’: Convert and copy a file 11.3 ‘install’: Copy files and set attributes 11.4 ‘mv’: Move (rename) files 11.5 ‘rm’: Remove files or directories 11.6 ‘shred’: Remove files more securely 12 Special file types 12.1 ‘link’: Make a hard link via the link syscall 12.2 ‘ln’: Make links between files 12.3 ‘mkdir’: Make directories 12.4 ‘mkfifo’: Make FIFOs (named pipes) 12.5 ‘mknod’: Make block or character special files 12.6 ‘readlink’: Print value of a symlink or canonical file name 12.7 ‘rmdir’: Remove empty directories 12.8 ‘unlink’: Remove files via the unlink syscall 13 Changing file attributes 13.1 ‘chown’: Change file owner and group 13.2 ‘chgrp’: Change group ownership 13.3 ‘chmod’: Change access permissions 13.4 ‘touch’: Change file timestamps 14 File space usage 14.1 ‘df’: Report file system space usage 14.2 ‘du’: Estimate file space usage 14.3 ‘stat’: Report file or file system status 14.4 ‘sync’: Synchronize cached writes to persistent storage 14.5 ‘truncate’: Shrink or extend the size of a file 15 Printing text 15.1 ‘echo’: Print a line of text 15.2 ‘printf’: Format and print data 15.3 ‘yes’: Print a string until interrupted 16 Conditions 16.1 ‘false’: Do nothing, unsuccessfully 16.2 ‘true’: Do nothing, successfully 16.3 ‘test’: Check file types and compare values 16.3.1 File type tests 16.3.2 Access permission tests 16.3.3 File characteristic tests 16.3.4 String tests 16.3.5 Numeric tests 16.3.6 Connectives for ‘test’ 16.4 ‘expr’: Evaluate expressions 16.4.1 String expressions 16.4.2 Numeric expressions 16.4.3 Relations for ‘expr’ 16.4.4 Examples of using ‘expr’ 17 Redirection 17.1 ‘tee’: Redirect output to multiple files or processes 18 File name manipulation 18.1 ‘basename’: Strip directory and suffix from a file name 18.2 ‘dirname’: Strip last file name component 18.3 ‘pathchk’: Check file name validity and portability 18.4 ‘mktemp’: Create temporary file or directory 18.5 ‘realpath’: Print the resolved file name. 18.5.1 Realpath usage examples 19 Working context 19.1 ‘pwd’: Print working directory 19.2 ‘stty’: Print or change terminal characteristics 19.2.1 Control settings 19.2.2 Input settings 19.2.3 Output settings 19.2.4 Local settings 19.2.5 Combination settings 19.2.6 Special characters 19.2.7 Special settings 19.3 ‘printenv’: Print all or some environment variables 19.4 ‘tty’: Print file name of terminal on standard input 20 User information 20.1 ‘id’: Print user identity 20.2 ‘logname’: Print current login name 20.3 ‘whoami’: Print effective user name 20.4 ‘groups’: Print group names a user is in 20.5 ‘users’: Print login names of users currently logged in 20.6 ‘who’: Print who is currently logged in 21 System context 21.1 ‘date’: Print or set system date and time 21.1.1 Specifying the format of ‘date’ output 21.1.1.1 Time conversion specifiers 21.1.1.2 Date conversion specifiers 21.1.1.3 Literal conversion specifiers 21.1.1.4 Padding and other flags 21.1.2 Setting the time 21.1.3 Options for ‘date’ 21.1.4 Examples of ‘date’ 21.2 ‘arch’: Print machine hardware name 21.3 ‘nproc’: Print the number of available processors 21.4 ‘uname’: Print system information 21.5 ‘hostname’: Print or set system name 21.6 ‘hostid’: Print numeric host identifier 21.7 ‘uptime’: Print system uptime and load 22 SELinux context 22.1 ‘chcon’: Change SELinux context of file 22.2 ‘runcon’: Run a command in specified SELinux context 23 Modified command invocation 23.1 ‘chroot’: Run a command with a different root directory 23.2 ‘env’: Run a command in a modified environment 23.2.1 General options 23.2.2 ‘-S’/‘--split-string’ usage in scripts Testing and troubleshooting 23.2.3 ‘-S’/‘--split-string’ syntax Splitting arguments by whitespace Escape sequences Comments Environment variable expansion 23.3 ‘nice’: Run a command with modified niceness 23.4 ‘nohup’: Run a command immune to hangups 23.5 ‘stdbuf’: Run a command with modified I/O stream buffering 23.6 ‘timeout’: Run a command with a time limit 24 Process control 24.1 ‘kill’: Send a signal to processes 25 Delaying 25.1 ‘sleep’: Delay for a specified time 26 Numeric operations 26.1 ‘factor’: Print prime factors 26.2 ‘numfmt’: Reformat numbers 26.2.1 General options 26.2.2 Possible UNITs: 26.2.3 Examples of using ‘numfmt’ 26.3 ‘seq’: Print numeric sequences 27 File permissions 27.1 Structure of File Mode Bits 27.2 Symbolic Modes 27.2.1 Setting Permissions 27.2.2 Copying Existing Permissions 27.2.3 Changing Special Mode Bits 27.2.4 Conditional Executability 27.2.5 Making Multiple Changes 27.2.6 The Umask and Protection 27.3 Numeric Modes 27.4 Operator Numeric Modes 27.5 Directories and the Set-User-ID and Set-Group-ID Bits 28 File timestamps 29 Date input formats 29.1 General date syntax 29.2 Calendar date items 29.3 Time of day items 29.4 Time zone items 29.5 Combined date and time of day items 29.6 Day of week items 29.7 Relative items in date strings 29.8 Pure numbers in date strings 29.9 Seconds since the Epoch 29.10 Specifying time zone rules 29.11 Authors of ‘parse_datetime’ 30 Version sort ordering 30.1 Version sort overview 30.1.1 Using version sort in GNU Coreutils 30.1.2 Version sort and natural sort 30.1.3 Variations in version sort order 30.2 Version sort implementation 30.2.1 Version-sort ordering rules 30.2.2 Version sort is not the same as numeric sort 30.2.3 Version sort punctuation 30.2.4 Punctuation vs letters 30.2.5 The tilde ‘~’ 30.2.6 Version sort ignores locale 30.3 Differences from Debian version sort 30.3.1 Hyphen-minus ‘-’ and colon ‘:’ 30.3.2 Special priority in GNU Coreutils version sort 30.3.3 Special handling of file extensions 30.3.4 Comparing two strings using Debian’s algorithm 30.4 Advanced Topics 30.4.1 Reporting version sort bugs 30.4.2 Other version/natural sort implementations 30.4.3 Related source code 31 Opening the Software Toolbox Toolbox Introduction I/O Redirection The ‘who’ Command The ‘cut’ Command The ‘sort’ Command The ‘uniq’ Command Putting the Tools Together Appendix A GNU Free Documentation License Index GNU Coreutils ************* This manual documents version 9.4 of the GNU core utilities, including the standard programs for text and file manipulation. Copyright © 1994–2023 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”. 1 Introduction ************** This manual is a work in progress: many sections make no attempt to explain basic concepts in a way suitable for novices. Thus, if you are interested, please get involved in improving this manual. The entire GNU community will benefit. The GNU utilities documented here are mostly compatible with the POSIX standard. Please report bugs to . Include the version number, machine architecture, input files, and any other information needed to reproduce the bug: your input, what you expected, what you got, and why it is wrong. If you have a problem with ‘sort’ or ‘date’, try using the ‘--debug’ option, as it can often help find and fix problems without having to wait for an answer to a bug report. If the debug output does not suffice to fix the problem on your own, please compress and attach it to the rest of your bug report. Although diffs are welcome, please include a description of the problem as well, since this is sometimes difficult to infer. *Note (gcc)Bugs::. This manual was originally derived from the Unix man pages in the distributions, which were written by David MacKenzie and updated by Jim Meyering. What you are reading now is the authoritative documentation for these utilities; the man pages are no longer being maintained. The original ‘fmt’ man page was written by Ross Paterson. François Pinard did the initial conversion to Texinfo format. Karl Berry did the indexing, some reorganization, and editing of the results. Brian Youmans of the Free Software Foundation office staff combined the manuals for textutils, fileutils, and sh-utils to produce the present omnibus manual. Richard Stallman contributed his usual invaluable insights to the overall process. 2 Common options **************** Certain options are available in all of these programs. Rather than writing identical descriptions for each of the programs, they are described here. (In fact, every GNU program accepts (or should accept) these options.) Normally options and operands can appear in any order, and programs act as if all the options appear before any operands. For example, ‘sort -r passwd -t :’ acts like ‘sort -r -t : passwd’, since ‘:’ is an option-argument of ‘-t’. However, if the ‘POSIXLY_CORRECT’ environment variable is set, options must appear before operands, unless otherwise specified for a particular command. A few programs can usefully have trailing operands with leading ‘-’. With such a program, options must precede operands even if ‘POSIXLY_CORRECT’ is not set, and this fact is noted in the program description. For example, the ‘env’ command’s options must appear before its operands, since in some cases the operands specify a command that itself contains options. Most programs that accept long options recognize unambiguous abbreviations of those options. For example, ‘rmdir --ignore-fail-on-non-empty’ can be invoked as ‘rmdir --ignore-fail’ or even ‘rmdir --i’. Ambiguous options, such as ‘ls --h’, are identified as such. Some of these programs recognize the ‘--help’ and ‘--version’ options only when one of them is the sole command line argument. For these programs, abbreviations of the long options are not always recognized. ‘--help’ Print a usage message listing all available options, then exit successfully. ‘--version’ Print the version number, then exit successfully. ‘--’ Delimit the option list. Later arguments, if any, are treated as operands even if they begin with ‘-’. For example, ‘sort -- -r’ reads from the file named ‘-r’. A single ‘-’ operand is not really an option, though it looks like one. It stands for a file operand, and some tools treat it as standard input, or as standard output if that is clear from the context. For example, ‘sort -’ reads from standard input, and is equivalent to plain ‘sort’. Unless otherwise specified, a ‘-’ can appear as any operand that requires a file name. 2.1 Backup options ================== Some GNU programs (at least ‘cp’, ‘install’, ‘ln’, and ‘mv’) optionally make backups of files before writing new versions. These options control the details of these backups. The options are also briefly mentioned in the descriptions of the particular programs. ‘-b’ ‘--backup[=METHOD]’ Make a backup of each file that would otherwise be overwritten or removed. Without this option, the original versions are destroyed. Use METHOD to determine the type of backups to make. When this option is used but METHOD is not specified, then the value of the ‘VERSION_CONTROL’ environment variable is used. And if ‘VERSION_CONTROL’ is not set, the default backup type is ‘existing’. Note that the short form of this option, ‘-b’ does not accept any argument. Using ‘-b’ is equivalent to using ‘--backup=existing’. This option corresponds to the Emacs variable ‘version-control’; the values for METHOD are the same as those used in Emacs. This option also accepts more descriptive names. The valid METHODs are (unique abbreviations are accepted): ‘none’ ‘off’ Never make backups. ‘numbered’ ‘t’ Always make numbered backups. ‘existing’ ‘nil’ Make numbered backups of files that already have them, simple backups of the others. ‘simple’ ‘never’ Always make simple backups. Please note ‘never’ is not to be confused with ‘none’. ‘-S SUFFIX’ ‘--suffix=SUFFIX’ Append SUFFIX to each backup file made with ‘-b’. If this option is not specified, the value of the ‘SIMPLE_BACKUP_SUFFIX’ environment variable is used. And if ‘SIMPLE_BACKUP_SUFFIX’ is not set, the default is ‘~’, just as in Emacs. 2.2 Block size ============== Some GNU programs (at least ‘df’, ‘du’, and ‘ls’) display sizes in “blocks”. You can adjust the block size and method of display to make sizes easier to read. The block size used for display is independent of any file system block size. Fractional block counts are rounded up to the nearest integer. The default block size is chosen by examining the following environment variables in turn; the first one that is set determines the block size. ‘DF_BLOCK_SIZE’ This specifies the default block size for the ‘df’ command. Similarly, ‘DU_BLOCK_SIZE’ specifies the default for ‘du’ and ‘LS_BLOCK_SIZE’ for ‘ls’. ‘BLOCK_SIZE’ This specifies the default block size for all three commands, if the above command-specific environment variables are not set. ‘BLOCKSIZE’ This specifies the default block size for all values that are normally printed as blocks, if neither ‘BLOCK_SIZE’ nor the above command-specific environment variables are set. Unlike the other environment variables, ‘BLOCKSIZE’ does not affect values that are normally printed as byte counts, e.g., the file sizes contained in ‘ls -l’ output. ‘POSIXLY_CORRECT’ If neither ‘COMMAND_BLOCK_SIZE’, nor ‘BLOCK_SIZE’, nor ‘BLOCKSIZE’ is set, but this variable is set, the block size defaults to 512. If none of the above environment variables are set, the block size currently defaults to 1024 bytes in most contexts, but this number may change in the future. For ‘ls’ file sizes, the block size defaults to 1 byte. A block size specification can be a positive integer specifying the number of bytes per block, or it can be ‘human-readable’ or ‘si’ to select a human-readable format. Integers may be followed by suffixes that are upward compatible with the SI prefixes (http://www.bipm.org/en/publications/si-brochure/chapter3.html) for decimal multiples and with the ISO/IEC 80000-13 (formerly IEC 60027-2) prefixes (https://physics.nist.gov/cuu/Units/binary.html) for binary multiples. With human-readable formats, output sizes are followed by a size letter such as ‘M’ for megabytes. ‘BLOCK_SIZE=human-readable’ uses powers of 1024; ‘M’ stands for 1,048,576 bytes. ‘BLOCK_SIZE=si’ is similar, but uses powers of 1000 and appends ‘B’; ‘MB’ stands for 1,000,000 bytes. A block size specification preceded by ‘'’ causes output sizes to be displayed with thousands separators. The ‘LC_NUMERIC’ locale specifies the thousands separator and grouping. For example, in an American English locale, ‘--block-size="'1kB"’ would cause a size of 1234000 bytes to be displayed as ‘1,234’. In the default C locale, there is no thousands separator so a leading ‘'’ has no effect. An integer block size can be followed by a suffix to specify a multiple of that size. A bare size letter, or one followed by ‘iB’, specifies a multiple using powers of 1024. A size letter followed by ‘B’ specifies powers of 1000 instead. For example, ‘1M’ and ‘1MiB’ are equivalent to ‘1048576’, whereas ‘1MB’ is equivalent to ‘1000000’. A plain suffix without a preceding integer acts as if ‘1’ were prepended, except that it causes a size indication to be appended to the output. For example, ‘--block-size="kB"’ displays 3000 as ‘3kB’. The following suffixes are defined. Large sizes like ‘1Q’ may be rejected by your computer due to limitations of its arithmetic. ‘kB’ kilobyte: 10^3 = 1000. ‘k’ ‘K’ ‘KiB’ kibibyte: 2^{10} = 1024. ‘K’ is special: the SI prefix is ‘k’ and the ISO/IEC 80000-13 prefix is ‘Ki’, but tradition and POSIX use ‘k’ to mean ‘KiB’. ‘MB’ megabyte: 10^6 = 1,000,000. ‘M’ ‘MiB’ mebibyte: 2^{20} = 1,048,576. ‘GB’ gigabyte: 10^9 = 1,000,000,000. ‘G’ ‘GiB’ gibibyte: 2^{30} = 1,073,741,824. ‘TB’ terabyte: 10^{12} = 1,000,000,000,000. ‘T’ ‘TiB’ tebibyte: 2^{40} = 1,099,511,627,776. ‘PB’ petabyte: 10^{15} = 1,000,000,000,000,000. ‘P’ ‘PiB’ pebibyte: 2^{50} = 1,125,899,906,842,624. ‘EB’ exabyte: 10^{18} = 1,000,000,000,000,000,000. ‘E’ ‘EiB’ exbibyte: 2^{60} = 1,152,921,504,606,846,976. ‘ZB’ zettabyte: 10^{21} = 1,000,000,000,000,000,000,000 ‘Z’ ‘ZiB’ zebibyte: 2^{70} = 1,180,591,620,717,411,303,424. ‘YB’ yottabyte: 10^{24} = 1,000,000,000,000,000,000,000,000. ‘Y’ ‘YiB’ yobibyte: 2^{80} = 1,208,925,819,614,629,174,706,176. ‘RB’ ronnabyte: 10^{27} = 1,000,000,000,000,000,000,000,000,000. ‘R’ ‘RiB’ robibyte: 2^{90} = 1,237,940,039,285,380,274,899,124,224. ‘QB’ quettabyte: 10^{30} = 1,000,000,000,000,000,000,000,000,000,000. ‘Q’ ‘QiB’ quebibyte: 2^{100} = 1,267,650,600,228,229,401,496,703,205,376. Block size defaults can be overridden by an explicit ‘--block-size=SIZE’ option. The ‘-k’ option is equivalent to ‘--block-size=1K’, which is the default unless the ‘POSIXLY_CORRECT’ environment variable is set. The ‘-h’ or ‘--human-readable’ option is equivalent to ‘--block-size=human-readable’. The ‘--si’ option is equivalent to ‘--block-size=si’. Note for ‘ls’ the ‘-k’ option does not control the display of the apparent file sizes, whereas the ‘--block-size’ option does. 2.3 Signal specifications ========================= A SIGNAL may be a signal name like ‘HUP’, or a signal number like ‘1’, or an exit status of a process terminated by the signal. A signal name can be given in canonical form or prefixed by ‘SIG’. The case of the letters is ignored. The following signal names and numbers are supported on all POSIX compliant systems: ‘HUP’ 1. Hangup. ‘INT’ 2. Terminal interrupt. ‘QUIT’ 3. Terminal quit. ‘ABRT’ 6. Process abort. ‘KILL’ 9. Kill (cannot be caught or ignored). ‘ALRM’ 14. Alarm Clock. ‘TERM’ 15. Termination. Other supported signal names have system-dependent corresponding numbers. All systems conforming to POSIX 1003.1-2001 also support the following signals: ‘BUS’ Access to an undefined portion of a memory object. ‘CHLD’ Child process terminated, stopped, or continued. ‘CONT’ Continue executing, if stopped. ‘FPE’ Erroneous arithmetic operation. ‘ILL’ Illegal Instruction. ‘PIPE’ Write on a pipe with no one to read it. ‘SEGV’ Invalid memory reference. ‘STOP’ Stop executing (cannot be caught or ignored). ‘TSTP’ Terminal stop. ‘TTIN’ Background process attempting read. ‘TTOU’ Background process attempting write. ‘URG’ High bandwidth data is available at a socket. ‘USR1’ User-defined signal 1. ‘USR2’ User-defined signal 2. POSIX 1003.1-2001 systems that support the XSI extension also support the following signals: ‘POLL’ Pollable event. ‘PROF’ Profiling timer expired. ‘SYS’ Bad system call. ‘TRAP’ Trace/breakpoint trap. ‘VTALRM’ Virtual timer expired. ‘XCPU’ CPU time limit exceeded. ‘XFSZ’ File size limit exceeded. POSIX 1003.1-2001 systems that support the XRT extension also support at least eight real-time signals called ‘RTMIN’, ‘RTMIN+1’, ..., ‘RTMAX-1’, ‘RTMAX’. 2.4 chown, chgrp, chroot, id: Disambiguating user names and IDs =============================================================== Since the USER and GROUP arguments to these commands may be specified as names or numeric IDs, there is an apparent ambiguity. What if a user or group _name_ is a string of digits? Should the command interpret it as a user name or as an ID? (Using a number as a user name is common in some environments.) POSIX requires that these commands first attempt to resolve the specified string as a name, and only once that fails, then try to interpret it as an ID. This is troublesome when you want to specify a numeric ID, say 42, and it must work even in a pathological situation where ‘42’ is a user name that maps to some other user ID, say 1000. Simply invoking ‘chown 42 F’, will set ‘F’s owner ID to 1000 – not what you intended. GNU ‘chown’, ‘chgrp’, ‘chroot’, and ‘id’ provide a way to work around this, that at the same time may result in a significant performance improvement by eliminating a database look-up. Simply precede each numeric user ID and/or group ID with a ‘+’, in order to force its interpretation as an integer: chown +42 F chgrp +$numeric_group_id another-file chown +0:+0 / The name look-up process is skipped for each ‘+’-prefixed string, because a string containing ‘+’ is never a valid user or group name. This syntax is accepted on most common Unix systems, but not on Solaris 10. 2.5 Sources of random data ========================== The ‘shuf’, ‘shred’, and ‘sort’ commands sometimes need random data to do their work. For example, ‘sort -R’ must choose a hash function at random, and it needs random data to make this selection. By default these commands use an internal pseudo-random generator initialized by a small amount of entropy, but can be directed to use an external source with the ‘--random-source=FILE’ option. An error is reported if FILE does not contain enough bytes. For example, the device file ‘/dev/urandom’ could be used as the source of random data. Typically, this device gathers environmental noise from device drivers and other sources into an entropy pool, and uses the pool to generate random bits. If the pool is short of data, the device reuses the internal pool to produce more bits, using a cryptographically secure pseudo-random number generator. But be aware that this device is not designed for bulk random data generation and is relatively slow. ‘/dev/urandom’ suffices for most practical uses, but applications requiring high-value or long-term protection of private data may require an alternate data source like ‘/dev/random’ or ‘/dev/arandom’. The set of available sources depends on your operating system. To reproduce the results of an earlier invocation of a command, you can save some random data into a file and then use that file as the random source in earlier and later invocations of the command. Rather than depending on a file, one can generate a reproducible arbitrary amount of pseudo-random data given a seed value, using for example: get_seeded_random() { seed="$1" openssl enc -aes-256-ctr -pass pass:"$seed" -nosalt \ /dev/null } shuf -i1-100 --random-source=<(get_seeded_random 42) 2.6 Target directory ==================== The ‘cp’, ‘install’, ‘ln’, and ‘mv’ commands normally treat the last operand specially when it is a directory or a symbolic link to a directory. For example, ‘cp source dest’ is equivalent to ‘cp source dest/source’ if ‘dest’ is a directory. Sometimes this behavior is not exactly what is wanted, so these commands support the following options to allow more fine-grained control: ‘-T’ ‘--no-target-directory’ Do not treat the last operand specially when it is a directory or a symbolic link to a directory. This can help avoid race conditions in programs that operate in a shared area. For example, when the command ‘mv /tmp/source /tmp/dest’ succeeds, there is no guarantee that ‘/tmp/source’ was renamed to ‘/tmp/dest’: it could have been renamed to ‘/tmp/dest/source’ instead, if some other process created ‘/tmp/dest’ as a directory. However, if ‘mv -T /tmp/source /tmp/dest’ succeeds, there is no question that ‘/tmp/source’ was renamed to ‘/tmp/dest’. In the opposite situation, where you want the last operand to be treated as a directory and want a diagnostic otherwise, you can use the ‘--target-directory’ (‘-t’) option. ‘-t DIRECTORY’ ‘--target-directory=DIRECTORY’ Use DIRECTORY as the directory component of each destination file name. The interface for most programs is that after processing options and a finite (possibly zero) number of fixed-position arguments, the remaining argument list is either expected to be empty, or is a list of items (usually files) that will all be handled identically. The ‘xargs’ program is designed to work well with this convention. The commands in the ‘mv’-family are unusual in that they take a variable number of arguments with a special case at the _end_ (namely, the target directory). This makes it nontrivial to perform some operations, e.g., “move all files from here to ../d/”, because ‘mv * ../d/’ might exhaust the argument space, and ‘ls | xargs ...’ doesn’t have a clean way to specify an extra final argument for each invocation of the subject command. (It can be done by going through a shell command, but that requires more human labor and brain power than it should.) The ‘--target-directory’ (‘-t’) option allows the ‘cp’, ‘install’, ‘ln’, and ‘mv’ programs to be used conveniently with ‘xargs’. For example, you can move the files from the current directory to a sibling directory, ‘d’ like this: ls | xargs mv -t ../d -- However, this doesn’t move files whose names begin with ‘.’. If you use the GNU ‘find’ program, you can move those files too, with this command: find . -mindepth 1 -maxdepth 1 \ | xargs mv -t ../d But both of the above approaches fail if there are no files in the current directory, or if any file has a name containing a blank or some other special characters. The following example removes those limitations and requires both GNU ‘find’ and GNU ‘xargs’: find . -mindepth 1 -maxdepth 1 -print0 \ | xargs --null --no-run-if-empty \ mv -t ../d The ‘--target-directory’ (‘-t’) and ‘--no-target-directory’ (‘-T’) options cannot be combined. 2.7 Trailing slashes ==================== Some GNU programs (at least ‘cp’ and ‘mv’) allow you to remove any trailing slashes from each SOURCE argument before operating on it. The ‘--strip-trailing-slashes’ option enables this behavior. This is useful when a SOURCE argument may have a trailing slash and specify a symbolic link to a directory. This scenario is in fact rather common because some shells can automatically append a trailing slash when performing file name completion on such symbolic links. Without this option, ‘mv’, for example, (via the system’s rename function) must interpret a trailing slash as a request to dereference the symbolic link and so must rename the indirectly referenced _directory_ and not the symbolic link. Although it may seem surprising that such behavior be the default, it is required by POSIX and is consistent with other parts of that standard. 2.8 Traversing symlinks ======================= The following options modify how ‘chown’ and ‘chgrp’ traverse a hierarchy when the ‘--recursive’ (‘-R’) option is also specified. If more than one of the following options is specified, only the final one takes effect. These options specify whether processing a symbolic link to a directory entails operating on just the symbolic link or on all files in the hierarchy rooted at that directory. These options are independent of ‘--dereference’ and ‘--no-dereference’ (‘-h’), which control whether to modify a symlink or its referent. ‘-H’ If ‘--recursive’ (‘-R’) is specified and a command line argument is a symbolic link to a directory, traverse it. ‘-L’ In a recursive traversal, traverse every symbolic link to a directory that is encountered. ‘-P’ Do not traverse any symbolic links. This is the default if none of ‘-H’, ‘-L’, or ‘-P’ is specified. 2.9 Treating ‘/’ specially ========================== Certain commands can operate destructively on entire hierarchies. For example, if a user with appropriate privileges mistakenly runs ‘rm -rf / tmp/junk’, that may remove all files on the entire system. Since there are so few legitimate uses for such a command, GNU ‘rm’ normally declines to operate on any directory that resolves to ‘/’. If you really want to try to remove all the files on your system, you can use the ‘--no-preserve-root’ option, but the default behavior, specified by the ‘--preserve-root’ option, is safer for most purposes. The commands ‘chgrp’, ‘chmod’ and ‘chown’ can also operate destructively on entire hierarchies, so they too support these options. Although, unlike ‘rm’, they don’t actually unlink files, these commands are arguably more dangerous when operating recursively on ‘/’, since they often work much more quickly, and hence damage more files before an alert user can interrupt them. Tradition and POSIX require these commands to operate recursively on ‘/’, so they default to ‘--no-preserve-root’, but using the ‘--preserve-root’ option makes them safer for most purposes. For convenience you can specify ‘--preserve-root’ in an alias or in a shell function. Note that the ‘--preserve-root’ option also ensures that ‘chgrp’ and ‘chown’ do not modify ‘/’ even when dereferencing a symlink pointing to ‘/’. 2.10 Special built-in utilities =============================== Some programs like ‘nice’ can invoke other programs; for example, the command ‘nice cat file’ invokes the program ‘cat’ by executing the command ‘cat file’. However, “special built-in utilities” like ‘exit’ cannot be invoked this way. For example, the command ‘nice exit’ does not have a well-defined behavior: it may generate an error message instead of exiting. Here is a list of the special built-in utilities that are standardized by POSIX 1003.1-2004. . : break continue eval exec exit export readonly return set shift times trap unset For example, because ‘.’, ‘:’, and ‘exec’ are special, the commands ‘nice . foo.sh’, ‘nice :’, and ‘nice exec pwd’ do not work as you might expect. Many shells extend this list. For example, Bash has several extra special built-in utilities like ‘history’, and ‘suspend’, and with Bash the command ‘nice suspend’ generates an error message instead of suspending. 2.11 Exit status ================ Nearly every command invocation yields an integral “exit status” that can be used to change how other commands work. For the vast majority of commands, an exit status of zero indicates success. Failure is indicated by a nonzero value – typically ‘1’, though it may differ on unusual platforms as POSIX requires only that it be nonzero. However, some of the programs documented here do produce other exit status values and a few associate different meanings with the values ‘0’ and ‘1’. Here are the exceptions: ‘chroot’, ‘env’, ‘expr’, ‘ls’, ‘nice’, ‘nohup’, ‘numfmt’, ‘printenv’, ‘runcon’, ‘sort’, ‘stdbuf’, ‘test’, ‘timeout’, ‘tty’. 2.12 Floating point numbers =========================== Commands that accept or produce floating point numbers employ the floating point representation of the underlying system, and suffer from rounding error, overflow, and similar floating-point issues. Almost all modern systems use IEEE-754 floating point, and it is typically portable to assume IEEE-754 behavior these days. IEEE-754 has positive and negative infinity, distinguishes positive from negative zero, and uses special values called NaNs to represent invalid computations such as dividing zero by itself. For more information, please see David Goldberg’s paper What Every Computer Scientist Should Know About Floating-Point Arithmetic (https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html). Commands that accept floating point numbers as options, operands or input use the standard C functions ‘strtod’ and ‘strtold’ to convert from text to floating point numbers. These floating point numbers therefore can use scientific notation like ‘1.0e-34’ and ‘-10e100’. Commands that parse floating point also understand case-insensitive ‘inf’, ‘infinity’, and ‘NaN’, although whether such values are useful depends on the command in question. Modern C implementations also accept hexadecimal floating point numbers such as ‘-0x.ep-3’, which stands for −14/16 times 2^-3, which equals −0.109375. *Note (libc)Parsing of Floats::. Normally the ‘LC_NUMERIC’ locale determines the decimal-point character. However, some commands’ descriptions specify that they accept numbers in either the current or the C locale; for example, they treat ‘3.14’ like ‘3,14’ if the current locale uses comma as a decimal point. 2.13 Standards conformance ========================== In a few cases, the GNU utilities’ default behavior is incompatible with the POSIX standard. To suppress these incompatibilities, define the ‘POSIXLY_CORRECT’ environment variable. Unless you are checking for POSIX conformance, you probably do not need to define ‘POSIXLY_CORRECT’. Newer versions of POSIX are occasionally incompatible with older versions. For example, older versions of POSIX required the command ‘sort +1’ to sort based on the second and succeeding fields in each input line, but in POSIX 1003.1-2001 the same command is required to sort the file named ‘+1’, and you must instead use the command ‘sort -k 2’ to get the field-based sort. To complicate things further, POSIX 1003.1-2008 allows an implementation to have either the old or the new behavior. The GNU utilities normally conform to the version of POSIX that is standard for your system. To cause them to conform to a different version of POSIX, define the ‘_POSIX2_VERSION’ environment variable to a value of the form YYYYMM specifying the year and month the standard was adopted. Three values are currently supported for ‘_POSIX2_VERSION’: ‘199209’ stands for POSIX 1003.2-1992, ‘200112’ stands for POSIX 1003.1-2001, and ‘200809’ stands for POSIX 1003.1-2008. For example, if you have a POSIX 1003.1-2001 system but are running software containing traditional usage like ‘sort +1’ or ‘tail +10’, you can work around the compatibility problems by setting ‘_POSIX2_VERSION=200809’ in your environment. 2.14 ‘coreutils’: Multi-call program ==================================== The ‘coreutils’ command invokes an individual utility, either implicitly selected by the last component of the name used to invoke ‘coreutils’, or explicitly with the ‘--coreutils-prog’ option. Synopsis: coreutils --coreutils-prog=PROGRAM ... The ‘coreutils’ command is not installed by default, so portable scripts should not rely on its existence. 3 Output of entire files ************************ These commands read and write entire files, possibly transforming them in some way. 3.1 ‘cat’: Concatenate and write files ====================================== ‘cat’ copies each FILE (‘-’ means standard input), or standard input if none are given, to standard output. Synopsis: cat [OPTION] [FILE]... The program accepts the following options. Also see *note Common options::. ‘-A’ ‘--show-all’ Equivalent to ‘-vET’. ‘-b’ ‘--number-nonblank’ Number all nonempty output lines, starting with 1. ‘-e’ Equivalent to ‘-vE’. ‘-E’ ‘--show-ends’ Display a ‘$’ after the end of each line. The ‘\r\n’ combination is shown as ‘^M$’. ‘-n’ ‘--number’ Number all output lines, starting with 1. This option is ignored if ‘-b’ is in effect. ‘-s’ ‘--squeeze-blank’ Suppress repeated adjacent blank lines; output just one empty line instead of several. ‘-t’ Equivalent to ‘-vT’. ‘-T’ ‘--show-tabs’ Display TAB characters as ‘^I’. ‘-u’ Ignored; for POSIX compatibility. ‘-v’ ‘--show-nonprinting’ Display control characters except for LFD and TAB using ‘^’ notation and precede characters that have the high bit set with ‘M-’. On systems like MS-DOS that distinguish between text and binary files, ‘cat’ normally reads and writes in binary mode. However, ‘cat’ reads in text mode if one of the options ‘-bensAE’ is used or if ‘cat’ is reading from standard input and standard input is a terminal. Similarly, ‘cat’ writes in text mode if one of the options ‘-bensAE’ is used or if standard output is a terminal. An exit status of zero indicates success, and a nonzero value indicates failure. Examples: # Output f's contents, then standard input, then g's contents. cat f - g # Copy standard input to standard output. cat 3.2 ‘tac’: Concatenate and write files in reverse ================================================= ‘tac’ copies each FILE (‘-’ means standard input), or standard input if none are given, to standard output, reversing the records (lines by default) in each separately. Synopsis: tac [OPTION]... [FILE]... “Records” are separated by instances of a string (newline by default). By default, this separator string is attached to the end of the record that it follows in the file. The program accepts the following options. Also see *note Common options::. ‘-b’ ‘--before’ The separator is attached to the beginning of the record that it precedes in the file. ‘-r’ ‘--regex’ Treat the separator string as a regular expression. ‘-s SEPARATOR’ ‘--separator=SEPARATOR’ Use SEPARATOR as the record separator, instead of newline. Note an empty SEPARATOR is treated as a zero byte. I.e., input and output items are delimited with ASCII NUL. On systems like MS-DOS that distinguish between text and binary files, ‘tac’ reads and writes in binary mode. An exit status of zero indicates success, and a nonzero value indicates failure. Example: # Reverse a file character by character. tac -r -s 'x\|[^x]' 3.3 ‘nl’: Number lines and write files ====================================== ‘nl’ writes each FILE (‘-’ means standard input), or standard input if none are given, to standard output, with line numbers added to some or all of the lines. Synopsis: nl [OPTION]... [FILE]... ‘nl’ decomposes its input into (logical) page sections; by default, the line number is reset to 1 at each logical page section. ‘nl’ treats all of the input files as a single document; it does not reset line numbers or logical pages between files. A logical page consists of three sections: header, body, and footer. Any of the sections can be empty. Each can be numbered in a different style from the others. The beginnings of the sections of logical pages are indicated in the input file by a line containing exactly one of these delimiter strings: ‘\:\:\:’ start of header; ‘\:\:’ start of body; ‘\:’ start of footer. The characters from which these strings are made can be changed from ‘\’ and ‘:’ via options (see below), but the pattern of each string cannot be changed. A section delimiter is replaced by an empty line on output. Any text that comes before the first section delimiter string in the input file is considered to be part of a body section, so ‘nl’ treats a file that contains no section delimiters as a single body section. The program accepts the following options. Also see *note Common options::. ‘-b STYLE’ ‘--body-numbering=STYLE’ Select the numbering style for lines in the body section of each logical page. When a line is not numbered, the current line number is not incremented, but the line number separator character is still prepended to the line. The styles are: ‘a’ number all lines, ‘t’ number only nonempty lines (default for body), ‘n’ do not number lines (default for header and footer), ‘pBRE’ number only lines that contain a match for the basic regular expression BRE. *Note Regular Expressions: (grep)Regular Expressions. ‘-d CD’ ‘--section-delimiter=CD’ Set the section delimiter characters to CD; default is ‘\:’. If only C is given, the second remains ‘:’. As a GNU extension more than two characters can be specified, and also if CD is empty (‘-d ''’), then section matching is disabled. (Remember to protect ‘\’ or other metacharacters from shell expansion with quotes or extra backslashes.) ‘-f STYLE’ ‘--footer-numbering=STYLE’ Analogous to ‘--body-numbering’. ‘-h STYLE’ ‘--header-numbering=STYLE’ Analogous to ‘--body-numbering’. ‘-i NUMBER’ ‘--line-increment=NUMBER’ Increment line numbers by NUMBER (default 1). NUMBER can be negative to decrement. ‘-l NUMBER’ ‘--join-blank-lines=NUMBER’ Consider NUMBER (default 1) consecutive empty lines to be one logical line for numbering, and only number the last one. Where fewer than NUMBER consecutive empty lines occur, do not number them. An empty line is one that contains no characters, not even spaces or tabs. ‘-n FORMAT’ ‘--number-format=FORMAT’ Select the line numbering format (default is ‘rn’): ‘ln’ left justified, no leading zeros; ‘rn’ right justified, no leading zeros; ‘rz’ right justified, leading zeros. ‘-p’ ‘--no-renumber’ Do not reset the line number at the start of a logical page. ‘-s STRING’ ‘--number-separator=STRING’ Separate the line number from the text line in the output with STRING (default is the TAB character). ‘-v NUMBER’ ‘--starting-line-number=NUMBER’ Set the initial line number on each logical page to NUMBER (default 1). The starting NUMBER can be negative. ‘-w NUMBER’ ‘--number-width=NUMBER’ Use NUMBER characters for line numbers (default 6). An exit status of zero indicates success, and a nonzero value indicates failure. 3.4 ‘od’: Write files in octal or other formats =============================================== ‘od’ writes an unambiguous representation of each FILE (‘-’ means standard input), or standard input if none are given. Synopses: od [OPTION]... [FILE]... od [-abcdfilosx]... [FILE] [[+]OFFSET[.][b]] od [OPTION]... --traditional [FILE] [[+]OFFSET[.][b] [[+]LABEL[.][b]]] Each line of output consists of the offset in the input, followed by groups of data from the file. By default, ‘od’ prints the offset in octal, and each group of file data is a C ‘short int’’s worth of input printed as a single octal number. If OFFSET is given, it specifies how many input bytes to skip before formatting and writing. By default, it is interpreted as an octal number, but the optional trailing decimal point causes it to be interpreted as decimal. If no decimal is specified and the offset begins with ‘0x’ or ‘0X’ it is interpreted as a hexadecimal number. If there is a trailing ‘b’, the number of bytes skipped will be OFFSET multiplied by 512. If a command is of both the first and second forms, the second form is assumed if the last operand begins with ‘+’ or (if there are two operands) a digit. For example, in ‘od foo 10’ and ‘od +10’ the ‘10’ is an offset, whereas in ‘od 10’ the ‘10’ is a file name. The program accepts the following options. Also see *note Common options::. ‘-A RADIX’ ‘--address-radix=RADIX’ Select the base in which file offsets are printed. RADIX can be one of the following: ‘d’ decimal; ‘o’ octal; ‘x’ hexadecimal; ‘n’ none (do not print offsets). The default is octal. ‘--endian=ORDER’ Reorder input bytes, to handle inputs with differing byte orders, or to provide consistent output independent of the endian convention of the current system. Swapping is performed according to the specified ‘--type’ size and endian ORDER, which can be ‘little’ or ‘big’. ‘-j BYTES’ ‘--skip-bytes=BYTES’ Skip BYTES input bytes before formatting and writing. If BYTES begins with ‘0x’ or ‘0X’, it is interpreted in hexadecimal; otherwise, if it begins with ‘0’, in octal; otherwise, in decimal. BYTES may be, or may be an integer optionally followed by, one of the following multiplicative suffixes: ‘b’ => 512 ("blocks") ‘KB’ => 1000 (KiloBytes) ‘K’ => 1024 (KibiBytes) ‘MB’ => 1000*1000 (MegaBytes) ‘M’ => 1024*1024 (MebiBytes) ‘GB’ => 1000*1000*1000 (GigaBytes) ‘G’ => 1024*1024*1024 (GibiBytes) and so on for ‘T’, ‘P’, ‘E’, ‘Z’, ‘Y’, ‘R’, and ‘Q’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on. ‘-N BYTES’ ‘--read-bytes=BYTES’ Output at most BYTES bytes of the input. Prefixes and suffixes on ‘bytes’ are interpreted as for the ‘-j’ option. ‘-S BYTES’ ‘--strings[=BYTES]’ Instead of the normal output, output only “string constants”: at least BYTES consecutive printable characters, followed by a zero byte (ASCII NUL). Prefixes and suffixes on BYTES are interpreted as for the ‘-j’ option. If BYTES is omitted with ‘--strings’, the default is 3. ‘-t TYPE’ ‘--format=TYPE’ Select the format in which to output the file data. TYPE is a string of one or more of the below type indicator characters. If you include more than one type indicator character in a single TYPE string, or use this option more than once, ‘od’ writes one copy of each output line using each of the data types that you specified, in the order that you specified. Adding a trailing “z” to any type specification appends a display of the single byte character representation of the printable characters to the output line generated by the type specification. ‘a’ named character, ignoring high-order bit ‘c’ printable single byte character, C backslash escape or a 3 digit octal sequence ‘d’ signed decimal ‘f’ floating point (*note Floating point::) ‘o’ octal ‘u’ unsigned decimal ‘x’ hexadecimal The type ‘a’ outputs things like ‘sp’ for space, ‘nl’ for newline, and ‘nul’ for a zero byte. Only the least significant seven bits of each byte is used; the high-order bit is ignored. Type ‘c’ outputs ‘ ’, ‘\n’, and ‘\0’, respectively. Except for types ‘a’ and ‘c’, you can specify the number of bytes to use in interpreting each number in the given data type by following the type indicator character with a decimal integer. Alternately, you can specify the size of one of the C compiler’s built-in data types by following the type indicator character with one of the following characters. For integers (‘d’, ‘o’, ‘u’, ‘x’): ‘C’ char ‘S’ short ‘I’ int ‘L’ long For floating point (‘f’): F float D double L long double ‘-v’ ‘--output-duplicates’ Output consecutive lines that are identical. By default, when two or more consecutive output lines would be identical, ‘od’ outputs only the first line, and puts just an asterisk on the following line to indicate the elision. ‘-w[N]’ ‘--width[=N]’ Dump ‘n’ input bytes per output line. This must be a multiple of the least common multiple of the sizes associated with the specified output types. If this option is not given at all, the default is 16. If N is omitted, the default is 32. The next several options are shorthands for format specifications. GNU ‘od’ accepts any combination of shorthands and format specification options. These options accumulate. ‘-a’ Output as named characters. Equivalent to ‘-t a’. ‘-b’ Output as octal bytes. Equivalent to ‘-t o1’. ‘-c’ Output as printable single byte characters, C backslash escapes or 3 digit octal sequences. Equivalent to ‘-t c’. ‘-d’ Output as unsigned decimal two-byte units. Equivalent to ‘-t u2’. ‘-f’ Output as floats. Equivalent to ‘-t fF’. ‘-i’ Output as decimal ints. Equivalent to ‘-t dI’. ‘-l’ Output as decimal long ints. Equivalent to ‘-t dL’. ‘-o’ Output as octal two-byte units. Equivalent to ‘-t o2’. ‘-s’ Output as decimal two-byte units. Equivalent to ‘-t d2’. ‘-x’ Output as hexadecimal two-byte units. Equivalent to ‘-t x2’. ‘--traditional’ Recognize the non-option label argument that traditional ‘od’ accepted. The following syntax: od --traditional [FILE] [[+]OFFSET[.][b] [[+]LABEL[.][b]]] can be used to specify at most one file and optional arguments specifying an offset and a pseudo-start address, LABEL. The LABEL argument is interpreted just like OFFSET, but it specifies an initial pseudo-address. The pseudo-addresses are displayed in parentheses following any normal address. An exit status of zero indicates success, and a nonzero value indicates failure. 3.5 ‘base32’: Transform data into printable data ================================================ ‘base32’ transforms data read from a file, or standard input, into (or from) base32 encoded form. The base32 encoded form uses printable ASCII characters to represent binary data. The usage and options of this command are precisely the same as for ‘base64’. *Note base64 invocation::. For more general encoding functionality see *note basenc invocation::. 3.6 ‘base64’: Transform data into printable data ================================================ ‘base64’ transforms data read from a file, or standard input, into (or from) base64 encoded form. The base64 encoded form uses printable ASCII characters to represent binary data. Synopses: base64 [OPTION]... [FILE] base64 --decode [OPTION]... [FILE] The base64 encoding expands data to roughly 133% of the original. The base32 encoding expands data to roughly 160% of the original. The format conforms to RFC 4648 (https://tools.ietf.org/search/rfc4648). For more general encoding functionality see *note basenc invocation::. The program accepts the following options. Also see *note Common options::. ‘-w COLS’ ‘--wrap=COLS’ During encoding, wrap lines after COLS characters. This must be a positive number. The default is to wrap after 76 characters. Use the value 0 to disable line wrapping altogether. ‘-d’ ‘--decode’ Change the mode of operation, from the default of encoding data, to decoding data. Input is expected to be base64 encoded data, and the output will be the original data. ‘-i’ ‘--ignore-garbage’ When decoding, newlines are always accepted. During decoding, ignore unrecognized bytes, to permit distorted data to be decoded. An exit status of zero indicates success, and a nonzero value indicates failure. 3.7 ‘basenc’: Transform data into printable data ================================================ ‘basenc’ transforms data read from a file, or standard input, into (or from) various common encoding forms. The encoded form uses printable ASCII characters to represent binary data. Synopses: basenc ENCODING [OPTION]... [FILE] basenc ENCODING --decode [OPTION]... [FILE] The ENCODING argument is required. If FILE is omitted, ‘basenc’ reads from standard input. The ‘-w/--wrap’,‘-i/--ignore-garbage’, ‘-d/--decode’ options of this command are precisely the same as for ‘base64’. *Note base64 invocation::. Supported ENCODINGs are: ‘--base64’ Encode into (or decode from with ‘-d/--decode’) base64 form. The format conforms to RFC 4648#4 (https://tools.ietf.org/search/rfc4648#section-4). Equivalent to the ‘base64’ command. ‘--base64url’ Encode into (or decode from with ‘-d/--decode’) file-and-url-safe base64 form (using ‘_’ and ‘-’ instead of ‘+’ and ‘/’). The format conforms to RFC 4648#5 (https://tools.ietf.org/search/rfc4648#section-5). ‘--base32’ Encode into (or decode from with ‘-d/--decode’) base32 form. The encoded data uses the ‘ABCDEFGHIJKLMNOPQRSTUVWXYZ234567=’ characters. The format conforms to RFC 4648#6 (https://tools.ietf.org/search/rfc4648#section-6). Equivalent to the ‘base32’ command. ‘--base32hex’ Encode into (or decode from with ‘-d/--decode’) Extended Hex Alphabet base32 form. The encoded data uses the ‘0123456789ABCDEFGHIJKLMNOPQRSTUV=’ characters. The format conforms to RFC 4648#7 (https://tools.ietf.org/search/rfc4648#section-7). ‘--base16’ Encode into (or decode from with ‘-d/--decode’) base16 (hexadecimal) form. The encoded data uses the ‘0123456789ABCDEF’ characters. The format conforms to RFC 4648#8 (https://tools.ietf.org/search/rfc4648#section-8). ‘--base2lsbf’ Encode into (or decode from with ‘-d/--decode’) binary string form (‘0’ and ‘1’) with the _least_ significant bit of every byte first. ‘--base2msbf’ Encode into (or decode from with ‘-d/--decode’) binary string form (‘0’ and ‘1’) with the _most_ significant bit of every byte first. ‘--z85’ Encode into (or decode from with ‘-d/--decode’) Z85 form (a modified Ascii85 form). The encoded data uses the ‘0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTU VWXYZ.-:+=^!/*?&<>()[]{}@%$#’. characters. The format conforms to ZeroMQ spec:32/Z85 (https://rfc.zeromq.org/spec:32/Z85/). When encoding with ‘--z85’, input length must be a multiple of 4; when decoding with ‘--z85’, input length must be a multiple of 5. Encoding/decoding examples: $ printf '\376\117\202' | basenc --base64 /k+C $ printf '\376\117\202' | basenc --base64url _k-C $ printf '\376\117\202' | basenc --base32 7ZHYE=== $ printf '\376\117\202' | basenc --base32hex VP7O4=== $ printf '\376\117\202' | basenc --base16 FE4F82 $ printf '\376\117\202' | basenc --base2lsbf 011111111111001001000001 $ printf '\376\117\202' | basenc --base2msbf 111111100100111110000010 $ printf '\376\117\202\000' | basenc --z85 @.FaC $ printf 01010100 | basenc --base2msbf --decode T $ printf 01010100 | basenc --base2lsbf --decode * 4 Formatting file contents ************************** These commands reformat the contents of files. 4.1 ‘fmt’: Reformat paragraph text ================================== ‘fmt’ fills and joins lines to produce output lines of (at most) a given number of characters (75 by default). Synopsis: fmt [OPTION]... [FILE]... ‘fmt’ reads from the specified FILE arguments (or standard input if none are given), and writes to standard output. By default, blank lines, spaces between words, and indentation are preserved in the output; successive input lines with different indentation are not joined; tabs are expanded on input and introduced on output. ‘fmt’ prefers breaking lines at the end of a sentence, and tries to avoid line breaks after the first word of a sentence or before the last word of a sentence. A “sentence break” is defined as either the end of a paragraph or a word ending in any of ‘.?!’, followed by two spaces or end of line, ignoring any intervening parentheses or quotes. Like TeX, ‘fmt’ reads entire “paragraphs” before choosing line breaks; the algorithm is a variant of that given by Donald E. Knuth and Michael F. Plass in “Breaking Paragraphs Into Lines”, ‘Software: Practice & Experience’ 11, 11 (November 1981), 1119–1184. The program accepts the following options. Also see *note Common options::. ‘-c’ ‘--crown-margin’ “Crown margin” mode: preserve the indentation of the first two lines within a paragraph, and align the left margin of each subsequent line with that of the second line. ‘-t’ ‘--tagged-paragraph’ “Tagged paragraph” mode: like crown margin mode, except that if indentation of the first line of a paragraph is the same as the indentation of the second, the first line is treated as a one-line paragraph. ‘-s’ ‘--split-only’ Split lines only. Do not join short lines to form longer ones. This prevents sample lines of code, and other such “formatted” text from being unduly combined. ‘-u’ ‘--uniform-spacing’ Uniform spacing. Reduce spacing between words to one space, and spacing between sentences to two spaces. ‘-WIDTH’ ‘-w WIDTH’ ‘--width=WIDTH’ Fill output lines up to WIDTH characters (default 75 or GOAL plus 10, if GOAL is provided). ‘-g GOAL’ ‘--goal=GOAL’ ‘fmt’ initially tries to make lines GOAL characters wide. By default, this is 7% shorter than WIDTH. ‘-p PREFIX’ ‘--prefix=PREFIX’ Only lines beginning with PREFIX (possibly preceded by whitespace) are subject to formatting. The prefix and any preceding whitespace are stripped for the formatting and then re-attached to each formatted output line. One use is to format certain kinds of program comments, while leaving the code unchanged. An exit status of zero indicates success, and a nonzero value indicates failure. 4.2 ‘pr’: Paginate or columnate files for printing ================================================== ‘pr’ writes each FILE (‘-’ means standard input), or standard input if none are given, to standard output, paginating and optionally outputting in multicolumn format; optionally merges all FILEs, printing all in parallel, one per column. Synopsis: pr [OPTION]... [FILE]... By default, a 5-line header is printed at each page: two blank lines; a line with the date, the file name, and the page count; and two more blank lines. A footer of five blank lines is also printed. The default PAGE_LENGTH is 66 lines. The default number of text lines is therefore 56. The text line of the header takes the form ‘DATE STRING PAGE’, with spaces inserted around STRING so that the line takes up the full PAGE_WIDTH. Here, DATE is the date (see the ‘-D’ or ‘--date-format’ option for details), STRING is the centered header string, and PAGE identifies the page number. The ‘LC_MESSAGES’ locale category affects the spelling of PAGE; in the default C locale, it is ‘Page NUMBER’ where NUMBER is the decimal page number. Form feeds in the input cause page breaks in the output. Multiple form feeds produce empty pages. Columns are of equal width, separated by an optional string (default is ‘space’). For multicolumn output, lines will always be truncated to PAGE_WIDTH (default 72), unless you use the ‘-J’ option. For single column output no line truncation occurs by default. Use ‘-W’ option to truncate lines in that case. The program accepts the following options. Also see *note Common options::. ‘+FIRST_PAGE[:LAST_PAGE]’ ‘--pages=FIRST_PAGE[:LAST_PAGE]’ Begin printing with page FIRST_PAGE and stop with LAST_PAGE. Missing ‘:LAST_PAGE’ implies end of file. While estimating the number of skipped pages each form feed in the input file results in a new page. Page counting with and without ‘+FIRST_PAGE’ is identical. By default, counting starts with the first page of input file (not first page printed). Line numbering may be altered by ‘-N’ option. ‘-COLUMN’ ‘--columns=COLUMN’ With each single FILE, produce COLUMN columns of output (default is 1) and print columns down, unless ‘-a’ is used. The column width is automatically decreased as COLUMN increases; unless you use the ‘-W/-w’ option to increase PAGE_WIDTH as well. This option might well cause some lines to be truncated. The number of lines in the columns on each page are balanced. The options ‘-e’ and ‘-i’ are on for multiple text-column output. Together with ‘-J’ option column alignment and line truncation is turned off. Lines of full length are joined in a free field format and ‘-S’ option may set field separators. ‘-COLUMN’ may not be used with ‘-m’ option. ‘-a’ ‘--across’ With each single FILE, print columns across rather than down. The ‘-COLUMN’ option must be given with COLUMN greater than one. If a line is too long to fit in a column, it is truncated. ‘-c’ ‘--show-control-chars’ Print control characters using hat notation (e.g., ‘^G’); print other nonprinting characters in octal backslash notation. By default, nonprinting characters are not changed. ‘-d’ ‘--double-space’ Double space the output. ‘-D FORMAT’ ‘--date-format=FORMAT’ Format header dates using FORMAT, using the same conventions as for the command ‘date +FORMAT’. *Note date invocation::. Except for directives, which start with ‘%’, characters in FORMAT are printed unchanged. You can use this option to specify an arbitrary string in place of the header date, e.g., ‘--date-format="Monday morning"’. The default date format is ‘%Y-%m-%d %H:%M’ (for example, ‘2020-07-09 23:59’); but if the ‘POSIXLY_CORRECT’ environment variable is set and the ‘LC_TIME’ locale category specifies the POSIX locale, the default is ‘%b %e %H:%M %Y’ (for example, ‘Jul 9 23:59 2020’. Timestamps are listed according to the time zone rules specified by the ‘TZ’ environment variable, or by the system default rules if ‘TZ’ is not set. *Note Specifying the Time Zone with ‘TZ’: (libc)TZ Variable. ‘-e[IN-TABCHAR[IN-TABWIDTH]]’ ‘--expand-tabs[=IN-TABCHAR[IN-TABWIDTH]]’ Expand TABs to spaces on input. Optional argument IN-TABCHAR is the input tab character (default is the TAB character). Second optional argument IN-TABWIDTH is the input tab character’s width (default is 8). ‘-f’ ‘-F’ ‘--form-feed’ Use a form feed instead of newlines to separate output pages. This does not alter the default page length of 66 lines. ‘-h HEADER’ ‘--header=HEADER’ Replace the file name in the header with the centered string HEADER. When using the shell, HEADER should be quoted and should be separated from ‘-h’ by a space. ‘-i[OUT-TABCHAR[OUT-TABWIDTH]]’ ‘--output-tabs[=OUT-TABCHAR[OUT-TABWIDTH]]’ Replace spaces with TABs on output. Optional argument OUT-TABCHAR is the output tab character (default is the TAB character). Second optional argument OUT-TABWIDTH is the output tab character’s width (default is 8). ‘-J’ ‘--join-lines’ Merge lines of full length. Used together with the column options ‘-COLUMN’, ‘-a -COLUMN’ or ‘-m’. Turns off ‘-W/-w’ line truncation; no column alignment used; may be used with ‘--sep-string[=STRING]’. ‘-J’ has been introduced (together with ‘-W’ and ‘--sep-string’) to disentangle the old (POSIX-compliant) options ‘-w’ and ‘-s’ along with the three column options. ‘-l PAGE_LENGTH’ ‘--length=PAGE_LENGTH’ Set the page length to PAGE_LENGTH (default 66) lines, including the lines of the header [and the footer]. If PAGE_LENGTH is less than or equal to 10, the header and footer are omitted, as if the ‘-t’ option had been given. ‘-m’ ‘--merge’ Merge and print all FILEs in parallel, one in each column. If a line is too long to fit in a column, it is truncated, unless the ‘-J’ option is used. ‘--sep-string[=STRING]’ may be used. Empty pages in some FILEs (form feeds set) produce empty columns, still marked by STRING. The result is a continuous line numbering and column marking throughout the whole merged file. Completely empty merged pages show no separators or line numbers. The default header becomes ‘DATE PAGE’ with spaces inserted in the middle; this may be used with the ‘-h’ or ‘--header’ option to fill up the middle blank part. ‘-n[NUMBER-SEPARATOR[DIGITS]]’ ‘--number-lines[=NUMBER-SEPARATOR[DIGITS]]’ Provide DIGITS digit line numbering (default for DIGITS is 5). With multicolumn output the number occupies the first DIGITS column positions of each text column or only each line of ‘-m’ output. With single column output the number precedes each line just as ‘-m’ does. Default counting of the line numbers starts with the first line of the input file (not the first line printed, compare the ‘--page’ option and ‘-N’ option). Optional argument NUMBER-SEPARATOR is the character appended to the line number to separate it from the text followed. The default separator is the TAB character. In a strict sense a TAB is always printed with single column output only. The TAB width varies with the TAB position, e.g., with the left MARGIN specified by ‘-o’ option. With multicolumn output priority is given to ‘equal width of output columns’ (a POSIX specification). The TAB width is fixed to the value of the first column and does not change with different values of left MARGIN. That means a fixed number of spaces is always printed in the place of the NUMBER-SEPARATOR TAB. The tabification depends upon the output position. ‘-N LINE_NUMBER’ ‘--first-line-number=LINE_NUMBER’ Start line counting with the number LINE_NUMBER at first line of first page printed (in most cases not the first line of the input file). ‘-o MARGIN’ ‘--indent=MARGIN’ Indent each line with a margin MARGIN spaces wide (default is zero). The total page width is the size of the margin plus the PAGE_WIDTH set with the ‘-W/-w’ option. A limited overflow may occur with numbered single column output (compare ‘-n’ option). ‘-r’ ‘--no-file-warnings’ Do not print a warning message when an argument FILE cannot be opened. (The exit status will still be nonzero, however.) ‘-s[CHAR]’ ‘--separator[=CHAR]’ Separate columns by a single character CHAR. The default for CHAR is the TAB character without ‘-w’ and ‘no character’ with ‘-w’. Without ‘-s’ the default separator ‘space’ is set. ‘-s[char]’ turns off line truncation of all three column options (‘-COLUMN’|‘-a -COLUMN’|‘-m’) unless ‘-w’ is set. This is a POSIX-compliant formulation. ‘-S[STRING]’ ‘--sep-string[=STRING]’ Use STRING to separate output columns. The ‘-S’ option doesn’t affect the ‘-W/-w’ option, unlike the ‘-s’ option which does. It does not affect line truncation or column alignment. Without ‘-S’, and with ‘-J’, ‘pr’ uses the default output separator, TAB. Without ‘-S’ or ‘-J’, ‘pr’ uses a ‘space’ (same as ‘-S" "’). If no ‘STRING’ argument is specified, ‘""’ is assumed. ‘-t’ ‘--omit-header’ Do not print the usual header [and footer] on each page, and do not fill out the bottom of pages (with blank lines or a form feed). No page structure is produced, but form feeds set in the input files are retained. The predefined pagination is not changed. ‘-t’ or ‘-T’ may be useful together with other options; e.g.: ‘-t -e4’, expand TAB characters in the input file to 4 spaces but don’t make any other changes. Use of ‘-t’ overrides ‘-h’. ‘-T’ ‘--omit-pagination’ Do not print header [and footer]. In addition eliminate all form feeds set in the input files. ‘-v’ ‘--show-nonprinting’ Print nonprinting characters in octal backslash notation. ‘-w PAGE_WIDTH’ ‘--width=PAGE_WIDTH’ Set page width to PAGE_WIDTH characters for multiple text-column output only (default for PAGE_WIDTH is 72). The specified PAGE_WIDTH is rounded down so that columns have equal width. ‘-s[CHAR]’ turns off the default page width and any line truncation and column alignment. Lines of full length are merged, regardless of the column options set. No PAGE_WIDTH setting is possible with single column output. A POSIX-compliant formulation. ‘-W PAGE_WIDTH’ ‘--page_width=PAGE_WIDTH’ Set the page width to PAGE_WIDTH characters, honored with and without a column option. With a column option, the specified PAGE_WIDTH is rounded down so that columns have equal width. Text lines are truncated, unless ‘-J’ is used. Together with one of the three column options (‘-COLUMN’, ‘-a -COLUMN’ or ‘-m’) column alignment is always used. The separator options ‘-S’ or ‘-s’ don’t disable the ‘-W’ option. Default is 72 characters. Without ‘-W PAGE_WIDTH’ and without any of the column options NO line truncation is used (defined to keep downward compatibility and to meet most frequent tasks). That’s equivalent to ‘-W 72 -J’. The header line is never truncated. An exit status of zero indicates success, and a nonzero value indicates failure. 4.3 ‘fold’: Wrap input lines to fit in specified width ====================================================== ‘fold’ writes each FILE (‘-’ means standard input), or standard input if none are given, to standard output, breaking long lines. Synopsis: fold [OPTION]... [FILE]... By default, ‘fold’ breaks lines wider than 80 columns. The output is split into as many lines as necessary. ‘fold’ counts screen columns by default; thus, a tab may count more than one column, backspace decreases the column count, and carriage return sets the column to zero. The program accepts the following options. Also see *note Common options::. ‘-b’ ‘--bytes’ Count bytes rather than columns, so that tabs, backspaces, and carriage returns are each counted as taking up one column, just like other characters. ‘-s’ ‘--spaces’ Break at word boundaries: the line is broken after the last blank before the maximum line length. If the line contains no such blanks, the line is broken at the maximum line length as usual. ‘-w WIDTH’ ‘--width=WIDTH’ Use a maximum line length of WIDTH columns instead of 80. For compatibility ‘fold’ supports an obsolete option syntax ‘-WIDTH’. New scripts should use ‘-w WIDTH’ instead. An exit status of zero indicates success, and a nonzero value indicates failure. 5 Output of parts of files ************************** These commands output pieces of the input. 5.1 ‘head’: Output the first part of files ========================================== ‘head’ prints the first part (10 lines by default) of each FILE; it reads from standard input if no files are given or when given a FILE of ‘-’. Synopsis: head [OPTION]... [FILE]... If more than one FILE is specified, ‘head’ prints a one-line header consisting of: ==> FILE NAME <== before the output for each FILE. The program accepts the following options. Also see *note Common options::. ‘-c [-]NUM’ ‘--bytes=[-]NUM’ Print the first NUM bytes, instead of initial lines. However, if NUM is prefixed with a ‘-’, print all but the last NUM bytes of each file. NUM may be, or may be an integer optionally followed by, one of the following multiplicative suffixes: ‘b’ => 512 ("blocks") ‘KB’ => 1000 (KiloBytes) ‘K’ => 1024 (KibiBytes) ‘MB’ => 1000*1000 (MegaBytes) ‘M’ => 1024*1024 (MebiBytes) ‘GB’ => 1000*1000*1000 (GigaBytes) ‘G’ => 1024*1024*1024 (GibiBytes) and so on for ‘T’, ‘P’, ‘E’, ‘Z’, ‘Y’, ‘R’, and ‘Q’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on. ‘-n [-]NUM’ ‘--lines=[-]NUM’ Output the first NUM lines. However, if NUM is prefixed with a ‘-’, print all but the last NUM lines of each file. Size multiplier suffixes are the same as with the ‘-c’ option. ‘-q’ ‘--quiet’ ‘--silent’ Never print file name headers. ‘-v’ ‘--verbose’ Always print file name headers. ‘-z’ ‘--zero-terminated’ Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters). For compatibility ‘head’ also supports an obsolete option syntax ‘-[NUM][bkm][cqv]’, which is recognized only if it is specified first. NUM is a decimal number optionally followed by a size letter (‘b’, ‘k’, ‘m’) as in ‘-c’, or ‘l’ to mean count by lines, or other option letters (‘cqv’). Scripts intended for standard hosts should use ‘-c NUM’ or ‘-n NUM’ instead. If your script must also run on hosts that support only the obsolete syntax, it is usually simpler to avoid ‘head’, e.g., by using ‘sed 5q’ instead of ‘head -5’. An exit status of zero indicates success, and a nonzero value indicates failure. 5.2 ‘tail’: Output the last part of files ========================================= ‘tail’ prints the last part (10 lines by default) of each FILE; it reads from standard input if no files are given or when given a FILE of ‘-’. Synopsis: tail [OPTION]... [FILE]... If more than one FILE is specified, ‘tail’ prints a one-line header before the output for each FILE, consisting of: ==> FILE NAME <== For further processing of tail output, it can be useful to convert the file headers to line prefixes, which can be done like: tail ... | awk ' /^==> .* <==$/ {prefix=substr($0,5,length-8)":"; next} {print prefix$0} ' | ... GNU ‘tail’ can output any amount of data (some other versions of ‘tail’ cannot). It also has no ‘-r’ option (print in reverse), since reversing a file is really a different job from printing the end of a file; BSD ‘tail’ (which is the one with ‘-r’) can only reverse files that are at most as large as its buffer, which is typically 32 KiB. A more reliable and versatile way to reverse files is the GNU ‘tac’ command. The program accepts the following options. Also see *note Common options::. ‘-c [+]NUM’ ‘--bytes=[+]NUM’ Output the last NUM bytes, instead of final lines. If NUM is prefixed with a ‘+’, start printing with byte NUM from the start of each file. For example to skip the first byte use ‘tail -c +2’, while to skip all but the last byte use ‘tail -c 1’. NUM may be, or may be an integer optionally followed by, one of the following multiplicative suffixes: ‘b’ => 512 ("blocks") ‘KB’ => 1000 (KiloBytes) ‘K’ => 1024 (KibiBytes) ‘MB’ => 1000*1000 (MegaBytes) ‘M’ => 1024*1024 (MebiBytes) ‘GB’ => 1000*1000*1000 (GigaBytes) ‘G’ => 1024*1024*1024 (GibiBytes) and so on for ‘T’, ‘P’, ‘E’, ‘Z’, ‘Y’, ‘R’, and ‘Q’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on. ‘-f’ ‘--follow[=HOW]’ Loop forever trying to read more characters at the end of the file, presumably because the file is growing. If more than one file is given, ‘tail’ prints a header whenever it gets output from a different file, to indicate which file that output is from. There are two ways to specify how you’d like to track files with this option, but that difference is noticeable only when a followed file is removed or renamed. If you’d like to continue to track the end of a growing file even after it has been unlinked, use ‘--follow=descriptor’. This is the default behavior, but it is not useful if you’re tracking a log file that may be rotated (removed or renamed, then reopened). In that case, use ‘--follow=name’ to track the named file, perhaps by reopening it periodically to see if it has been removed and recreated by some other program. Note that the inotify-based implementation handles this case without the need for any periodic reopening. No matter which method you use, if the tracked file is determined to have shrunk, ‘tail’ prints a message saying the file has been truncated and resumes tracking from the start of the file, assuming it has been truncated to 0, which is the usual truncation operation for log files. When a file is removed, ‘tail’’s behavior depends on whether it is following the name or the descriptor. When following by name, tail can detect that a file has been removed and gives a message to that effect, and if ‘--retry’ has been specified it will continue checking periodically to see if the file reappears. When following a descriptor, tail does not detect that the file has been unlinked or renamed and issues no message; even though the file may no longer be accessible via its original name, it may still be growing. The option values ‘descriptor’ and ‘name’ may be specified only with the long form of the option, not with ‘-f’. The ‘-f’ option is ignored if no FILE operand is specified and standard input is a FIFO or a pipe. Likewise, the ‘-f’ option has no effect for any operand specified as ‘-’, when standard input is a FIFO or a pipe. With kernel inotify support, output is triggered by file changes and is generally very prompt. Otherwise, ‘tail’ sleeps for one second between checks – use ‘--sleep-interval=N’ to change that default – which can make the output appear slightly less responsive or bursty. When using tail without inotify support, you can make it more responsive by using a sub-second sleep interval, e.g., via an alias like this: alias tail='tail -s.1' ‘-F’ This option is the same as ‘--follow=name --retry’. That is, tail will attempt to reopen a file when it is removed. Should this fail, tail will keep trying until it becomes accessible again. ‘--max-unchanged-stats=N’ When tailing a file by name, if there have been N (default n=5) consecutive iterations for which the file has not changed, then ‘open’/‘fstat’ the file to determine if that file name is still associated with the same device/inode-number pair as before. When following a log file that is rotated, this is approximately the number of seconds between when tail prints the last pre-rotation lines and when it prints the lines that have accumulated in the new log file. This option is meaningful only when polling (i.e., without inotify) and when following by name. ‘-n [+]NUM’ ‘--lines=[+]’ Output the last NUM lines. If NUM is prefixed with a ‘+’, start printing with line NUM from the start of each file. For example to skip the first line use ‘tail -n +2’, while to skip all but the last line use ‘tail -n 1’. Size multiplier suffixes are the same as with the ‘-c’ option. ‘--pid=PID’ When following by name or by descriptor, you may specify the process ID, PID, of the sole writer of all FILE arguments. Then, shortly after that process terminates, tail will also terminate. This will work properly only if the writer and the tailing process are running on the same machine. For example, to save the output of a build in a file and to watch the file grow, if you invoke ‘make’ and ‘tail’ like this then the tail process will stop when your build completes. Without this option, you would have had to kill the ‘tail -f’ process yourself. $ make >& makerr & tail --pid=$! -f makerr If you specify a PID that is not in use or that does not correspond to the process that is writing to the tailed files, then ‘tail’ may terminate long before any FILEs stop growing or it may not terminate until long after the real writer has terminated. Note that ‘--pid’ cannot be supported on some systems; ‘tail’ will print a warning if this is the case. ‘-q’ ‘--quiet’ ‘--silent’ Never print file name headers. ‘--retry’ Indefinitely try to open the specified file. This option is useful mainly when following (and otherwise issues a warning). When following by file descriptor (i.e., with ‘--follow=descriptor’), this option only affects the initial open of the file, as after a successful open, ‘tail’ will start following the file descriptor. When following by name (i.e., with ‘--follow=name’), ‘tail’ infinitely retries to re-open the given files until killed. Without this option, when ‘tail’ encounters a file that doesn’t exist or is otherwise inaccessible, it reports that fact and never checks it again. ‘-s NUMBER’ ‘--sleep-interval=NUMBER’ Change the number of seconds to wait between iterations (the default is 1.0). During one iteration, every specified file is checked to see if it has changed size. When ‘tail’ uses inotify, this polling-related option is usually ignored. However, if you also specify ‘--pid=P’, ‘tail’ checks whether process P is alive at least every NUMBER seconds. The NUMBER must be non-negative and can be a floating-point number in either the current or the C locale. *Note Floating point::. ‘-v’ ‘--verbose’ Always print file name headers. ‘-z’ ‘--zero-terminated’ Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters). For compatibility ‘tail’ also supports an obsolete usage ‘tail -[NUM][bcl][f] [FILE]’, which is recognized only if it does not conflict with the usage described above. This obsolete form uses exactly one option and at most one file. In the option, NUM is an optional decimal number optionally followed by a size letter (‘b’, ‘c’, ‘l’) to mean count by 512-byte blocks, bytes, or lines, optionally followed by ‘f’ which has the same meaning as ‘-f’. On systems not conforming to POSIX 1003.1-2001, the leading ‘-’ can be replaced by ‘+’ in the traditional option syntax with the same meaning as in counts, and on obsolete systems predating POSIX 1003.1-2001 traditional usage overrides normal usage when the two conflict. This behavior can be controlled with the ‘_POSIX2_VERSION’ environment variable (*note Standards conformance::). Scripts intended for use on standard hosts should avoid traditional syntax and should use ‘-c NUM[b]’, ‘-n NUM’, and/or ‘-f’ instead. If your script must also run on hosts that support only the traditional syntax, you can often rewrite it to avoid problematic usages, e.g., by using ‘sed -n '$p'’ rather than ‘tail -1’. If that’s not possible, the script can use a test like ‘if tail -c +1 /dev/null 2>&1; then ...’ to decide which syntax to use. Even if your script assumes the standard behavior, you should still beware usages whose behaviors differ depending on the POSIX version. For example, avoid ‘tail - main.c’, since it might be interpreted as either ‘tail main.c’ or as ‘tail -- - main.c’; avoid ‘tail -c 4’, since it might mean either ‘tail -c4’ or ‘tail -c 10 4’; and avoid ‘tail +4’, since it might mean either ‘tail ./+4’ or ‘tail -n +4’. An exit status of zero indicates success, and a nonzero value indicates failure. 5.3 ‘split’: Split a file into pieces. ====================================== ‘split’ creates output files containing consecutive or interleaved sections of INPUT (standard input if none is given or INPUT is ‘-’). Synopsis: split [OPTION] [INPUT [PREFIX]] By default, ‘split’ puts 1000 lines of INPUT (or whatever is left over for the last section), into each output file. The output files’ names consist of PREFIX (‘x’ by default) followed by a group of characters (‘aa’, ‘ab’, ... by default), such that concatenating the output files in traditional sorted order by file name produces the original input file (except ‘-nr/N’). By default split will initially create files with two generated suffix characters, and will increase this width by two when the next most significant position reaches the last character. (‘yz’, ‘zaaa’, ‘zaab’, ...). In this way an arbitrary number of output files are supported, which sort as described above, even in the presence of an ‘--additional-suffix’ option. If the ‘-a’ option is specified and the output file names are exhausted, ‘split’ reports an error without deleting the output files that it did create. The program accepts the following options. Also see *note Common options::. ‘-l LINES’ ‘--lines=LINES’ Put LINES lines of INPUT into each output file. If ‘--separator’ is specified, then LINES determines the number of records. For compatibility ‘split’ also supports an obsolete option syntax ‘-LINES’. New scripts should use ‘-l LINES’ instead. ‘-b SIZE’ ‘--bytes=SIZE’ Put SIZE bytes of INPUT into each output file. SIZE may be, or may be an integer optionally followed by, one of the following multiplicative suffixes: ‘b’ => 512 ("blocks") ‘KB’ => 1000 (KiloBytes) ‘K’ => 1024 (KibiBytes) ‘MB’ => 1000*1000 (MegaBytes) ‘M’ => 1024*1024 (MebiBytes) ‘GB’ => 1000*1000*1000 (GigaBytes) ‘G’ => 1024*1024*1024 (GibiBytes) and so on for ‘T’, ‘P’, ‘E’, ‘Z’, ‘Y’, ‘R’, and ‘Q’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on. ‘-C SIZE’ ‘--line-bytes=SIZE’ Put into each output file as many complete lines of INPUT as possible without exceeding SIZE bytes. Individual lines or records longer than SIZE bytes are broken into multiple files. SIZE has the same format as for the ‘--bytes’ option. If ‘--separator’ is specified, then LINES determines the number of records. ‘--filter=COMMAND’ With this option, rather than simply writing to each output file, write through a pipe to the specified shell COMMAND for each output file. COMMAND should use the $FILE environment variable, which is set to a different output file name for each invocation of the command. For example, imagine that you have a 1TiB compressed file that, if uncompressed, would be too large to reside on secondary storage, yet you must split it into individually-compressed pieces of a more manageable size. To do that, you might run this command: xz -dc BIG.xz | split -b200G --filter='xz > $FILE.xz' - big- Assuming a 10:1 compression ratio, that would create about fifty 20GiB files with names ‘big-aa.xz’, ‘big-ab.xz’, ‘big-ac.xz’, etc. ‘-n CHUNKS’ ‘--number=CHUNKS’ Split INPUT to CHUNKS output files where CHUNKS may be: N generate N files based on current size of INPUT K/N output only Kth of N to standard output l/N generate N files without splitting lines or records l/K/N likewise but output only Kth of N to stdout r/N like ‘l’ but use round robin distribution r/K/N likewise but output only Kth of N to stdout If the input size is not a multiple of N, early output files are one byte longer than later output files, to make up the difference. Any excess bytes appearing after the initial calculation are discarded (except when using ‘r’ mode). All N files are created even if there are fewer than N lines, or the INPUT is truncated. For ‘l’ mode, chunks are approximately INPUT size / N. Although the INPUT is still partitioned as before into N regions of approximately equal size, if a line _starts_ within a partition it is written completely to the corresponding file. Since lines or records are not split even if they overlap a partition, the files written can be larger or smaller than the partition size, and even empty if a line/record is so long as to completely overlap the partition. When the input is a pipe or some other special file where the size cannot easily be determined, there is no trouble for ‘r’ mode because the size of the input is irrelevant. For other modes, such an input is first copied to a temporary to determine its size. ‘-a LENGTH’ ‘--suffix-length=LENGTH’ Use suffixes of length LENGTH. If a LENGTH of 0 is specified, this is the same as if (any previous) ‘-a’ was not specified, and thus enables the default behavior, which starts the suffix length at 2, and unless ‘-n’ or ‘--numeric-suffixes=FROM’ is specified, will auto increase the length by 2 as required. ‘-d’ ‘--numeric-suffixes[=FROM]’ Use digits in suffixes rather than lower-case letters. The numerical suffix counts from FROM if specified, 0 otherwise. FROM is supported with the long form option, and is used to either set the initial suffix for a single run, or to set the suffix offset for independently split inputs, and consequently the auto suffix length expansion described above is disabled. Therefore you may also want to use option ‘-a’ to allow suffixes beyond ‘99’. Note if option ‘--number’ is specified and the number of files is less than FROM, a single run is assumed and the minimum suffix length required is automatically determined. ‘-x’ ‘--hex-suffixes[=FROM]’ Like ‘--numeric-suffixes’, but use hexadecimal numbers (in lower case). ‘--additional-suffix=SUFFIX’ Append an additional SUFFIX to output file names. SUFFIX must not contain slash. ‘-e’ ‘--elide-empty-files’ Suppress the generation of zero-length output files. This can happen with the ‘--number’ option if a file is (truncated to be) shorter than the number requested, or if a line is so long as to completely span a chunk. The output file sequence numbers, always run consecutively even when this option is specified. ‘-t SEPARATOR’ ‘--separator=SEPARATOR’ Use character SEPARATOR as the record separator instead of the default newline character (ASCII LF). To specify ASCII NUL as the separator, use the two-character string ‘\0’, e.g., ‘split -t '\0'’. ‘-u’ ‘--unbuffered’ Immediately copy input to output in ‘--number r/...’ mode, which is a much slower mode of operation. ‘--verbose’ Write a diagnostic just before each output file is opened. An exit status of zero indicates success, and a nonzero value indicates failure. Here are a few examples to illustrate how the ‘--number’ (‘-n’) option works: Notice how, by default, one line may be split onto two or more: $ seq -w 6 10 > k; split -n3 k; head xa? ==> xaa <== 06 07 ==> xab <== 08 0 ==> xac <== 9 10 Use the "l/" modifier to suppress that: $ seq -w 6 10 > k; split -nl/3 k; head xa? ==> xaa <== 06 07 ==> xab <== 08 09 ==> xac <== 10 Use the "r/" modifier to distribute lines in a round-robin fashion: $ seq -w 6 10 > k; split -nr/3 k; head xa? ==> xaa <== 06 09 ==> xab <== 07 10 ==> xac <== 08 You can also extract just the Kth chunk. This extracts and prints just the 7th "chunk" of 33: $ seq 100 > k; split -nl/7/33 k 20 21 22 5.4 ‘csplit’: Split a file into context-determined pieces ========================================================= ‘csplit’ creates zero or more output files containing sections of INPUT (standard input if INPUT is ‘-’). Synopsis: csplit [OPTION]... INPUT PATTERN... The contents of the output files are determined by the PATTERN arguments, as detailed below. An error occurs if a PATTERN argument refers to a nonexistent line of the input file (e.g., if no remaining line matches a given regular expression). After every PATTERN has been matched, any remaining input is copied into one last output file. By default, ‘csplit’ prints the number of bytes written to each output file after it has been created. The types of pattern arguments are: ‘N’ Create an output file containing the input up to but not including line N (a positive integer). If followed by a repeat count, also create an output file containing the next N lines of the input file once for each repeat. ‘/REGEXP/[OFFSET]’ Create an output file containing the current line up to (but not including) the next line of the input file that contains a match for REGEXP. The optional OFFSET is an integer, that can be preceded by ‘+’ or ‘-’. If it is given, the input up to (but not including) the matching line plus or minus OFFSET is put into the output file, and the line after that begins the next section of input. Note lines within a negative offset of a regexp pattern are not matched in subsequent regexp patterns. ‘%REGEXP%[OFFSET]’ Like the previous type, except that it does not create an output file, so that section of the input file is effectively ignored. ‘{REPEAT-COUNT}’ Repeat the previous pattern REPEAT-COUNT additional times. The REPEAT-COUNT can either be a positive integer or an asterisk, meaning repeat as many times as necessary until the input is exhausted. The output files’ names consist of a prefix (‘xx’ by default) followed by a suffix. By default, the suffix is an ascending sequence of two-digit decimal numbers from ‘00’ to ‘99’. In any case, concatenating the output files in sorted order by file name produces the original input file, excluding portions skipped with a %REGEXP% pattern or the ‘--suppress-matched’ option. By default, if ‘csplit’ encounters an error or receives a hangup, interrupt, quit, or terminate signal, it removes any output files that it has created so far before it exits. The program accepts the following options. Also see *note Common options::. ‘-f PREFIX’ ‘--prefix=PREFIX’ Use PREFIX as the output file name prefix. ‘-b FORMAT’ ‘--suffix-format=FORMAT’ Use FORMAT as the output file name suffix. When this option is specified, the suffix string must include exactly one ‘printf(3)’-style conversion specification, possibly including format specification flags, a field width, a precision specification, or all of these kinds of modifiers. The format letter must convert a binary unsigned integer argument to readable form. The format letters ‘d’ and ‘i’ are aliases for ‘u’, and the ‘u’, ‘o’, ‘x’, and ‘X’ conversions are allowed. The entire FORMAT is given (with the current output file number) to ‘sprintf(3)’ to form the file name suffixes for each of the individual output files in turn. If this option is used, the ‘--digits’ option is ignored. ‘-n DIGITS’ ‘--digits=DIGITS’ Use output file names containing numbers that are DIGITS digits long instead of the default 2. ‘-k’ ‘--keep-files’ Do not remove output files when errors are encountered. ‘--suppress-matched’ Do not output lines matching the specified PATTERN. I.e., suppress the boundary line from the start of the second and subsequent splits. ‘-z’ ‘--elide-empty-files’ Suppress the generation of zero-length output files. (In cases where the section delimiters of the input file are supposed to mark the first lines of each of the sections, the first output file will generally be a zero-length file unless you use this option.) The output file sequence numbers always run consecutively starting from 0, even when this option is specified. ‘-s’ ‘-q’ ‘--silent’ ‘--quiet’ Do not print counts of output file sizes. An exit status of zero indicates success, and a nonzero value indicates failure. Here is an example of its usage. First, create an empty directory for the exercise, and cd into it: $ mkdir d && cd d Now, split the sequence of 1..14 on lines that end with 0 or 5: $ seq 14 | csplit - '/[05]$/' '{*}' 8 10 15 Each number printed above is the size of an output file that csplit has just created. List the names of those output files: $ ls xx00 xx01 xx02 Use ‘head’ to show their contents: $ head xx* ==> xx00 <== 1 2 3 4 ==> xx01 <== 5 6 7 8 9 ==> xx02 <== 10 11 12 13 14 Example of splitting input by empty lines: $ csplit --suppress-matched INPUT.TXT '/^$/' '{*}' 6 Summarizing files ******************* These commands generate just a few numbers representing entire contents of files. 6.1 ‘wc’: Print newline, word, and byte counts ============================================== ‘wc’ counts the number of bytes, characters, words, and newlines in each given FILE, or standard input if none are given or for a FILE of ‘-’. A word is a nonzero length sequence of printable characters delimited by white space. Synopsis: wc [OPTION]... [FILE]... ‘wc’ prints one line of counts for each file, and if the file was given as an argument, it prints the file name following the counts. By default if more than one FILE is given, ‘wc’ prints a final line containing the cumulative counts, with the file name ‘total’. This ‘total’ line can be controlled with the ‘--total’ option, which is a GNU extension. The counts are printed in this order: newlines, words, characters, bytes, maximum line length. Each count is printed right-justified in a field with at least one space between fields so that the numbers and file names normally line up nicely in columns. The width of the count fields varies depending on the inputs, so you should not depend on a particular field width. However, as a GNU extension, if only one count is printed, it is guaranteed to be printed without leading spaces. By default, ‘wc’ prints three counts: the newline, words, and byte counts. Options can specify that only certain counts be printed. Options do not undo others previously given, so wc --bytes --words prints both the byte counts and the word counts. With the ‘--max-line-length’ option, ‘wc’ prints the length of the longest line per file, and if there is more than one file it prints the maximum (not the sum) of those lengths. The line lengths here are measured in screen columns, according to the current locale and assuming tab positions in every 8th column. The program accepts the following options. Also see *note Common options::. ‘-c’ ‘--bytes’ Print only the byte counts. ‘-m’ ‘--chars’ Print only the character counts, as per the current locale. Invalid characters are not counted. ‘-w’ ‘--words’ Print only the word counts. A word is a nonzero length sequence of printable characters separated by white space. ‘-l’ ‘--lines’ Print only the newline character counts. Note a file without a trailing newline character, will not have that last portion included in the line count. ‘-L’ ‘--max-line-length’ Print only the maximum display widths. Tabs are set at every 8th column. Display widths of wide characters are considered. Non-printable characters are given 0 width. ‘--total=WHEN’ Control when and how the final line with cumulative counts is printed. WHEN is one of: • auto - This is the default mode of ‘wc’ when no ‘--total’ option is specified. Output a total line if more than one FILE is specified. • always - Always output a total line, irrespective of the number of files processed. • only - Only output total counts. I.e., don’t print individual file counts, suppress any leading spaces, and don’t print the ‘total’ word itself, to simplify subsequent processing. • never - Never output a total line. ‘--files0-from=FILE’ Disallow processing files named on the command line, and instead process those named in file FILE; each name being terminated by a zero byte (ASCII NUL). This is useful when the list of file names is so long that it may exceed a command line length limitation. In such cases, running ‘wc’ via ‘xargs’ is undesirable because it splits the list into pieces and makes ‘wc’ print a total for each sublist rather than for the entire list. One way to produce a list of ASCII NUL terminated file names is with GNU ‘find’, using its ‘-print0’ predicate. If FILE is ‘-’ then the ASCII NUL terminated file names are read from standard input. For example, to find the length of the longest line in any ‘.c’ or ‘.h’ file in the current hierarchy, do this: find . -name '*.[ch]' -print0 | wc -L --files0-from=- | tail -n1 An exit status of zero indicates success, and a nonzero value indicates failure. 6.2 ‘sum’: Print checksum and block counts ========================================== ‘sum’ computes a 16-bit checksum for each given FILE, or standard input if none are given or for a FILE of ‘-’. Synopsis: sum [OPTION]... [FILE]... ‘sum’ prints the checksum for each FILE followed by the number of blocks in the file (rounded up). If at least one FILE is given, file names are also printed. By default, GNU ‘sum’ computes checksums using an algorithm compatible with BSD ‘sum’ and prints file sizes in units of 1024-byte blocks. The program accepts the following options. Also see *note Common options::. ‘-r’ Use the default (BSD compatible) algorithm. This option is included for compatibility with the System V ‘sum’. Unless ‘-s’ was also given, it has no effect. ‘-s’ ‘--sysv’ Compute checksums using an algorithm compatible with System V ‘sum’’s default, and print file sizes in units of 512-byte blocks. ‘sum’ is provided for compatibility; the ‘cksum’ program (see next section) is preferable in new applications. An exit status of zero indicates success, and a nonzero value indicates failure. 6.3 ‘cksum’: Print and verify file checksums ============================================ ‘cksum’ by default computes a 32-bit cyclic redundancy check (CRC) checksum for each given FILE, or standard input if none are given or for a FILE of ‘-’. cksum also supports the ‘-a/--algorithm’ option to select the digest algorithm to use. ‘cksum’ is the preferred interface to these digests, subsuming the other standalone checksumming utilities, which can be emulated using ‘cksum -a md5 --untagged "$@"’ etc. Synopsis: cksum [OPTION]... [FILE]... ‘cksum’ is typically used to ensure that files have not been corrupted, by comparing the ‘cksum’ output for the received files with the ‘cksum’ output for the original files (typically given in the distribution). 6.3.1 cksum output modes ------------------------ Legacy output format ‘cksum’ by default prints the POSIX standard CRC checksum for each file along with the number of bytes in the file, and the file name unless no arguments were given. The 32-bit CRC used is based on the polynomial used for CRC error checking in the ISO/IEC 8802-3:1996 standard (Ethernet). Similar output formats are used for the other legacy checksums selectable with ‘--algorithm=sysv’ or ‘--algorithm=bsd’, detailed at *note sum invocation::. Tagged output format With the ‘--algorithm’ option selecting non legacy checksums, the ‘cksum’ command defaults to output of the form: DIGEST_NAME (FILE NAME) = DIGEST Note the standalone checksum utilities can select this output mode by using the ‘--tag’ option. Untagged output format With the ‘--untagged’ option and the ‘--algorithm’ option selecting non legacy checksums, the following output format is used. Note this is the default output format of the standalone checksum utilities. For each FILE, we print the checksum, a space, a flag indicating binary or text input mode, and the file name. Binary mode is indicated with ‘*’, text mode with ‘ ’ (space). Binary mode is the default on systems where it’s significant, otherwise text mode is the default. Note without ‘--zero’, and with non legacy output formats, if FILE contains a backslash, newline, or carriage return, the line is started with a backslash, and each problematic character in the file name is escaped with a backslash, making the output unambiguous even in the presence of arbitrary file names. Since the backslash character itself is escaped, any other backslash escape sequences are reserved for future use. 6.3.2 cksum general options --------------------------- ‘-a’ ‘--algorithm’ Compute checksums using the specified digest algorithm. Supported legacy checksums (which are not supported by ‘--check’): ‘sysv’ equivalent to sum -s ‘bsd’ equivalent to sum -r ‘crc’ equivalent to cksum (the default) Supported more modern digest algorithms are: ‘md5’ equivalent to md5sum ‘sha1’ equivalent to sha1sum ‘sha224’ equivalent to sha224sum ‘sha256’ equivalent to sha256sum ‘sha384’ equivalent to sha384sum ‘sha512’ equivalent to sha512sum ‘blake2b’ equivalent to b2sum ‘sm3’ only available through cksum ‘--base64’ Print base64-encoded digests not hexadecimal. This option is ignored with ‘--check’. The format conforms to RFC 4648#4 (https://tools.ietf.org/search/rfc4648#section-4). Note that each base64-encoded digest has zero, one or two trailing padding (‘=’) bytes. The length of that padding is the checksum-bit-length modulo 3, and the ‘--check’ parser requires precisely the same input digest string as what is output. I.e., removing or adding any ‘=’ padding renders a digest non-matching. ‘--debug’ Output extra information to stderr, like the checksum implementation being used. ‘-l’ ‘--length’ Change (shorten) the default digest length. This is specified in bits and thus must be a multiple of 8. This option is ignored when ‘--check’ is specified, as the length is automatically determined when checking. ‘--raw’ Print only the unencoded raw binary digest for a single input. Do not output the file name or anything else. Use network byte order (big endian) where applicable: for ‘bsd’, ‘crc’, and ‘sysv’. This option works only with a single input. Unlike other output formats, ‘cksum’ provides no way to ‘--check’ a ‘--raw’ checksum. ‘--untagged’ Output using the original Coreutils format used by the other standalone checksum utilities like ‘md5sum’ for example. This format has the checksum at the start of the line, and may be more amenable to further processing by other utilities, especially in combination with the ‘--zero’ option. Note this does not identify the digest algorithm used for the checksum. *Note cksum output modes:: for details of this format. 6.3.3 cksum common options -------------------------- ‘-b’ ‘--binary’ Note this option is not supported by the ‘cksum’ command, as it operates in binary mode exclusively. Treat each input file as binary, by reading it in binary mode and outputting a ‘*’ flag. This is the inverse of ‘--text’. On systems like GNU that do not distinguish between binary and text files, this option merely flags each input mode as binary: the checksum is unaffected. This option is the default on systems like MS-DOS that distinguish between binary and text files, except for reading standard input when standard input is a terminal. ‘-c’ ‘--check’ Read file names and checksum information (not data) from each FILE (or from standard input if no FILE was specified) and report whether the checksums match the contents of the named files. The input to this mode is usually the output of a prior, checksum-generating run of the command. Three input formats are supported. Either the default output format described above, the ‘--tag’ output format, or the BSD reversed mode format which is similar to the default mode, but doesn’t use a character to distinguish binary and text modes. For the ‘cksum’ command, the ‘--check’ option supports auto-detecting the digest algorithm to use, when presented with checksum information in the ‘--tag’ output format. Also for the ‘cksum’ command, the ‘--check’ option auto-detects the digest encoding, accepting both standard hexadecimal checksums and those generated via ‘cksum’ with its ‘--base64’ option. Output with ‘--zero’ enabled is not supported by ‘--check’. For each such line, ‘cksum’ reads the named file and computes its checksum. Then, if the computed message digest does not match the one on the line with the file name, the file is noted as having failed the test. Otherwise, the file passes the test. By default, for each valid line, one line is written to standard output indicating whether the named file passed the test. After all checks have been performed, if there were any failures, a warning is issued to standard error. Use the ‘--status’ option to inhibit that output. If any listed file cannot be opened or read, if any valid line has a checksum inconsistent with the associated file, or if no valid line is found, ‘cksum’ exits with nonzero status. Otherwise, it exits successfully. Note the ‘cksum’ command doesn’t support ‘--check’ with the older ‘sysv’, ‘bsd’, or ‘crc’ algorithms. ‘--ignore-missing’ This option is useful only when verifying checksums. When verifying checksums, don’t fail or report any status for missing files. This is useful when verifying a subset of downloaded files given a larger list of checksums. ‘--quiet’ This option is useful only when verifying checksums. When verifying checksums, don’t generate an ’OK’ message per successfully checked file. Files that fail the verification are reported in the default one-line-per-file format. If there is any checksum mismatch, print a warning summarizing the failures to standard error. ‘--status’ This option is useful only when verifying checksums. When verifying checksums, don’t generate the default one-line-per-file diagnostic and don’t output the warning summarizing any failures. Failures to open or read a file still evoke individual diagnostics to standard error. If all listed files are readable and are consistent with the associated checksums, exit successfully. Otherwise exit with a status code indicating there was a failure. ‘--tag’ Output BSD style checksums, which indicate the checksum algorithm used. As a GNU extension, if ‘--zero’ is not used, file names with problematic characters are escaped as described above, using the same escaping indicator of ‘\’ at the start of the line, as used with the other output format. The ‘--tag’ option implies binary mode, and is disallowed with ‘--text’ mode as supporting that would unnecessarily complicate the output format, while providing little benefit. *Note cksum output modes:: for details of this format. The ‘cksum’ command, uses ‘--tag’ as its default output format. ‘-t’ ‘--text’ Note this option is not supported by the ‘cksum’ command. Treat each input file as text, by reading it in text mode and outputting a ‘ ’ flag. This is the inverse of ‘--binary’. This option is the default on systems like GNU that do not distinguish between binary and text files. On other systems, it is the default for reading standard input when standard input is a terminal. This mode is never defaulted to if ‘--tag’ is used. ‘-w’ ‘--warn’ When verifying checksums, warn about improperly formatted checksum lines. This option is useful only if all but a few lines in the checked input are valid. ‘--strict’ When verifying checksums, if one or more input line is invalid, exit nonzero after all warnings have been issued. ‘-z’ ‘--zero’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. Also file name escaping is not used. 6.4 ‘md5sum’: Print or check MD5 digests ======================================== ‘md5sum’ computes a 128-bit checksum (or “fingerprint” or “message-digest”) for each specified FILE. Note: The MD5 digest is more reliable than a simple CRC (provided by the ‘cksum’ command) for detecting accidental file corruption, as the chances of accidentally having two files with identical MD5 are vanishingly small. However, it should not be considered secure against malicious tampering: although finding a file with a given MD5 fingerprint is considered infeasible at the moment, it is known how to modify certain files, including digital certificates, so that they appear valid when signed with an MD5 digest. For more secure hashes, consider using SHA-2, or the newer ‘b2sum’ command. *Note sha2 utilities::. *Note b2sum invocation::. If a FILE is specified as ‘-’ or if no files are given ‘md5sum’ computes the checksum for the standard input. ‘md5sum’ can also determine whether a file and checksum are consistent. Synopsis: md5sum [OPTION]... [FILE]... ‘md5sum’ uses the ‘Untagged output format’ for each specified file, as described at *note cksum output modes::. The program accepts *note cksum common options::. Also see *note Common options::. An exit status of zero indicates success, and a nonzero value indicates failure. 6.5 ‘b2sum’: Print or check BLAKE2 digests ========================================== ‘b2sum’ computes a 512-bit checksum for each specified FILE. If a FILE is specified as ‘-’ or if no files are given ‘b2sum’ computes the checksum for the standard input. ‘b2sum’ can also determine whether a file and checksum are consistent. Synopsis: b2sum [OPTION]... [FILE]... ‘b2sum’ uses the ‘Untagged output format’ for each specified file, as described at *note cksum output modes::. The program accepts *note cksum common options::. Also see *note Common options::. In addition ‘b2sum’ supports the following options. ‘-l’ ‘--length’ Change (shorten) the default digest length. This is specified in bits and thus must be a multiple of 8. This option is ignored when ‘--check’ is specified, as the length is automatically determined when checking. 6.6 ‘sha1sum’: Print or check SHA-1 digests =========================================== ‘sha1sum’ computes a 160-bit checksum for each specified FILE. Note: The SHA-1 digest is more reliable than a simple CRC (provided by the ‘cksum’ command) for detecting accidental file corruption, as the chances of accidentally having two files with identical SHA-1 are vanishingly small. However, it should not be considered secure against malicious tampering: although finding a file with a given SHA-1 fingerprint is considered infeasible at the moment, it is known how to modify certain files, including digital certificates, so that they appear valid when signed with an SHA-1 digest. For more secure hashes, consider using SHA-2, or the newer ‘b2sum’ command. *Note sha2 utilities::. *Note b2sum invocation::. If a FILE is specified as ‘-’ or if no files are given ‘sha1sum’ computes the checksum for the standard input. ‘sha1sum’ can also determine whether a file and checksum are consistent. Synopsis: sha1sum [OPTION]... [FILE]... ‘sha1sum’ uses the ‘Untagged output format’ for each specified file, as described at *note cksum output modes::. The program accepts *note cksum common options::. Also see *note Common options::. 6.7 sha2 utilities: Print or check SHA-2 digests ================================================ The commands ‘sha224sum’, ‘sha256sum’, ‘sha384sum’ and ‘sha512sum’ compute checksums of various lengths (respectively 224, 256, 384 and 512 bits), collectively known as the SHA-2 hashes. If a FILE is specified as ‘-’ or if no files are given ‘sha???sum’ computes the checksum for the standard input. ‘sha???sum’ can also determine whether a file and checksum are consistent. Synopsis: sha???sum [OPTION]... [FILE]... ‘sha???sum’ uses the ‘Untagged output format’ for each specified file, as described at *note cksum output modes::. The program accepts *note cksum common options::. Also see *note Common options::. 7 Operating on sorted files *************************** These commands work with (or produce) sorted files. 7.1 ‘sort’: Sort text files =========================== ‘sort’ sorts, merges, or compares all the lines from the given files, or standard input if none are given or for a FILE of ‘-’. By default, ‘sort’ writes the results to standard output. Synopsis: sort [OPTION]... [FILE]... Many options affect how ‘sort’ compares lines; if the results are unexpected, try the ‘--debug’ option to see what happened. A pair of lines is compared as follows: ‘sort’ compares each pair of fields (see ‘--key’), in the order specified on the command line, according to the associated ordering options, until a difference is found or no fields are left. If no key fields are specified, ‘sort’ uses a default key of the entire line. Finally, as a last resort when all keys compare equal, ‘sort’ compares entire lines as if no ordering options other than ‘--reverse’ (‘-r’) were specified. The ‘--stable’ (‘-s’) option disables this “last-resort comparison” so that lines in which all fields compare equal are left in their original relative order. The ‘--unique’ (‘-u’) option also disables the last-resort comparison. Unless otherwise specified, all comparisons use the character collating sequence specified by the ‘LC_COLLATE’ locale.(1) A line’s trailing newline is not part of the line for comparison purposes. If the final byte of an input file is not a newline, GNU ‘sort’ silently supplies one. GNU ‘sort’ (as specified for all GNU utilities) has no limit on input line length or restrictions on bytes allowed within lines. ‘sort’ has three modes of operation: sort (the default), merge, and check for order. The following options change the operation mode: ‘-c’ ‘--check’ ‘--check=diagnose-first’ Check whether the given file is already sorted: if it is not all sorted, print a diagnostic containing the first out-of-order line and exit with a status of 1. Otherwise, exit successfully. At most one input file can be given. ‘-C’ ‘--check=quiet’ ‘--check=silent’ Exit successfully if the given file is already sorted, and exit with status 1 otherwise. At most one input file can be given. This is like ‘-c’, except it does not print a diagnostic. ‘-m’ ‘--merge’ Merge the given files by sorting them as a group. Each input file must always be individually sorted. It always works to sort instead of merge; merging is provided because it is faster, in the case where it works. Exit status: 0 if no error occurred 1 if invoked with ‘-c’ or ‘-C’ and the input is not sorted 2 if an error occurred If the environment variable ‘TMPDIR’ is set, ‘sort’ uses its value as the directory for temporary files instead of ‘/tmp’. The ‘--temporary-directory’ (‘-T’) option in turn overrides the environment variable. The following options affect the ordering of output lines. They may be specified globally or as part of a specific key field. If no key fields are specified, global options apply to comparison of entire lines; otherwise the global options are inherited by key fields that do not specify any special options of their own. In pre-POSIX versions of ‘sort’, global options affect only later key fields, so portable shell scripts should specify global options first. ‘-b’ ‘--ignore-leading-blanks’ Ignore leading blanks when finding sort keys in each line. By default a blank is a space or a tab, but the ‘LC_CTYPE’ locale can change this. Note blanks may be ignored by your locale’s collating rules, but without this option they will be significant for character positions specified in keys with the ‘-k’ option. ‘-d’ ‘--dictionary-order’ Sort in “phone directory” order: ignore all characters except letters, digits and blanks when sorting. By default letters and digits are those of ASCII and a blank is a space or a tab, but the ‘LC_CTYPE’ locale can change this. ‘-f’ ‘--ignore-case’ Fold lowercase characters into the equivalent uppercase characters when comparing so that, for example, ‘b’ and ‘B’ sort as equal. The ‘LC_CTYPE’ locale determines character types. When used with ‘--unique’ those lower case equivalent lines are thrown away. (There is currently no way to throw away the upper case equivalent instead. (Any ‘--reverse’ given would only affect the final result, after the throwing away.)) ‘-g’ ‘--general-numeric-sort’ ‘--sort=general-numeric’ Sort numerically, converting a prefix of each line to a long double-precision floating point number. *Note Floating point::. Do not report overflow, underflow, or conversion errors. Use the following collating sequence: • Lines that do not start with numbers (all considered to be equal). • NaNs (“Not a Number” values, in IEEE floating point arithmetic) in a consistent but machine-dependent order. • Minus infinity. • Finite numbers in ascending numeric order (with -0 and +0 equal). • Plus infinity. Use this option only if there is no alternative; it is much slower than ‘--numeric-sort’ (‘-n’) and it can lose information when converting to floating point. You can use this option to sort hexadecimal numbers prefixed with ‘0x’ or ‘0X’, where those numbers are not fixed width, or of varying case. However for hex numbers of consistent case, and left padded with ‘0’ to a consistent width, a standard lexicographic sort will be faster. ‘-h’ ‘--human-numeric-sort’ ‘--sort=human-numeric’ Sort numerically, first by numeric sign (negative, zero, or positive); then by SI suffix (either empty, or ‘k’ or ‘K’, or one of ‘MGTPEZYRQ’, in that order; *note Block size::); and finally by numeric value. For example, ‘1023M’ sorts before ‘1G’ because ‘M’ (mega) precedes ‘G’ (giga) as an SI suffix. This option sorts values that are consistently scaled to the nearest suffix, regardless of whether suffixes denote powers of 1000 or 1024, and it therefore sorts the output of any single invocation of the ‘df’, ‘du’, or ‘ls’ commands that are invoked with their ‘--human-readable’ or ‘--si’ options. The syntax for numbers is the same as for the ‘--numeric-sort’ option; the SI suffix must immediately follow the number. Note also the ‘numfmt’ command, which can be used to reformat numbers to human format _after_ the sort, thus often allowing sort to operate on more accurate numbers. ‘-i’ ‘--ignore-nonprinting’ Ignore nonprinting characters. The ‘LC_CTYPE’ locale determines character types. This option has no effect if the stronger ‘--dictionary-order’ (‘-d’) option is also given. ‘-M’ ‘--month-sort’ ‘--sort=month’ An initial string, consisting of any amount of blanks, followed by a month name abbreviation, is folded to UPPER case and compared in the order ‘JAN’ < ‘FEB’ < ... < ‘DEC’. Invalid names compare low to valid names. The ‘LC_TIME’ locale category determines the month spellings. By default a blank is a space or a tab, but the ‘LC_CTYPE’ locale can change this. ‘-n’ ‘--numeric-sort’ ‘--sort=numeric’ Sort numerically. The number begins each line and consists of optional blanks, an optional ‘-’ sign, and zero or more digits possibly separated by thousands separators, optionally followed by a decimal-point character and zero or more digits. An empty number is treated as ‘0’. The ‘LC_NUMERIC’ locale specifies the decimal-point character and thousands separator. By default a blank is a space or a tab, but the ‘LC_CTYPE’ locale can change this. Comparison is exact; there is no rounding error. Neither a leading ‘+’ nor exponential notation is recognized. To compare such strings numerically, use the ‘--general-numeric-sort’ (‘-g’) option. ‘-V’ ‘--version-sort’ Sort by version name and number. It behaves like a standard sort, except that each sequence of decimal digits is treated numerically as an index/version number. (*Note Version sort ordering::.) ‘-r’ ‘--reverse’ Reverse the result of comparison, so that lines with greater key values appear earlier in the output instead of later. ‘-R’ ‘--random-sort’ ‘--sort=random’ Sort by hashing the input keys and then sorting the hash values. Choose the hash function at random, ensuring that it is free of collisions so that differing keys have differing hash values. This is like a random permutation of the inputs (*note shuf invocation::), except that keys with the same value sort together. If multiple random sort fields are specified, the same random hash function is used for all fields. To use different random hash functions for different fields, you can invoke ‘sort’ more than once. The choice of hash function is affected by the ‘--random-source’ option. Other options are: ‘--compress-program=PROG’ Compress any temporary files with the program PROG. With no arguments, PROG must compress standard input to standard output, and when given the ‘-d’ option it must decompress standard input to standard output. Terminate with an error if PROG exits with nonzero status. White space and the backslash character should not appear in PROG; they are reserved for future use. ‘--files0-from=FILE’ Disallow processing files named on the command line, and instead process those named in file FILE; each name being terminated by a zero byte (ASCII NUL). This is useful when the list of file names is so long that it may exceed a command line length limitation. In such cases, running ‘sort’ via ‘xargs’ is undesirable because it splits the list into pieces and makes ‘sort’ print sorted output for each sublist rather than for the entire list. One way to produce a list of ASCII NUL terminated file names is with GNU ‘find’, using its ‘-print0’ predicate. If FILE is ‘-’ then the ASCII NUL terminated file names are read from standard input. ‘-k POS1[,POS2]’ ‘--key=POS1[,POS2]’ Specify a sort field that consists of the part of the line between POS1 and POS2 (or the end of the line, if POS2 is omitted), _inclusive_. In its simplest form POS specifies a field number (starting with 1), with fields being separated by runs of blank characters, and by default those blanks being included in the comparison at the start of each field. To adjust the handling of blank characters see the ‘-b’ and ‘-t’ options. More generally, each POS has the form ‘F[.C][OPTS]’, where F is the number of the field to use, and C is the number of the first character from the beginning of the field. Fields and character positions are numbered starting with 1; a character position of zero in POS2 indicates the field’s last character. If ‘.C’ is omitted from POS1, it defaults to 1 (the beginning of the field); if omitted from POS2, it defaults to 0 (the end of the field). OPTS are ordering options, allowing individual keys to be sorted according to different rules; see below for details. Keys can span multiple fields. Example: To sort on the second field, use ‘--key=2,2’ (‘-k 2,2’). See below for more notes on keys and more examples. See also the ‘--debug’ option to help determine the part of the line being used in the sort. ‘--debug’ Highlight the portion of each line used for sorting. Also issue warnings about questionable usage to standard error. ‘--batch-size=NMERGE’ Merge at most NMERGE inputs at once. When ‘sort’ has to merge more than NMERGE inputs, it merges them in groups of NMERGE, saving the result in a temporary file, which is then used as an input in a subsequent merge. A large value of NMERGE may improve merge performance and decrease temporary storage utilization at the expense of increased memory usage and I/O. Conversely a small value of NMERGE may reduce memory requirements and I/O at the expense of temporary storage consumption and merge performance. The value of NMERGE must be at least 2. The default value is currently 16, but this is implementation-dependent and may change in the future. The value of NMERGE may be bounded by a resource limit for open file descriptors. The commands ‘ulimit -n’ or ‘getconf OPEN_MAX’ may display limits for your systems; these limits may be modified further if your program already has some files open, or if the operating system has other limits on the number of open files. If the value of NMERGE exceeds the resource limit, ‘sort’ silently uses a smaller value. ‘-o OUTPUT-FILE’ ‘--output=OUTPUT-FILE’ Write output to OUTPUT-FILE instead of standard output. Normally, ‘sort’ reads all input before opening OUTPUT-FILE, so you can sort a file in place by using commands like ‘sort -o F F’ and ‘cat F | sort -o F’. However, it is often safer to output to an otherwise-unused file, as data may be lost if the system crashes or ‘sort’ encounters an I/O or other serious error while a file is being sorted in place. Also, ‘sort’ with ‘--merge’ (‘-m’) can open the output file before reading all input, so a command like ‘cat F | sort -m -o F - G’ is not safe as ‘sort’ might start writing ‘F’ before ‘cat’ is done reading it. On newer systems, ‘-o’ cannot appear after an input file if ‘POSIXLY_CORRECT’ is set, e.g., ‘sort F -o F’. Portable scripts should specify ‘-o OUTPUT-FILE’ before any input files. ‘--random-source=FILE’ Use FILE as a source of random data used to determine which random hash function to use with the ‘-R’ option. *Note Random sources::. ‘-s’ ‘--stable’ Make ‘sort’ stable by disabling its last-resort comparison. This option has no effect if no fields or global ordering options other than ‘--reverse’ (‘-r’) are specified. ‘-S SIZE’ ‘--buffer-size=SIZE’ Use a main-memory sort buffer of the given SIZE. By default, SIZE is in units of 1024 bytes. Appending ‘%’ causes SIZE to be interpreted as a percentage of physical memory. Appending ‘K’ multiplies SIZE by 1024 (the default), ‘M’ by 1,048,576, ‘G’ by 1,073,741,824, and so on for ‘T’, ‘P’, ‘E’, ‘Z’, ‘Y’, ‘R’, and ‘Q’. Appending ‘b’ causes SIZE to be interpreted as a byte count, with no multiplication. This option can improve the performance of ‘sort’ by causing it to start with a larger or smaller sort buffer than the default. However, this option affects only the initial buffer size. The buffer grows beyond SIZE if ‘sort’ encounters input lines larger than SIZE. ‘-t SEPARATOR’ ‘--field-separator=SEPARATOR’ Use character SEPARATOR as the field separator when finding the sort keys in each line. By default, fields are separated by the empty string between a non-blank character and a blank character. By default a blank is a space or a tab, but the ‘LC_CTYPE’ locale can change this. That is, given the input line ‘ foo bar’, ‘sort’ breaks it into fields ‘ foo’ and ‘ bar’. The field separator is not considered to be part of either the field preceding or the field following, so with ‘sort -t " "’ the same input line has three fields: an empty field, ‘foo’, and ‘bar’. However, fields that extend to the end of the line, as ‘-k 2’, or fields consisting of a range, as ‘-k 2,3’, retain the field separators present between the endpoints of the range. To specify ASCII NUL as the field separator, use the two-character string ‘\0’, e.g., ‘sort -t '\0'’. ‘-T TEMPDIR’ ‘--temporary-directory=TEMPDIR’ Use directory TEMPDIR to store temporary files, overriding the ‘TMPDIR’ environment variable. If this option is given more than once, temporary files are stored in all the directories given. If you have a large sort or merge that is I/O-bound, you can often improve performance by using this option to specify directories on different file systems. ‘--parallel=N’ Set the number of sorts run in parallel to N. By default, N is set to the number of available processors, but limited to 8, as there are diminishing performance gains after that. Note also that using N threads increases the memory usage by a factor of log N. Also see *note nproc invocation::. ‘-u’ ‘--unique’ Normally, output only the first of a sequence of lines that compare equal. For the ‘--check’ (‘-c’ or ‘-C’) option, check that no pair of consecutive lines compares equal. This option also disables the default last-resort comparison. The commands ‘sort -u’ and ‘sort | uniq’ are equivalent, but this equivalence does not extend to arbitrary ‘sort’ options. For example, ‘sort -n -u’ inspects only the value of the initial numeric string when checking for uniqueness, whereas ‘sort -n | uniq’ inspects the entire line. *Note uniq invocation::. ‘-z’ ‘--zero-terminated’ Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters). Historical (BSD and System V) implementations of ‘sort’ have differed in their interpretation of some options, particularly ‘-b’, ‘-f’, and ‘-n’. GNU sort follows the POSIX behavior, which is usually (but not always!) like the System V behavior. According to POSIX, ‘-n’ no longer implies ‘-b’. For consistency, ‘-M’ has been changed in the same way. This may affect the meaning of character positions in field specifications in obscure cases. The only fix is to add an explicit ‘-b’. A position in a sort field specified with ‘-k’ may have any of the option letters ‘MbdfghinRrV’ appended to it, in which case no global ordering options are inherited by that particular field. The ‘-b’ option may be independently attached to either or both of the start and end positions of a field specification, and if it is inherited from the global options it will be attached to both. If input lines can contain leading or adjacent blanks and ‘-t’ is not used, then ‘-k’ is typically combined with ‘-b’ or an option that implicitly ignores leading blanks (‘Mghn’) as otherwise the varying numbers of leading blanks in fields can cause confusing results. If the start position in a sort field specifier falls after the end of the line or after the end field, the field is empty. If the ‘-b’ option was specified, the ‘.C’ part of a field specification is counted from the first nonblank character of the field. On systems not conforming to POSIX 1003.1-2001, ‘sort’ supports a traditional origin-zero syntax ‘+POS1 [-POS2]’ for specifying sort keys. The traditional command ‘sort +A.X -B.Y’ is equivalent to ‘sort -k A+1.X+1,B’ if Y is ‘0’ or absent, otherwise it is equivalent to ‘sort -k A+1.X+1,B+1.Y’. This traditional behavior can be controlled with the ‘_POSIX2_VERSION’ environment variable (*note Standards conformance::); it can also be enabled when ‘POSIXLY_CORRECT’ is not set by using the traditional syntax with ‘-POS2’ present. Scripts intended for use on standard hosts should avoid traditional syntax and should use ‘-k’ instead. For example, avoid ‘sort +2’, since it might be interpreted as either ‘sort ./+2’ or ‘sort -k 3’. If your script must also run on hosts that support only the traditional syntax, it can use a test like ‘if sort -k 1 /dev/null 2>&1; then ...’ to decide which syntax to use. Here are some examples to illustrate various combinations of options. • Sort in descending (reverse) numeric order. sort -n -r • Run no more than 4 sorts concurrently, using a buffer size of 10M. sort --parallel=4 -S 10M • Sort alphabetically, omitting the first and second fields and the blanks at the start of the third field. This uses a single key composed of the characters beginning at the start of the first nonblank character in field three and extending to the end of each line. sort -k 3b • Sort numerically on the second field and resolve ties by sorting alphabetically on the third and fourth characters of field five. Use ‘:’ as the field delimiter. sort -t : -k 2,2n -k 5.3,5.4 Note that if you had written ‘-k 2n’ instead of ‘-k 2,2n’ ‘sort’ would have used all characters beginning in the second field and extending to the end of the line as the primary _numeric_ key. For the large majority of applications, treating keys spanning more than one field as numeric will not do what you expect. Also note that the ‘n’ modifier was applied to the field-end specifier for the first key. It would have been equivalent to specify ‘-k 2n,2’ or ‘-k 2n,2n’. All modifiers except ‘b’ apply to the associated _field_, regardless of whether the modifier character is attached to the field-start and/or the field-end part of the key specifier. • Sort the password file on the fifth field and ignore any leading blanks. Sort lines with equal values in field five on the numeric user ID in field three. Fields are separated by ‘:’. sort -t : -k 5b,5 -k 3,3n /etc/passwd sort -t : -n -k 5b,5 -k 3,3 /etc/passwd sort -t : -b -k 5,5 -k 3,3n /etc/passwd These three commands have equivalent effect. The first specifies that the first key’s start position ignores leading blanks and the second key is sorted numerically. The other two commands rely on global options being inherited by sort keys that lack modifiers. The inheritance works in this case because ‘-k 5b,5b’ and ‘-k 5b,5’ are equivalent, as the location of a field-end lacking a ‘.C’ character position is not affected by whether initial blanks are skipped. • Sort a set of log files, primarily by IPv4 address and secondarily by timestamp. If two lines’ primary and secondary keys are identical, output the lines in the same order that they were input. The log files contain lines that look like this: 4.150.156.3 - - [01/Apr/2020:06:31:51 +0000] message 1 211.24.3.231 - - [24/Apr/2020:20:17:39 +0000] message 2 Fields are separated by exactly one space. Sort IPv4 addresses lexicographically, e.g., 212.61.52.2 sorts before 212.129.233.201 because 61 is less than 129. sort -s -t ' ' -k 4.9n -k 4.5M -k 4.2n -k 4.14,4.21 file*.log | sort -s -t '.' -k 1,1n -k 2,2n -k 3,3n -k 4,4n This example cannot be done with a single POSIX ‘sort’ invocation, since IPv4 address components are separated by ‘.’ while dates come just after a space. So it is broken down into two invocations of ‘sort’: the first sorts by timestamp and the second by IPv4 address. The timestamp is sorted by year, then month, then day, and finally by hour-minute-second field, using ‘-k’ to isolate each field. Except for hour-minute-second there’s no need to specify the end of each key field, since the ‘n’ and ‘M’ modifiers sort based on leading prefixes that cannot cross field boundaries. The IPv4 addresses are sorted lexicographically. The second sort uses ‘-s’ so that ties in the primary key are broken by the secondary key; the first sort uses ‘-s’ so that the combination of the two sorts is stable. Note as a GNU extension, the above example could be achieved in a single ‘sort’ invocation by sorting the IPv4 address field using a ‘V’ version type, like ‘-k1,1V’. • Generate a tags file in case-insensitive sorted order. find src -type f -print0 | sort -z -f | xargs -0 etags --append The use of ‘-print0’, ‘-z’, and ‘-0’ in this case means that file names that contain blanks or other special characters are not broken up by the sort operation. • Use the common DSU, Decorate Sort Undecorate idiom to sort lines according to their length. awk '{print length, $0}' /etc/passwd | sort -n | cut -f2- -d' ' In general this technique can be used to sort data that the ‘sort’ command does not support, or is inefficient at, sorting directly. • Shuffle a list of directories, but preserve the order of files within each directory. For instance, one could use this to generate a music playlist in which albums are shuffled but the songs of each album are played in order. ls */* | sort -t / -k 1,1R -k 2,2 ---------- Footnotes ---------- (1) If you use a non-POSIX locale (e.g., by setting ‘LC_ALL’ to ‘en_US’), then ‘sort’ may produce output that is sorted differently than you’re accustomed to. In that case, set the ‘LC_ALL’ environment variable to ‘C’. Note that setting only ‘LC_COLLATE’ has two problems. First, it is ineffective if ‘LC_ALL’ is also set. Second, it has undefined behavior if ‘LC_CTYPE’ (or ‘LANG’, if ‘LC_CTYPE’ is unset) is set to an incompatible value. For example, you get undefined behavior if ‘LC_CTYPE’ is ‘ja_JP.PCK’ but ‘LC_COLLATE’ is ‘en_US.UTF-8’. 7.2 ‘shuf’: Shuffling text ========================== ‘shuf’ shuffles its input by outputting a random permutation of its input lines. Each output permutation is equally likely. Synopses: shuf [OPTION]... [FILE] shuf -e [OPTION]... [ARG]... shuf -i LO-HI [OPTION]... ‘shuf’ has three modes of operation that affect where it obtains its input lines. By default, it reads lines from standard input. The following options change the operation mode: ‘-e’ ‘--echo’ Treat each command-line operand as an input line. ‘-i LO-HI’ ‘--input-range=LO-HI’ Act as if input came from a file containing the range of unsigned decimal integers LO...HI, one per line. ‘shuf’’s other options can affect its behavior in all operation modes: ‘-n COUNT’ ‘--head-count=COUNT’ Output at most COUNT lines. By default, all input lines are output. ‘-o OUTPUT-FILE’ ‘--output=OUTPUT-FILE’ Write output to OUTPUT-FILE instead of standard output. ‘shuf’ reads all input before opening OUTPUT-FILE, so you can safely shuffle a file in place by using commands like ‘shuf -o F file1 $ printf '%s\n' b c d e f g > file2 $ comm --total -123 file1 file2 1 2 4 total This option is a GNU extension. Portable scripts should use ‘wc’ to get the totals, e.g. for the above example files: $ comm -23 file1 file2 | wc -l # number of lines only in file1 1 $ comm -13 file1 file2 | wc -l # number of lines only in file2 2 $ comm -12 file1 file2 | wc -l # number of lines common to both files 4 ‘-z’ ‘--zero-terminated’ Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters). 7.5 ‘ptx’: Produce permuted indexes =================================== ‘ptx’ reads a text file and essentially produces a permuted index, with each keyword in its context. The calling sketch is either one of: ptx [OPTION ...] [FILE ...] ptx -G [OPTION ...] [INPUT [OUTPUT]] The ‘-G’ (or its equivalent: ‘--traditional’) option disables all GNU extensions and reverts to traditional mode, thus introducing some limitations and changing several of the program’s default option values. When ‘-G’ is not specified, GNU extensions are always enabled. GNU extensions to ‘ptx’ are documented wherever appropriate in this document. *Note Compatibility in ptx::, for the full list. Individual options are explained in the following sections. When GNU extensions are enabled, there may be zero, one or several FILEs after the options. If there is no FILE, the program reads the standard input. If there is one or several FILEs, they give the name of input files which are all read in turn, as if all the input files were concatenated. However, there is a full contextual break between each file and, when automatic referencing is requested, file names and line numbers refer to individual text input files. In all cases, the program outputs the permuted index to the standard output. When GNU extensions are _not_ enabled, that is, when the program operates in traditional mode, there may be zero, one or two parameters besides the options. If there are no parameters, the program reads the standard input and outputs the permuted index to the standard output. If there is only one parameter, it names the text INPUT to be read instead of the standard input. If two parameters are given, they give respectively the name of the INPUT file to read and the name of the OUTPUT file to produce. _Be very careful_ to note that, in this case, the contents of file given by the second parameter is destroyed. This behavior is dictated by System V ‘ptx’ compatibility; GNU Standards normally discourage output parameters not introduced by an option. Note that for _any_ file named as the value of an option or as an input text file, a single dash ‘-’ may be used, in which case standard input is assumed. However, it would not make sense to use this convention more than once per program invocation. 7.5.1 General options --------------------- ‘-G’ ‘--traditional’ As already explained, this option disables all GNU extensions to ‘ptx’ and switches to traditional mode. ‘--help’ Print a short help on standard output, then exit without further processing. ‘--version’ Print the program version on standard output, then exit without further processing. An exit status of zero indicates success, and a nonzero value indicates failure. 7.5.2 Charset selection ----------------------- As it is set up now, ‘ptx’ assumes that the input file is coded using 8-bit characters, and it may not work well in multibyte locales. In a single-byte locale, the default regular expression for a keyword allows foreign or diacriticized letters. Keyword sorting, however, is still crude; it obeys the underlying character set ordering quite blindly. The output of ‘ptx’ assumes the locale’s character encoding. For example, with ‘ptx’’s ‘-T’ option, if the locale uses the Latin-1 encoding you may need a LaTeX directive like ‘\usepackage[latin1]{inputenc}’ to render non-ASCII characters correctly. ‘-f’ ‘--ignore-case’ Fold lower case letters to upper case for sorting. 7.5.3 Word selection and input processing ----------------------------------------- ‘-b FILE’ ‘--break-file=FILE’ This option provides an alternative (to ‘-W’) method of describing which characters make up words. It introduces the name of a file which contains a list of characters which can_not_ be part of one word; this file is called the “Break file”. Any character which is not part of the Break file is a word constituent. If both options ‘-b’ and ‘-W’ are specified, then ‘-W’ has precedence and ‘-b’ is ignored. When GNU extensions are enabled, the only way to avoid newline as a break character is to write all the break characters in the file with no newline at all, not even at the end of the file. When GNU extensions are disabled, spaces, tabs and newlines are always considered as break characters even if not included in the Break file. ‘-i FILE’ ‘--ignore-file=FILE’ The file associated with this option contains a list of words which will never be taken as keywords in concordance output. It is called the “Ignore file”. The file contains exactly one word in each line; the end of line separation of words is not subject to the value of the ‘-S’ option. ‘-o FILE’ ‘--only-file=FILE’ The file associated with this option contains a list of words which will be retained in concordance output; any word not mentioned in this file is ignored. The file is called the “Only file”. The file contains exactly one word in each line; the end of line separation of words is not subject to the value of the ‘-S’ option. There is no default for the Only file. When both an Only file and an Ignore file are specified, a word is considered a keyword only if it is listed in the Only file and not in the Ignore file. ‘-r’ ‘--references’ On each input line, the leading sequence of non-white space characters will be taken to be a reference that has the purpose of identifying this input line in the resulting permuted index. *Note Output formatting in ptx::, for more information about reference production. Using this option changes the default value for option ‘-S’. Using this option, the program does not try very hard to remove references from contexts in output, but it succeeds in doing so _when_ the context ends exactly at the newline. If option ‘-r’ is used with ‘-S’ default value, or when GNU extensions are disabled, this condition is always met and references are completely excluded from the output contexts. ‘-S REGEXP’ ‘--sentence-regexp=REGEXP’ This option selects which regular expression will describe the end of a line or the end of a sentence. In fact, this regular expression is not the only distinction between end of lines or end of sentences, and input line boundaries have no special significance outside this option. By default, when GNU extensions are enabled and if ‘-r’ option is not used, end of sentences are used. In this case, this REGEX is imported from GNU Emacs: [.?!][]\"')}]*\\($\\|\t\\| \\)[ \t\n]* Whenever GNU extensions are disabled or if ‘-r’ option is used, end of lines are used; in this case, the default REGEXP is just: \n Using an empty REGEXP is equivalent to completely disabling end of line or end of sentence recognition. In this case, the whole file is considered to be a single big line or sentence. The user might want to disallow all truncation flag generation as well, through option ‘-F ""’. *Note Syntax of Regular Expressions: (emacs)Regexps. When the keywords happen to be near the beginning of the input line or sentence, this often creates an unused area at the beginning of the output context line; when the keywords happen to be near the end of the input line or sentence, this often creates an unused area at the end of the output context line. The program tries to fill those unused areas by wrapping around context in them; the tail of the input line or sentence is used to fill the unused area on the left of the output line; the head of the input line or sentence is used to fill the unused area on the right of the output line. As a matter of convenience to the user, many usual backslashed escape sequences from the C language are recognized and converted to the corresponding characters by ‘ptx’ itself. ‘-W REGEXP’ ‘--word-regexp=REGEXP’ This option selects which regular expression will describe each keyword. By default, if GNU extensions are enabled, a word is a sequence of letters; the REGEXP used is ‘\w+’. When GNU extensions are disabled, a word is by default anything which ends with a space, a tab or a newline; the REGEXP used is ‘[^ \t\n]+’. An empty REGEXP is equivalent to not using this option. *Note Syntax of Regular Expressions: (emacs)Regexps. As a matter of convenience to the user, many usual backslashed escape sequences, as found in the C language, are recognized and converted to the corresponding characters by ‘ptx’ itself. 7.5.4 Output formatting ----------------------- Output format is mainly controlled by the ‘-O’ and ‘-T’ options described in the table below. When neither ‘-O’ nor ‘-T’ are selected, and if GNU extensions are enabled, the program chooses an output format suitable for a dumb terminal. Each keyword occurrence is output to the center of one line, surrounded by its left and right contexts. Each field is properly justified, so the concordance output can be readily observed. As a special feature, if automatic references are selected by option ‘-A’ and are output before the left context, that is, if option ‘-R’ is _not_ selected, then a colon is added after the reference; this nicely interfaces with GNU Emacs ‘next-error’ processing. In this default output format, each white space character, like newline and tab, is merely changed to exactly one space, with no special attempt to compress consecutive spaces. This might change in the future. Except for those white space characters, every other character of the underlying set of 256 characters is transmitted verbatim. Output format is further controlled by the following options. ‘-g NUMBER’ ‘--gap-size=NUMBER’ Select the size of the minimum white space gap between the fields on the output line. ‘-w NUMBER’ ‘--width=NUMBER’ Select the maximum output width of each final line. If references are used, they are included or excluded from the maximum output width depending on the value of option ‘-R’. If this option is not selected, that is, when references are output before the left context, the maximum output width takes into account the maximum length of all references. If this option is selected, that is, when references are output after the right context, the maximum output width does not take into account the space taken by references, nor the gap that precedes them. ‘-A’ ‘--auto-reference’ Select automatic references. Each input line will have an automatic reference made up of the file name and the line ordinal, with a single colon between them. However, the file name will be empty when standard input is being read. If both ‘-A’ and ‘-r’ are selected, then the input reference is still read and skipped, but the automatic reference is used at output time, overriding the input reference. ‘-R’ ‘--right-side-refs’ In the default output format, when option ‘-R’ is not used, any references produced by the effect of options ‘-r’ or ‘-A’ are placed to the far right of output lines, after the right context. With default output format, when the ‘-R’ option is specified, references are rather placed at the beginning of each output line, before the left context. For any other output format, option ‘-R’ is ignored, with one exception: with ‘-R’ the width of references is _not_ taken into account in total output width given by ‘-w’. This option is automatically selected whenever GNU extensions are disabled. ‘-F STRING’ ‘--flag-truncation=STRING’ This option will request that any truncation in the output be reported using the string STRING. Most output fields theoretically extend towards the beginning or the end of the current line, or current sentence, as selected with option ‘-S’. But there is a maximum allowed output line width, changeable through option ‘-w’, which is further divided into space for various output fields. When a field has to be truncated because it cannot extend beyond the beginning or the end of the current line to fit in, then a truncation occurs. By default, the string used is a single slash, as in ‘-F /’. STRING may have more than one character, as in ‘-F ...’. Also, in the particular case when STRING is empty (‘-F ""’), truncation flagging is disabled, and no truncation marks are appended in this case. As a matter of convenience to the user, many usual backslashed escape sequences, as found in the C language, are recognized and converted to the corresponding characters by ‘ptx’ itself. ‘-M STRING’ ‘--macro-name=STRING’ Select another STRING to be used instead of ‘xx’, while generating output suitable for ‘nroff’, ‘troff’ or TeX. ‘-O’ ‘--format=roff’ Choose an output format suitable for ‘nroff’ or ‘troff’ processing. Each output line will look like: .xx "TAIL" "BEFORE" "KEYWORD_AND_AFTER" "HEAD" "REF" so it will be possible to write a ‘.xx’ roff macro to take care of the output typesetting. This is the default output format when GNU extensions are disabled. Option ‘-M’ can be used to change ‘xx’ to another macro name. In this output format, each non-graphical character, like newline and tab, is merely changed to exactly one space, with no special attempt to compress consecutive spaces. Each quote character ‘"’ is doubled so it will be correctly processed by ‘nroff’ or ‘troff’. ‘-T’ ‘--format=tex’ Choose an output format suitable for TeX processing. Each output line will look like: \xx {TAIL}{BEFORE}{KEYWORD}{AFTER}{HEAD}{REF} so it will be possible to write a ‘\xx’ definition to take care of the output typesetting. Note that when references are not being produced, that is, neither option ‘-A’ nor option ‘-r’ is selected, the last parameter of each ‘\xx’ call is inhibited. Option ‘-M’ can be used to change ‘xx’ to another macro name. In this output format, some special characters, like ‘$’, ‘%’, ‘&’, ‘#’ and ‘_’ are automatically protected with a backslash. Curly brackets ‘{’, ‘}’ are protected with a backslash and a pair of dollar signs (to force mathematical mode). The backslash itself produces the sequence ‘\backslash{}’. Circumflex and tilde diacritical marks produce the sequence ‘^\{ }’ and ‘~\{ }’ respectively. Other diacriticized characters of the underlying character set produce an appropriate TeX sequence as far as possible. The other non-graphical characters, like newline and tab, and all other characters which are not part of ASCII, are merely changed to exactly one space, with no special attempt to compress consecutive spaces. Let me know how to improve this special character processing for TeX. 7.5.5 The GNU extensions to ‘ptx’ --------------------------------- This version of ‘ptx’ contains a few features which do not exist in System V ‘ptx’. These extra features are suppressed by using the ‘-G’ command line option, unless overridden by other command line options. Some GNU extensions cannot be recovered by overriding, so the simple rule is to avoid ‘-G’ if you care about GNU extensions. Here are the differences between this program and System V ‘ptx’. • This program can read many input files at once, it always writes the resulting concordance on standard output. On the other hand, System V ‘ptx’ reads only one file and sends the result to standard output or, if a second FILE parameter is given on the command, to that FILE. Having output parameters not introduced by options is a dangerous practice which GNU avoids as far as possible. So, for using ‘ptx’ portably between GNU and System V, you should always use it with a single input file, and always expect the result on standard output. You might also want to automatically configure in a ‘-G’ option to ‘ptx’ calls in products using ‘ptx’, if the configurator finds that the installed ‘ptx’ accepts ‘-G’. • The only options available in System V ‘ptx’ are options ‘-b’, ‘-f’, ‘-g’, ‘-i’, ‘-o’, ‘-r’, ‘-t’ and ‘-w’. All other options are GNU extensions and are not repeated in this enumeration. Moreover, some options have a slightly different meaning when GNU extensions are enabled, as explained below. • By default, concordance output is not formatted for ‘troff’ or ‘nroff’. It is rather formatted for a dumb terminal. ‘troff’ or ‘nroff’ output may still be selected through option ‘-O’. • Unless ‘-R’ option is used, the maximum reference width is subtracted from the total output line width. With GNU extensions disabled, width of references is not taken into account in the output line width computations. • All 256 bytes, even ASCII NUL bytes, are always read and processed from input file with no adverse effect, even if GNU extensions are disabled. However, System V ‘ptx’ does not accept 8-bit characters, a few control characters are rejected, and the tilde ‘~’ is also rejected. • Input line length is only limited by available memory, even if GNU extensions are disabled. However, System V ‘ptx’ processes only the first 200 characters in each line. • The break (non-word) characters default to be every character except all letters of the underlying character set, diacriticized or not. When GNU extensions are disabled, the break characters default to space, tab and newline only. • The program makes better use of output line width. If GNU extensions are disabled, the program rather tries to imitate System V ‘ptx’, but still, there are some slight disposition glitches this program does not completely reproduce. • The user can specify both an Ignore file and an Only file. This is not allowed with System V ‘ptx’. 7.6 ‘tsort’: Topological sort ============================= ‘tsort’ performs a topological sort on the given FILE, or standard input if no input file is given or for a FILE of ‘-’. For more details and some history, see *note tsort background::. Synopsis: tsort [OPTION] [FILE] ‘tsort’ reads its input as pairs of strings, separated by blanks, indicating a partial ordering. The output is a total ordering that corresponds to the given partial ordering. For example tsort < file1.sorted $ sort -k 1b,1 file2 > file2.sorted $ join file1.sorted file2.sorted > file3 Normally, the sort order is that of the collating sequence specified by the ‘LC_COLLATE’ locale. Unless the ‘-t’ option is given, the sort comparison ignores blanks at the start of the join field, as in ‘sort -b’. If the ‘--ignore-case’ option is given, the sort comparison ignores the case of characters in the join field, as in ‘sort -f’: $ sort -k 1bf,1 file1 > file1.sorted $ sort -k 1bf,1 file2 > file2.sorted $ join --ignore-case file1.sorted file2.sorted > file3 The ‘sort’ and ‘join’ commands should use consistent locales and options if the output of ‘sort’ is fed to ‘join’. You can use a command like ‘sort -k 1b,1’ to sort a file on its default join field, but if you select a non-default locale, join field, separator, or comparison options, then you should do so consistently between ‘join’ and ‘sort’. To avoid any locale-related issues, it is recommended to use the ‘C’ locale for both commands: $ LC_ALL=C sort -k 1b,1 file1 > file1.sorted $ LC_ALL=C sort -k 1b,1 file2 > file2.sorted $ LC_ALL=C join file1.sorted file2.sorted > file3 8.3.3 Working with fields ------------------------- Use ‘-1’,‘-2’ to set the key fields for each of the input files. Ensure the preceding ‘sort’ commands operated on the same fields. The following example joins two files, using the values from seventh field of the first file and the third field of the second file: $ sort -k 7b,7 file1 > file1.sorted $ sort -k 3b,3 file2 > file2.sorted $ join -1 7 -2 3 file1.sorted file2.sorted > file3 If the field number is the same for both files, use ‘-j’: $ sort -k4b,4 file1 > file1.sorted $ sort -k4b,4 file2 > file2.sorted $ join -j4 file1.sorted file2.sorted > file3 Both ‘sort’ and ‘join’ operate of whitespace-delimited fields. To specify a different delimiter, use ‘-t’ in _both_: $ sort -t, -k3b,3 file1 > file1.sorted $ sort -t, -k3b,3 file2 > file2.sorted $ join -t, -j3 file1.sorted file2.sorted > file3 To specify a tab (ASCII 0x09) character instead of whitespace, use:(1) $ sort -t$'\t' -k3b,3 file1 > file1.sorted $ sort -t$'\t' -k3b,3 file2 > file2.sorted $ join -t$'\t' -j3 file1.sorted file2.sorted > file3 If ‘join -t ''’ is specified then the whole line is considered which matches the default operation of sort: $ sort file1 > file1.sorted $ sort file2 > file2.sorted $ join -t '' file1.sorted file2.sorted > file3 ---------- Footnotes ---------- (1) the ‘$'\t'’ is supported in most modern shells. For older shells, use a literal tab. 8.3.4 Controlling ‘join’’s field matching ----------------------------------------- In this section the ‘sort’ commands are omitted for brevity. Sorting the files before joining is still required. ‘join’’s default behavior is to print only lines common to both input files. Use ‘-a’ and ‘-v’ to print unpairable lines from one or both files. All examples below use the following two (pre-sorted) input files: $ cat file1 $ cat file2 a 1 a A b 2 c C Command Outcome -------------------------------------------------------------------------- $ join file1 file2 common lines (_intersection_) a 1 A $ join -a 1 file1 file2 common lines _and_ unpaired lines a 1 A from the first file b 2 $ join -a 2 file1 file2 common lines _and_ unpaired lines a 1 A from the second file c C $ join -a 1 -a 2 file1 file2 all lines (paired and unpaired) a 1 A from both files (_union_). b 2 see note below regarding ‘-o c C auto’. $ join -v 1 file1 file2 unpaired lines from the first file b 2 (_difference_) $ join -v 2 file1 file2 unpaired lines from the second c C file (_difference_) $ join -v 1 -v 2 file1 file2 unpaired lines from both files, b 2 omitting common lines (_symmetric c C difference_). The ‘-o auto -e X’ options are useful when dealing with unpaired lines. The following example prints all lines (common and unpaired) from both files. Without ‘-o auto’ it is not easy to discern which fields originate from which file: $ join -a 1 -a 2 file1 file2 a 1 A b 2 c C $ join -o auto -e X -a 1 -a 2 file1 file2 a 1 A b 2 X c X C If the input has no unpairable lines, a GNU extension is available; the sort order can be any order that considers two fields to be equal if and only if the sort comparison described above considers them to be equal. For example: $ cat file1 a a1 c c1 b b1 $ cat file2 a a2 c c2 b b2 $ join file1 file2 a a1 a2 c c1 c2 b b1 b2 8.3.5 Header lines ------------------ The ‘--header’ option can be used when the files to join have a header line which is not sorted: $ cat file1 Name Age Alice 25 Charlie 34 $ cat file2 Name Country Alice France Bob Spain $ join --header -o auto -e NA -a1 -a2 file1 file2 Name Age Country Alice 25 France Bob NA Spain Charlie 34 NA To sort a file with a header line, use GNU ‘sed -u’. The following example sort the files but keeps the first line of each file in place: $ ( sed -u 1q ; sort -k2b,2 ) < file1 > file1.sorted $ ( sed -u 1q ; sort -k2b,2 ) < file2 > file2.sorted $ join --header -o auto -e NA -a1 -a2 file1.sorted file2.sorted > file3 8.3.6 Union, Intersection and Difference of files ------------------------------------------------- Combine ‘sort’, ‘uniq’ and ‘join’ to perform the equivalent of set operations on files: Command outcome -------------------------------------------------------------------------- ‘sort -u file1 file2’ Union of unsorted files ‘sort file1 file2 | uniq -d’ Intersection of unsorted files ‘sort file1 file1 file2 | uniq -u’ Difference of unsorted files ‘sort file1 file2 | uniq -u’ Symmetric Difference of unsorted files ‘join -t '' -a1 -a2 file1 file2’ Union of sorted files ‘join -t '' file1 file2’ Intersection of sorted files ‘join -t '' -v2 file1 file2’ Difference of sorted files ‘join -t '' -v1 -v2 file1 file2’ Symmetric Difference of sorted files All examples above operate on entire lines and not on specific fields: ‘sort’ without ‘-k’ and ‘join -t ''’ both consider entire lines as the key. 9 Operating on characters ************************* These commands operate on individual characters. 9.1 ‘tr’: Translate, squeeze, and/or delete characters ====================================================== Synopsis: tr [OPTION]... STRING1 [STRING2] ‘tr’ copies standard input to standard output, performing one of the following operations: • translate, and optionally squeeze repeated characters in the result, • squeeze repeated characters, • delete characters, • delete characters, then squeeze repeated characters from the result. The STRING1 and STRING2 operands define arrays of characters ARRAY1 and ARRAY2. By default ARRAY1 lists input characters that ‘tr’ operates on, and ARRAY2 lists corresponding translations. In some cases the second operand is omitted. The program accepts the following options. Also see *note Common options::. Options must precede operands. ‘-c’ ‘-C’ ‘--complement’ Instead of ARRAY1, use its complement (all characters not specified by STRING1), in ascending order. Use this option with caution in multibyte locales where its meaning is not always clear or portable; see *note Character arrays::. ‘-d’ ‘--delete’ Delete characters in ARRAY1; do not translate. ‘-s’ ‘--squeeze-repeats’ Replace each sequence of a repeated character that is listed in the last specified ARRAY, with a single occurrence of that character. ‘-t’ ‘--truncate-set1’ Truncate ARRAY1 to the length of ARRAY2. An exit status of zero indicates success, and a nonzero value indicates failure. 9.1.1 Specifying arrays of characters ------------------------------------- The STRING1 and STRING2 operands are not regular expressions, even though they may look similar. Instead, they merely represent arrays of characters. As a GNU extension to POSIX, an empty string operand represents an empty array of characters. The interpretation of STRING1 and STRING2 depends on locale. GNU ‘tr’ fully supports only safe single-byte locales, where each possible input byte represents a single character. Unfortunately, this means GNU ‘tr’ will not handle commands like ‘tr ö Ł’ the way you might expect, since (assuming a UTF-8 encoding) this is equivalent to ‘tr '\303\266' '\305\201'’ and GNU ‘tr’ will simply transliterate all ‘\303’ bytes to ‘\305’ bytes, etc. POSIX does not clearly specify the behavior of ‘tr’ in locales where characters are represented by byte sequences instead of by individual bytes, or where data might contain invalid bytes that are encoding errors. To avoid problems in this area, you can run ‘tr’ in a safe single-byte locale by using a shell command like ‘LC_ALL=C tr’ instead of plain ‘tr’. Although most characters simply represent themselves in STRING1 and STRING2, the strings can contain shorthands listed below, for convenience. Some shorthands can be used only in STRING1 or STRING2, as noted below. Backslash escapes The following backslash escape sequences are recognized: ‘\a’ Bell (BEL, Control-G). ‘\b’ Backspace (BS, Control-H). ‘\f’ Form feed (FF, Control-L). ‘\n’ Newline (LF, Control-J). ‘\r’ Carriage return (CR, Control-M). ‘\t’ Tab (HT, Control-I). ‘\v’ Vertical tab (VT, Control-K). ‘\OOO’ The eight-bit byte with the value given by OOO, which is the longest sequence of one to three octal digits following the backslash. For portability, OOO should represent a value that fits in eight bits. As a GNU extension to POSIX, if the value would not fit, then only the first two digits of OOO are used, e.g., ‘\400’ is equivalent to ‘\0400’ and represents a two-byte sequence. ‘\\’ A backslash. It is an error if no character follows an unescaped backslash. As a GNU extension, a backslash followed by a character not listed above is interpreted as that character, removing any special significance; this can be used to escape the characters ‘[’ and ‘-’ when they would otherwise be special. Ranges The notation ‘M-N’ expands to the characters from M through N, in ascending order. M should not collate after N; if it does, an error results. As an example, ‘0-9’ is the same as ‘0123456789’. GNU ‘tr’ does not support the System V syntax that uses square brackets to enclose ranges. Translations specified in that format sometimes work as expected, since the brackets are often transliterated to themselves. However, they should be avoided because they sometimes behave unexpectedly. For example, ‘tr -d '[0-9]'’ deletes brackets as well as digits. Many historically common and even accepted uses of ranges are not fully portable. For example, on EBCDIC hosts using the ‘A-Z’ range will not do what most would expect because ‘A’ through ‘Z’ are not contiguous as they are in ASCII. One way to work around this is to use character classes (see below). Otherwise, it is most portable (and most ugly) to enumerate the members of the ranges. Repeated characters The notation ‘[C*N]’ in STRING2 expands to N copies of character C. Thus, ‘[y*6]’ is the same as ‘yyyyyy’. The notation ‘[C*]’ in STRING2 expands to as many copies of C as are needed to make ARRAY2 as long as ARRAY1. If N begins with ‘0’, it is interpreted in octal, otherwise in decimal. A zero-valued N is treated as if it were absent. Character classes The notation ‘[:CLASS:]’ expands to all characters in the (predefined) class CLASS. When the ‘--delete’ (‘-d’) and ‘--squeeze-repeats’ (‘-s’) options are both given, any character class can be used in STRING2. Otherwise, only the character classes ‘lower’ and ‘upper’ are accepted in STRING2, and then only if the corresponding character class (‘upper’ and ‘lower’, respectively) is specified in the same relative position in STRING1. Doing this specifies case conversion. Except for case conversion, a class’s characters appear in no particular order. The class names are given below; an error results when an invalid class name is given. ‘alnum’ Letters and digits. ‘alpha’ Letters. ‘blank’ Horizontal whitespace. ‘cntrl’ Control characters. ‘digit’ Digits. ‘graph’ Printable characters, not including space. ‘lower’ Lowercase letters. ‘print’ Printable characters, including space. ‘punct’ Punctuation characters. ‘space’ Horizontal or vertical whitespace. ‘upper’ Uppercase letters. ‘xdigit’ Hexadecimal digits. Equivalence classes The syntax ‘[=C=]’ expands to all characters equivalent to C, in no particular order. These equivalence classes are allowed in STRING2 only when ‘--delete’ (‘-d’) and ‘--squeeze-repeats’ ‘-s’ are both given. Although equivalence classes are intended to support non-English alphabets, there seems to be no standard way to define them or determine their contents. Therefore, they are not fully implemented in GNU ‘tr’; each character’s equivalence class consists only of that character, which is of no particular use. 9.1.2 Translating ----------------- ‘tr’ performs translation when STRING1 and STRING2 are both given and the ‘--delete’ (‘-d’) option is not given. ‘tr’ translates each character of its input that is in ARRAY1 to the corresponding character in ARRAY2. Characters not in ARRAY1 are passed through unchanged. As a GNU extension to POSIX, when a character appears more than once in ARRAY1, only the final instance is used. For example, these two commands are equivalent: tr aaa xyz tr a z A common use of ‘tr’ is to convert lowercase characters to uppercase. This can be done in many ways. Here are three of them: tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ tr a-z A-Z tr '[:lower:]' '[:upper:]' However, ranges like ‘a-z’ are not portable outside the C locale. When ‘tr’ is performing translation, ARRAY1 and ARRAY2 typically have the same length. If ARRAY1 is shorter than ARRAY2, the extra characters at the end of ARRAY2 are ignored. On the other hand, making ARRAY1 longer than ARRAY2 is not portable; POSIX says that the result is undefined. In this situation, BSD ‘tr’ pads ARRAY2 to the length of ARRAY1 by repeating the last character of ARRAY2 as many times as necessary. System V ‘tr’ truncates ARRAY1 to the length of ARRAY2. By default, GNU ‘tr’ handles this case like BSD ‘tr’. When the ‘--truncate-set1’ (‘-t’) option is given, GNU ‘tr’ handles this case like the System V ‘tr’ instead. This option is ignored for operations other than translation. Acting like System V ‘tr’ in this case breaks the relatively common BSD idiom: tr -cs A-Za-z0-9 '\012' because it converts only zero bytes (the first element in the complement of ARRAY1), rather than all non-alphanumerics, to newlines. By the way, the above idiom is not portable because it uses ranges, and it assumes that the octal code for newline is 012. Here is a better way to write it: tr -cs '[:alnum:]' '[\n*]' 9.1.3 Squeezing repeats and deleting ------------------------------------ When given just the ‘--delete’ (‘-d’) option, ‘tr’ removes any input characters that are in ARRAY1. When given just the ‘--squeeze-repeats’ (‘-s’) option and not translating, ‘tr’ replaces each input sequence of a repeated character that is in ARRAY1 with a single occurrence of that character. When given both ‘--delete’ and ‘--squeeze-repeats’, ‘tr’ first performs any deletions using ARRAY1, then squeezes repeats from any remaining characters using ARRAY2. The ‘--squeeze-repeats’ option may also be used when translating, in which case ‘tr’ first performs translation, then squeezes repeats from any remaining characters using ARRAY2. Here are some examples to illustrate various combinations of options: • Remove all zero bytes: tr -d '\0' • Put all words on lines by themselves. This converts all non-alphanumeric characters to newlines, then squeezes each string of repeated newlines into a single newline: tr -cs '[:alnum:]' '[\n*]' • Convert each sequence of repeated newlines to a single newline. I.e., delete empty lines: tr -s '\n' • Find doubled occurrences of words in a document. For example, people often write “the the” with the repeated words separated by a newline. The Bourne shell script below works first by converting each sequence of punctuation and blank characters to a single newline. That puts each “word” on a line by itself. Next it maps all uppercase characters to lower case, and finally it runs ‘uniq’ with the ‘-d’ option to print out only the words that were repeated. #!/bin/sh cat -- "$@" \ | tr -s '[:punct:][:blank:]' '[\n*]' \ | tr '[:upper:]' '[:lower:]' \ | uniq -d • Deleting a small set of characters is usually straightforward. For example, to remove all ‘a’s, ‘x’s, and ‘M’s you would do this: tr -d axM However, when ‘-’ is one of those characters, it can be tricky because ‘-’ has special meanings. Performing the same task as above but also removing all ‘-’ characters, we might try ‘tr -d -axM’, but that would fail because ‘tr’ would try to interpret ‘-a’ as a command-line option. Alternatively, we could try putting the hyphen inside the string, ‘tr -d a-xM’, but that wouldn’t work either because it would make ‘tr’ interpret ‘a-x’ as the range of characters ‘a’...‘x’ rather than the three. One way to solve the problem is to put the hyphen at the end of the list of characters: tr -d axM- Or you can use ‘--’ to terminate option processing: tr -d -- -axM 9.2 ‘expand’: Convert tabs to spaces ==================================== ‘expand’ writes the contents of each given FILE, or standard input if none are given or for a FILE of ‘-’, to standard output, with tab characters converted to the appropriate number of spaces. Synopsis: expand [OPTION]... [FILE]... By default, ‘expand’ converts all tabs to spaces. It preserves backspace characters in the output; they decrement the column count for tab calculations. The default action is equivalent to ‘-t 8’ (set tabs every 8 columns). The program accepts the following options. Also see *note Common options::. ‘-t TAB1[,TAB2]...’ ‘--tabs=TAB1[,TAB2]...’ If only one tab stop is given, set the tabs TAB1 spaces apart (default is 8). Otherwise, set the tabs at columns TAB1, TAB2, ... (numbered from 0), and replace any tabs beyond the last tab stop given with single spaces. Tab stops can be separated by blanks as well as by commas. As a GNU extension the last TAB specified can be prefixed with a ‘/’ to indicate a tab size to use for remaining positions. For example, ‘--tabs=2,4,/8’ will set tab stops at position 2 and 4, and every multiple of 8 after that. Also the last TAB specified can be prefixed with a ‘+’ to indicate a tab size to use for remaining positions, offset from the final explicitly specified tab stop. For example, to ignore the 1 character gutter present in diff output, one can specify a 1 character offset using ‘--tabs=1,+8’, which will set tab stops at positions 1,9,17,... For compatibility, GNU ‘expand’ also accepts the obsolete option syntax, ‘-T1[,T2]...’. New scripts should use ‘-t T1[,T2]...’ instead. ‘-i’ ‘--initial’ Only convert initial tabs (those that precede all non-space or non-tab characters) on each line to spaces. An exit status of zero indicates success, and a nonzero value indicates failure. 9.3 ‘unexpand’: Convert spaces to tabs ====================================== ‘unexpand’ writes the contents of each given FILE, or standard input if none are given or for a FILE of ‘-’, to standard output, converting blanks at the beginning of each line into as many tab characters as needed. In the default POSIX locale, a “blank” is a space or a tab; other locales may specify additional blank characters. Synopsis: unexpand [OPTION]... [FILE]... By default, ‘unexpand’ converts only initial blanks (those that precede all non-blank characters) on each line. It preserves backspace characters in the output; they decrement the column count for tab calculations. By default, tabs are set at every 8th column. The program accepts the following options. Also see *note Common options::. ‘-t TAB1[,TAB2]...’ ‘--tabs=TAB1[,TAB2]...’ If only one tab stop is given, set the tabs TAB1 columns apart instead of the default 8. Otherwise, set the tabs at columns TAB1, TAB2, ... (numbered from 0), and leave blanks beyond the tab stops given unchanged. Tab stops can be separated by blanks as well as by commas. As a GNU extension the last TAB specified can be prefixed with a ‘/’ to indicate a tab size to use for remaining positions. For example, ‘--tabs=2,4,/8’ will set tab stops at position 2 and 4, and every multiple of 8 after that. Also the last TAB specified can be prefixed with a ‘+’ to indicate a tab size to use for remaining positions, offset from the final explicitly specified tab stop. For example, to ignore the 1 character gutter present in diff output, one can specify a 1 character offset using ‘--tabs=1,+8’, which will set tab stops at positions 1,9,17,... This option implies the ‘-a’ option. For compatibility, GNU ‘unexpand’ supports the obsolete option syntax, ‘-TAB1[,TAB2]...’, where tab stops must be separated by commas. (Unlike ‘-t’, this obsolete option does not imply ‘-a’.) New scripts should use ‘--first-only -t TAB1[,TAB2]...’ instead. ‘-a’ ‘--all’ Also convert all sequences of two or more blanks just before a tab stop, even if they occur after non-blank characters in a line. An exit status of zero indicates success, and a nonzero value indicates failure. 10 Directory listing ******************** This chapter describes the ‘ls’ command and its variants ‘dir’ and ‘vdir’, which list information about files. 10.1 ‘ls’: List directory contents ================================== The ‘ls’ program lists information about files (of any type, including directories). Options and file arguments can be intermixed arbitrarily, as usual. Later options override earlier options that are incompatible. For non-option command-line arguments that are directories, by default ‘ls’ lists the contents of directories, not recursively, and omitting files with names beginning with ‘.’. For other non-option arguments, by default ‘ls’ lists just the file name. If no non-option argument is specified, ‘ls’ operates on the current directory, acting as if it had been invoked with a single argument of ‘.’. By default, the output is sorted alphabetically, according to the locale settings in effect.(1) If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is listed one per line and control characters are output as-is. Because ‘ls’ is such a fundamental program, it has accumulated many options over the years. They are described in the subsections below; within each section, options are listed alphabetically (ignoring case). The division of options into the subsections is not absolute, since some options affect more than one aspect of ‘ls’’s operation. Exit status: 0 success 1 minor problems (e.g., failure to access a file or directory not specified as a command line argument. This happens when listing a directory in which entries are actively being removed or renamed.) 2 serious trouble (e.g., memory exhausted, invalid option, failure to access a file or directory specified as a command line argument or a directory loop) Also see *note Common options::. ---------- Footnotes ---------- (1) If you use a non-POSIX locale (e.g., by setting ‘LC_ALL’ to ‘en_US’), then ‘ls’ may produce output that is sorted differently than you’re accustomed to. In that case, set the ‘LC_ALL’ environment variable to ‘C’. 10.1.1 Which files are listed ----------------------------- These options determine which files ‘ls’ lists information for. By default, ‘ls’ lists files and the contents of any directories on the command line, except that in directories it ignores files whose names start with ‘.’. ‘-a’ ‘--all’ In directories, do not ignore file names that start with ‘.’. ‘-A’ ‘--almost-all’ In directories, do not ignore all file names that start with ‘.’; ignore only ‘.’ and ‘..’. The ‘--all’ (‘-a’) option overrides this option. ‘-B’ ‘--ignore-backups’ In directories, ignore files that end with ‘~’. This option is equivalent to ‘--ignore='*~' --ignore='.*~'’. ‘-d’ ‘--directory’ List just the names of directories, as with other types of files, rather than listing their contents. Do not follow symbolic links listed on the command line unless the ‘--dereference-command-line’ (‘-H’), ‘--dereference’ (‘-L’), or ‘--dereference-command-line-symlink-to-dir’ options are specified. ‘-H’ ‘--dereference-command-line’ If a command line argument specifies a symbolic link, show information for the file the link references rather than for the link itself. ‘--dereference-command-line-symlink-to-dir’ Do not dereference symbolic links, with one exception: if a command line argument specifies a symbolic link that refers to a directory, show information for that directory rather than for the link itself. This is the default behavior unless long format is being used or any of the following options is in effect: ‘--classify’ (‘-F’), ‘--directory’ (‘-d’), ‘--dereference’ (‘-L’), or ‘--dereference-command-line’ (‘-H’)). ‘--group-directories-first’ Group all the directories before the files and then sort the directories and the files separately using the selected sort key (see ‘--sort’ option). That is, this option specifies a primary sort key, and the ‘--sort’ option specifies a secondary key. However, any use of ‘--sort=none’ (‘-U’) disables this option altogether. ‘--hide=PATTERN’ In directories, ignore files whose names match the shell pattern PATTERN, unless the ‘--all’ (‘-a’) or ‘--almost-all’ (‘-A’) is also given. This option acts like ‘--ignore=PATTERN’ except that it has no effect if ‘--all’ (‘-a’) or ‘--almost-all’ (‘-A’) is also given. This option can be useful in shell aliases. For example, if ‘lx’ is an alias for ‘ls --hide='*~'’ and ‘ly’ is an alias for ‘ls --ignore='*~'’, then the command ‘lx -A’ lists the file ‘README~’ even though ‘ly -A’ would not. ‘-I PATTERN’ ‘--ignore=PATTERN’ In directories, ignore files whose names match the shell pattern (not regular expression) PATTERN. As in the shell, an initial ‘.’ in a file name does not match a wildcard at the start of PATTERN. Sometimes it is useful to give this option several times. For example, $ ls --ignore='.??*' --ignore='.[^.]' --ignore='#*' The first option ignores names of length 3 or more that start with ‘.’, the second ignores all two-character names that start with ‘.’ except ‘..’, and the third ignores names that start with ‘#’. ‘-L’ ‘--dereference’ When showing file information for a symbolic link, show information for the file the link references rather than the link itself. However, even with this option, ‘ls’ still prints the name of the link itself, not the name of the file that the link points to. ‘-R’ ‘--recursive’ List the contents of all directories recursively. 10.1.2 What information is listed --------------------------------- These options affect the information that ‘ls’ displays. By default, only file names are shown. ‘--author’ In long format, list each file’s author. In GNU/Hurd, file authors can differ from their owners, but in other operating systems the two are the same. ‘-D’ ‘--dired’ Print an additional line after the main output: //DIRED// BEG1 END1 BEG2 END2 ... The BEGN and ENDN are unsigned integers that record the byte position of the beginning and end of each file name in the output. This makes it easy for Emacs to find the names, even when they contain unusual characters such as space or newline, without fancy searching. If directories are being listed recursively via ‘--recursive’ (‘-R’), output a similar line with offsets for each subdirectory name: //SUBDIRED// BEG1 END1 ... Finally, output a line of the form: //DIRED-OPTIONS// --quoting-style=WORD where WORD is the quoting style (*note Formatting the file names::). Here is an actual example: $ mkdir -p a/sub/deeper a/sub2 $ touch a/f1 a/f2 $ touch a/sub/deeper/file $ ls -gloRF --dired a a: total 8 -rw-r--r-- 1 0 Jun 10 12:27 f1 -rw-r--r-- 1 0 Jun 10 12:27 f2 drwxr-xr-x 3 4096 Jun 10 12:27 sub/ drwxr-xr-x 2 4096 Jun 10 12:27 sub2/ a/sub: total 4 drwxr-xr-x 2 4096 Jun 10 12:27 deeper/ a/sub/deeper: total 0 -rw-r--r-- 1 0 Jun 10 12:27 file a/sub2: total 0 //DIRED// 48 50 84 86 120 123 158 162 217 223 282 286 //SUBDIRED// 2 3 167 172 228 240 290 296 //DIRED-OPTIONS// --quoting-style=literal The pairs of offsets on the ‘//DIRED//’ line above delimit these names: ‘f1’, ‘f2’, ‘sub’, ‘sub2’, ‘deeper’, ‘file’. The offsets on the ‘//SUBDIRED//’ line delimit the following directory names: ‘a’, ‘a/sub’, ‘a/sub/deeper’, ‘a/sub2’. Here is an example of how to extract the fifth entry name, ‘deeper’, corresponding to the pair of offsets, 222 and 228: $ ls -gloRF --dired a > out $ dd bs=1 skip=222 count=6 < out 2>/dev/null; echo deeper Although the listing above includes a trailing slash for the ‘deeper’ entry, the offsets select the name without the trailing slash. However, if you invoke ‘ls’ with ‘--dired’ (‘-D’) along with an option like ‘--escape’ (‘-b’) and operate on a file whose name contains special characters, the backslash _is_ included: $ touch 'a b' $ ls -blog --dired 'a b' -rw-r--r-- 1 0 Jun 10 12:28 a\ b //DIRED// 30 34 //DIRED-OPTIONS// --quoting-style=escape If you use a quoting style like ‘--quoting-style=c’ (‘-Q’) that adds quote marks, then the offsets include the quote marks. So beware that the user may select the quoting style via the environment variable ‘QUOTING_STYLE’. Hence, applications using ‘--dired’ should either specify an explicit ‘--quoting-style=literal’ (‘-N’) option on the command line, or else be prepared to parse the escaped names. The ‘--dired’ (‘-D’) option has well-defined behavior only when long format is in effect and hyperlinks are disabled (e.g., ‘--hyperlink=none’). ‘--full-time’ Produce long format, and list times in full. It is equivalent to using ‘--format=long’ (‘-l’) with ‘--time-style=full-iso’ (*note Formatting file timestamps::). ‘-g’ Produce long format, but omit owner information. ‘-G’ ‘--no-group’ Inhibit display of group information in long format. (This is the default in some non-GNU versions of ‘ls’, so we provide this option for compatibility.) ‘-h’ ‘--human-readable’ Append a size letter to each size, such as ‘M’ for mebibytes. Powers of 1024 are used, not 1000; ‘M’ stands for 1,048,576 bytes. This option is equivalent to ‘--block-size=human-readable’. Use the ‘--si’ option if you prefer powers of 1000. ‘-i’ ‘--inode’ Print the inode number (also called the file serial number and index number) of each file to the left of the file name. (This number uniquely identifies each file within a particular file system.) ‘-l’ ‘--format=long’ ‘--format=verbose’ Produce long format. In addition to the name of each file, print the file type, file mode bits, number of hard links, owner name, group name, size, and timestamp (*note Formatting file timestamps::), normally the modification timestamp (the mtime, *note File timestamps::). If the owner or group name cannot be determined, print the owner or group ID instead, right-justified as a cue that it is a number rather than a textual name. Print question marks for other information that cannot be determined. Normally the size is printed as a byte count without punctuation, but this can be overridden (*note Block size::). For example, ‘--human-readable’ (‘-h’) prints an abbreviated, human-readable count, and ‘--block-size="'1"’ prints a byte count with the thousands separator of the current locale. For each directory that is listed, preface the files with a line ‘total BLOCKS’, where BLOCKS is the file system allocation for all files in that directory. The block size currently defaults to 1024 bytes, but this can be overridden (*note Block size::). The BLOCKS computed counts each hard link separately; this is arguably a deficiency. The file type is one of the following characters: ‘-’ regular file ‘b’ block special file ‘c’ character special file ‘C’ high performance (“contiguous data”) file ‘d’ directory ‘D’ door (Solaris) ‘l’ symbolic link ‘M’ off-line (“migrated”) file (Cray DMF) ‘n’ network special file (HP-UX) ‘p’ FIFO (named pipe) ‘P’ port (Solaris) ‘s’ socket ‘?’ some other file type The file mode bits listed are similar to symbolic mode specifications (*note Symbolic Modes::). But ‘ls’ combines multiple bits into the third character of each set of permissions as follows: ‘s’ If the set-user-ID or set-group-ID bit and the corresponding executable bit are both set. ‘S’ If the set-user-ID or set-group-ID bit is set but the corresponding executable bit is not set. ‘t’ If the restricted deletion flag or sticky bit, and the other-executable bit, are both set. The restricted deletion flag is another name for the sticky bit. *Note Mode Structure::. ‘T’ If the restricted deletion flag or sticky bit is set but the other-executable bit is not set. ‘x’ If the executable bit is set and none of the above apply. ‘-’ Otherwise. Following the file mode bits is a single character that specifies whether an alternate access method such as an access control list applies to the file. When the character following the file mode bits is a space, there is no alternate access method. When it is a printing character, then there is such a method. GNU ‘ls’ uses a ‘.’ character to indicate a file with a security context, but no other alternate access method. A file with any other combination of alternate access methods is marked with a ‘+’ character. ‘-n’ ‘--numeric-uid-gid’ Produce long format, but display right-justified numeric user and group IDs instead of left-justified owner and group names. ‘-o’ Produce long format, but omit group information. It is equivalent to using ‘--format=long’ (‘-l’) with ‘--no-group’ (‘-G’). ‘-s’ ‘--size’ Print the file system allocation of each file to the left of the file name. This is the amount of file system space used by the file, which is usually a bit more than the file’s size, but it can be less if the file has holes. Normally the allocation is printed in units of 1024 bytes, but this can be overridden (*note Block size::). For files that are NFS-mounted from an HP-UX system to a BSD system, this option reports sizes that are half the correct values. On HP-UX systems, it reports sizes that are twice the correct values for files that are NFS-mounted from BSD systems. This is due to a flaw in HP-UX; it also affects the HP-UX ‘ls’ program. ‘--si’ Append an SI-style abbreviation to each size, such as ‘M’ for megabytes. Powers of 1000 are used, not 1024; ‘M’ stands for 1,000,000 bytes. This option is equivalent to ‘--block-size=si’. Use the ‘-h’ or ‘--human-readable’ option if you prefer powers of 1024. ‘-Z’ ‘--context’ Display the SELinux security context or ‘?’ if none is found. In long format, print the security context to the left of the size column. 10.1.3 Sorting the output ------------------------- These options change the order in which ‘ls’ sorts the information it outputs. By default, sorting is done by character code (e.g., ASCII order). ‘-c’ ‘--time=ctime’ ‘--time=status’ In long format, print the status change timestamp (the ctime) instead of the mtime. When sorting by time or when not using long format, sort according to the ctime. *Note File timestamps::. ‘-f’ Produce an unsorted directory listing. This is equivalent to the combination of ‘--all’ (‘-a’), ‘--sort=none’ (‘-U’), ‘-1’, ‘--color=none’, and ‘--hyperlink=none’, while also disabling any previous use of ‘--size’ (‘-s’). ‘-r’ ‘--reverse’ Reverse whatever the sorting method is – e.g., list files in reverse alphabetical order, youngest first, smallest first, or whatever. This option has no effect when ‘--sort=none’ (‘-U’) is in effect. ‘-S’ ‘--sort=size’ Sort by file size, largest first. ‘-t’ ‘--sort=time’ Sort by modification timestamp (mtime) by default, newest first. The timestamp to order by can be changed with the ‘--time’ option. *Note File timestamps::. ‘-u’ ‘--time=atime’ ‘--time=access’ ‘--time=use’ In long format, print the last access timestamp (the atime). When sorting by time or when not using long format, sort according to the atime. *Note File timestamps::. ‘--time=mtime’ ‘--time=modification’ This is the default timestamp display and sorting mode. In long format, print the last data modification timestamp (the mtime). When sorting by time or when not using long format, sort according to the mtime. *Note File timestamps::. ‘--time=birth’ ‘--time=creation’ In long format, print the file creation timestamp if available, falling back to the file modification timestamp (mtime) if not. When sorting by time or when not using long format, sort according to the birth time. *Note File timestamps::. ‘-U’ ‘--sort=none’ Do not sort; list the files in whatever order they are stored in the directory. (Do not do any of the other unrelated things that ‘-f’ does.) This can be useful when listing large directories, where sorting can take some time. ‘-v’ ‘--sort=version’ Sort by version name and number, lowest first. It behaves like a default sort, except that each sequence of decimal digits is treated numerically as an index/version number. *Note Version sort ordering::. ‘--sort=width’ Sort by printed width of file names. This can be useful with the ‘--format=vertical’ (‘-C’) output format, to most densely display the listed files. ‘-X’ ‘--sort=extension’ Sort directory contents alphabetically by file extension (characters after the last ‘.’); files with no extension are sorted first. 10.1.4 General output formatting -------------------------------- These options affect the appearance of the overall output. ‘--format=single-column’ List one file name per line, with no other information. This is the default for ‘ls’ when standard output is not a terminal. See also the ‘--escape’ (‘-b’), ‘--hide-control-chars’ (‘-q’), and ‘--zero’ options to disambiguate output of file names containing newline characters. ‘-1’ List one file per line. This is like ‘--format=single-column’ except that it has no effect if long format is also in effect. ‘-C’ ‘--format=vertical’ List files in columns, sorted vertically, with no other information. This is the default for ‘ls’ if standard output is a terminal. It is always the default for the ‘dir’ program. GNU ‘ls’ uses variable width columns to display as many files as possible in the fewest lines. ‘--color [=WHEN]’ Specify whether to use color for distinguishing file types; WHEN may be omitted, or one of: • none - Do not use color at all. This is the default. • auto - Only use color if standard output is a terminal. • always - Always use color. Specifying ‘--color’ and no WHEN is equivalent to ‘--color=always’. If piping a colored listing through a pager like ‘less’, use the pager’s ‘-R’ option to pass the color codes to the terminal. Using the ‘--color’ option may incur a noticeable performance penalty when run in a large directory, because the default settings require that ‘ls’ ‘stat’ every single file it lists. However, if you would like most of the file-type coloring but can live without the other coloring options (e.g., executable, orphan, sticky, other-writable, capability), use ‘dircolors’ to set the ‘LS_COLORS’ environment variable like this, eval $(dircolors -p | perl -pe \ 's/^((CAP|S[ET]|O[TR]|M|E)\w+).*/$1 00/' | dircolors -) and on a ‘dirent.d_type’-capable file system, ‘ls’ will perform only one ‘stat’ call per command line argument. ‘-F’ ‘--classify [=WHEN]’ ‘--indicator-style=classify’ Append a character to each file name indicating the file type. Also, for regular files that are executable, append ‘*’. The file type indicators are ‘/’ for directories, ‘@’ for symbolic links, ‘|’ for FIFOs, ‘=’ for sockets, ‘>’ for doors, and nothing for regular files. WHEN may be omitted, or one of: • none - Do not classify. This is the default. • auto - Only classify if standard output is a terminal. • always - Always classify. Specifying ‘--classify’ and no WHEN is equivalent to ‘--classify=always’. Do not follow symbolic links listed on the command line unless the ‘--dereference-command-line’ (‘-H’), ‘--dereference’ (‘-L’), or ‘--dereference-command-line-symlink-to-dir’ options are specified. ‘--file-type’ ‘--indicator-style=file-type’ Append a character to each file name indicating the file type. This is like ‘--classify’ (‘-F’, except that executables are not marked. ‘--hyperlink [=WHEN]’ Output codes recognized by some terminals to link to files using the ‘file://’ URI format. WHEN may be omitted, or one of: • none - Do not use hyperlinks at all. This is the default. • auto - Only use hyperlinks if standard output is a terminal. • always - Always use hyperlinks. Specifying ‘--hyperlink’ and no WHEN is equivalent to ‘--hyperlink=always’. ‘--indicator-style=WORD’ Append a character indicator with style WORD to entry names, as follows: ‘none’ Do not append any character indicator; this is the default. ‘slash’ Append ‘/’ for directories. This is the same as the ‘-p’ option. ‘file-type’ Append ‘/’ for directories, ‘@’ for symbolic links, ‘|’ for FIFOs, ‘=’ for sockets, and nothing for regular files. This is the same as the ‘--file-type’ option. ‘classify’ Append ‘*’ for executable regular files, otherwise behave as for ‘file-type’. This is the same as the ‘--classify’ (‘-F’) option. ‘-k’ ‘--kibibytes’ Set the default block size to its normal value of 1024 bytes, overriding any contrary specification in environment variables (*note Block size::). If ‘--block-size’, ‘--human-readable’ (‘-h’), or ‘--si’ options are used, they take precedence even if ‘--kibibytes’ (‘-k’) is placed after The ‘--kibibytes’ (‘-k’) option affects the per-directory block count written in long format, and the file system allocation written by the ‘--size’ (‘-s’) option. It does not affect the file size in bytes that is written in long format. ‘-m’ ‘--format=commas’ List files horizontally, with as many as will fit on each line, separated by ‘, ’ (a comma and a space), and with no other information. ‘-p’ ‘--indicator-style=slash’ Append a ‘/’ to directory names. ‘-x’ ‘--format=across’ ‘--format=horizontal’ List the files in columns, sorted horizontally. ‘-T COLS’ ‘--tabsize=COLS’ Assume that each tab stop is COLS columns wide. The default is 8. ‘ls’ uses tabs where possible in the output, for efficiency. If COLS is zero, do not use tabs at all. Some terminal emulators might not properly align columns to the right of a TAB following a non-ASCII byte. You can avoid that issue by using the ‘-T0’ option or put ‘TABSIZE=0’ in your environment, to tell ‘ls’ to align using spaces, not tabs. If you set a terminal’s hardware tabs to anything other than the default, you should also use a ‘--tabsize’ option or ‘TABSIZE’ environment variable either to match the hardware tabs, or to disable the use of hardware tabs. Otherwise, the output of ‘ls’ may not line up. For example, if you run the shell command ‘tabs -4’ to set hardware tabs to every four columns, you should also run ‘export TABSIZE=4’ or ‘export TABSIZE=0’, or use the corresponding ‘--tabsize’ options. ‘-w COLS’ ‘--width=COLS’ Assume the screen is COLS columns wide. The default is taken from the terminal settings if possible; otherwise the environment variable ‘COLUMNS’ is used if it is set; otherwise the default is 80. With a COLS value of ‘0’, there is no limit on the length of the output line, and that single output line will be delimited with spaces, not tabs. ‘--zero’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. This option is incompatible with the ‘--dired’ (‘-D’) option. This option also implies the options ‘--show-control-chars’, ‘-1’, ‘--color=none’, and ‘--quoting-style=literal’ (‘-N’). 10.1.5 Formatting file timestamps --------------------------------- By default, file timestamps are listed in abbreviated form, using a date like ‘Mar 30 2020’ for non-recent timestamps, and a date-without-year and time like ‘Mar 30 23:45’ for recent timestamps. This format can change depending on the current locale as detailed below. A timestamp is considered to be “recent” if it is less than six months old, and is not dated in the future. If a timestamp dated today is not listed in recent form, the timestamp is in the future, which means you probably have clock skew problems which may break programs like ‘make’ that rely on file timestamps. *Note File timestamps::. Timestamps are listed according to the time zone rules specified by the ‘TZ’ environment variable, or by the system default rules if ‘TZ’ is not set. *Note Specifying the Time Zone with ‘TZ’: (libc)TZ Variable. The following option changes how file timestamps are printed. ‘--time-style=STYLE’ List timestamps in style STYLE. The STYLE should be one of the following: ‘+FORMAT’ List timestamps using FORMAT, where FORMAT is interpreted like the format argument of ‘date’ (*note date invocation::). For example, ‘--time-style="+%Y-%m-%d %H:%M:%S"’ causes ‘ls’ to list timestamps like ‘2020-03-30 23:45:56’. As with ‘date’, FORMAT’s interpretation is affected by the ‘LC_TIME’ locale category. If FORMAT contains two format strings separated by a newline, the former is used for non-recent files and the latter for recent files; if you want output columns to line up, you may need to insert spaces in one of the two formats. ‘full-iso’ List timestamps in full using ISO 8601-like date, time, and time zone components with nanosecond precision, e.g., ‘2020-07-21 23:45:56.477817180 -0400’. This style is equivalent to ‘+%Y-%m-%d %H:%M:%S.%N %z’. This is useful because the time output includes all the information that is available from the operating system. For example, this can help explain ‘make’’s behavior, since GNU ‘make’ uses the full timestamp to determine whether a file is out of date. ‘long-iso’ List ISO 8601 date and time components with minute precision, e.g., ‘2020-03-30 23:45’. These timestamps are shorter than ‘full-iso’ timestamps, and are usually good enough for everyday work. This style is equivalent to ‘+%Y-%m-%d %H:%M’. ‘iso’ List ISO 8601 dates for non-recent timestamps (e.g., ‘2020-03-30 ’), and ISO 8601-like month, day, hour, and minute for recent timestamps (e.g., ‘03-30 23:45’). These timestamps are uglier than ‘long-iso’ timestamps, but they carry nearly the same information in a smaller space and their brevity helps ‘ls’ output fit within traditional 80-column output lines. The following two ‘ls’ invocations are equivalent: newline=' ' ls -l --time-style="+%Y-%m-%d $newline%m-%d %H:%M" ls -l --time-style="iso" ‘locale’ List timestamps in a locale-dependent form. For example, a French locale might list non-recent timestamps like ‘30 mars 2020’ and recent timestamps like ‘30 mars 23:45’. Locale-dependent timestamps typically consume more space than ‘iso’ timestamps and are harder for programs to parse because locale conventions vary so widely, but they are easier for many people to read. The ‘LC_TIME’ locale category specifies the timestamp format. The default POSIX locale uses timestamps like ‘Mar 30 2020’ and ‘Mar 30 23:45’; in this locale, the following two ‘ls’ invocations are equivalent: newline=' ' ls -l --time-style="+%b %e %Y$newline%b %e %H:%M" ls -l --time-style="locale" Other locales behave differently. For example, in a German locale, ‘--time-style="locale"’ might be equivalent to ‘--time-style="+%e. %b %Y $newline%e. %b %H:%M"’ and might generate timestamps like ‘30. Mär 2020 ’ and ‘30. Mär 23:45’. ‘posix-STYLE’ List POSIX-locale timestamps if the ‘LC_TIME’ locale category is POSIX, STYLE timestamps otherwise. For example, the ‘posix-long-iso’ style lists timestamps like ‘Mar 30 2020’ and ‘Mar 30 23:45’ when in the POSIX locale, and like ‘2020-03-30 23:45’ otherwise. You can specify the default value of the ‘--time-style’ option with the environment variable ‘TIME_STYLE’; if ‘TIME_STYLE’ is not set the default style is ‘locale’. GNU Emacs 21.3 and later use the ‘--dired’ option and therefore can parse any date format, but if you are using Emacs 21.1 or 21.2 and specify a non-POSIX locale you may need to set ‘TIME_STYLE="posix-long-iso"’. To avoid certain denial-of-service attacks, timestamps that would be longer than 1000 bytes may be treated as errors. 10.1.6 Formatting the file names -------------------------------- These options change how file names themselves are printed. ‘-b’ ‘--escape’ ‘--quoting-style=escape’ Quote nongraphic characters in file names using alphabetic and octal backslash sequences like those used in C. ‘-N’ ‘--literal’ ‘--quoting-style=literal’ Do not quote file names. However, with ‘ls’ nongraphic characters are still printed as question marks if the output is a terminal and you do not specify the ‘--show-control-chars’ option. ‘-q’ ‘--hide-control-chars’ Print question marks instead of nongraphic characters in file names. This is the default if the output is a terminal and the program is ‘ls’. ‘-Q’ ‘--quote-name’ ‘--quoting-style=c’ Enclose file names in double quotes and quote nongraphic characters as in C. ‘--quoting-style=WORD’ Use style WORD to quote file names and other strings that may contain arbitrary characters. The WORD should be one of the following: ‘literal’ Output strings as-is; this is the same as the ‘--literal’ (‘-N’) option. ‘shell’ Quote strings for the shell if they contain shell metacharacters or would cause ambiguous output. The quoting is suitable for POSIX-compatible shells like ‘bash’, but it does not always work for incompatible shells like ‘csh’. ‘shell-always’ Quote strings for the shell, even if they would normally not require quoting. ‘shell-escape’ Like ‘shell’, but also quoting non-printable characters using the POSIX proposed ‘$''’ syntax suitable for most shells. ‘shell-escape-always’ Like ‘shell-escape’, but quote strings even if they would normally not require quoting. ‘c’ Quote strings as for C character string literals, including the surrounding double-quote characters; this is the same as the ‘--quote-name’ (‘-Q’) option. ‘escape’ Quote strings as for C character string literals, except omit the surrounding double-quote characters; this is the same as the ‘--escape’ (‘-b’) option. ‘clocale’ Quote strings as for C character string literals, except use surrounding quotation marks appropriate for the locale. ‘locale’ Quote strings as for C character string literals, except use surrounding quotation marks appropriate for the locale, and quote 'like this' instead of "like this" in the default C locale. This looks nicer on many displays. You can specify the default value of the ‘--quoting-style’ option with the environment variable ‘QUOTING_STYLE’. If that environment variable is not set, the default value is ‘shell-escape’ when the output is a terminal, and ‘literal’ otherwise. ‘--show-control-chars’ Print nongraphic characters as-is in file names. This is the default unless the output is a terminal and the program is ‘ls’. 10.2 ‘dir’: Briefly list directory contents =========================================== ‘dir’ is equivalent to ‘ls -C -b’; that is, by default files are listed in columns, sorted vertically, and special characters are represented by backslash escape sequences. *Note ‘ls’: ls invocation. 10.3 ‘vdir’: Verbosely list directory contents ============================================== ‘vdir’ is equivalent to ‘ls -l -b’; that is, by default files are listed in long format and special characters are represented by backslash escape sequences. *Note ‘ls’: ls invocation. 10.4 ‘dircolors’: Color setup for ‘ls’ ====================================== ‘dircolors’ outputs a sequence of shell commands to set up the terminal for color output from ‘ls’ (and ‘dir’, etc.). Typical usage: eval "$(dircolors [OPTION]... [FILE])" If FILE is specified, ‘dircolors’ reads it to determine which colors to use for which file types and extensions. Otherwise, a precompiled database is used. For details on the format of these files, run ‘dircolors --print-database’. To make ‘dircolors’ read a ‘~/.dircolors’ file if it exists, you can put the following lines in your ‘~/.bashrc’ (or adapt them to your favorite shell): d=.dircolors test -r $d && eval "$(dircolors $d)" The output is a shell command to set the ‘LS_COLORS’ environment variable. You can specify the shell syntax to use on the command line, or ‘dircolors’ will guess it from the value of the ‘SHELL’ environment variable. The program accepts the following options. Also see *note Common options::. ‘-b’ ‘--sh’ ‘--bourne-shell’ Output Bourne shell commands. This is the default if the ‘SHELL’ environment variable is set and does not end with ‘csh’ or ‘tcsh’. ‘-c’ ‘--csh’ ‘--c-shell’ Output C shell commands. This is the default if ‘SHELL’ ends with ‘csh’ or ‘tcsh’. ‘-p’ ‘--print-database’ Print the (compiled-in) default color configuration database. This output is itself a valid configuration file, and is fairly descriptive of the possibilities. ‘--print-ls-colors’ Print the LS_COLORS entries on separate lines, each colored as per the color they represent. An exit status of zero indicates success, and a nonzero value indicates failure. 11 Basic operations ******************* This chapter describes the commands for basic file manipulation: copying, moving (renaming), and deleting (removing). 11.1 ‘cp’: Copy files and directories ===================================== ‘cp’ copies files (or, optionally, directories). The copy is completely independent of the original. You can either copy one file to another, or copy arbitrarily many files to a destination directory. Synopses: cp [OPTION]... [-T] SOURCE DEST cp [OPTION]... SOURCE... DIRECTORY cp [OPTION]... -t DIRECTORY SOURCE... • If two file names are given, ‘cp’ copies the first file to the second. • If the ‘--target-directory’ (‘-t’) option is given, or failing that if the last file is a directory and the ‘--no-target-directory’ (‘-T’) option is not given, ‘cp’ copies each SOURCE file to the specified directory, using the SOURCEs’ names. Generally, files are written just as they are read. For exceptions, see the ‘--sparse’ option below. By default, ‘cp’ does not copy directories. However, the ‘-R’, ‘-a’, and ‘-r’ options cause ‘cp’ to copy recursively by descending into source directories and copying files to corresponding destination directories. When copying from a symbolic link, ‘cp’ normally follows the link only when not copying recursively or when ‘--link’ (‘-l’) is used. This default can be overridden with the ‘--archive’ (‘-a’), ‘-d’, ‘--dereference’ (‘-L’), ‘--no-dereference’ (‘-P’), and ‘-H’ options. If more than one of these options is specified, the last one silently overrides the others. When copying to a symbolic link, ‘cp’ follows the link only when it refers to an existing regular file. However, when copying to a dangling symbolic link, ‘cp’ refuses by default, and fails with a diagnostic, since the operation is inherently dangerous. This behavior is contrary to historical practice and to POSIX. Set ‘POSIXLY_CORRECT’ to make ‘cp’ attempt to create the target of a dangling destination symlink, in spite of the possible risk. Also, when an option like ‘--backup’ or ‘--link’ acts to rename or remove the destination before copying, ‘cp’ renames or removes the symbolic link rather than the file it points to. By default, ‘cp’ copies the contents of special files only when not copying recursively. This default can be overridden with the ‘--copy-contents’ option. ‘cp’ generally refuses to copy a file onto itself, with the following exception: if ‘--force --backup’ is specified with SOURCE and DEST identical, and referring to a regular file, ‘cp’ will make a backup file, either regular or numbered, as specified in the usual ways (*note Backup options::). This is useful when you simply want to make a backup of an existing file before changing it. The program accepts the following options. Also see *note Common options::. ‘-a’ ‘--archive’ Preserve as much as possible of the structure and attributes of the original files in the copy (but do not attempt to preserve internal directory structure; i.e., ‘ls -U’ may list the entries in a copied directory in a different order). Try to preserve SELinux security context and extended attributes (xattr), but ignore any failure to do that and print no corresponding diagnostic. Equivalent to ‘-dR --preserve=all’ with the reduced diagnostics. ‘--attributes-only’ Copy only the specified attributes of the source file to the destination. If the destination already exists, do not alter its contents. See the ‘--preserve’ option for controlling which attributes to copy. ‘-b’ ‘--backup[=METHOD]’ *Note Backup options::. Make a backup of each file that would otherwise be overwritten or removed. As a special case, ‘cp’ makes a backup of SOURCE when the force and backup options are given and SOURCE and DEST are the same name for an existing, regular file. One useful application of this combination of options is this tiny Bourne shell script: #!/bin/sh # Usage: backup FILE... # Create a GNU-style backup of each listed FILE. fail=0 for i; do cp --backup --force --preserve=all -- "$i" "$i" || fail=1 done exit $fail ‘--copy-contents’ If copying recursively, copy the contents of any special files (e.g., FIFOs and device files) as if they were regular files. This means trying to read the data in each source file and writing it to the destination. It is usually a mistake to use this option, as it normally has undesirable effects on special files like FIFOs and the ones typically found in the ‘/dev’ directory. In most cases, ‘cp -R --copy-contents’ will hang indefinitely trying to read from FIFOs and special files like ‘/dev/console’, and it will fill up your destination file system if you use it to copy ‘/dev/zero’. This option has no effect unless copying recursively, and it does not affect the copying of symbolic links. ‘-d’ Copy symbolic links as symbolic links rather than copying the files that they point to, and preserve hard links between source files in the copies. Equivalent to ‘--no-dereference --preserve=links’. ‘--debug’ Print extra information to stdout, explaining how files are copied. This option implies the ‘--verbose’ option. ‘-f’ ‘--force’ When copying without this option and an existing destination file cannot be opened for writing, the copy fails. However, with ‘--force’, when a destination file cannot be opened, ‘cp’ then tries to recreate the file by first removing it. Note ‘--force’ alone will not remove dangling symlinks. When this option is combined with ‘--link’ (‘-l’) or ‘--symbolic-link’ (‘-s’), the destination link is replaced, and unless ‘--backup’ (‘-b’) is also given there is no brief moment when the destination does not exist. Also see the description of ‘--remove-destination’. This option is independent of the ‘--interactive’ or ‘-i’ option: neither cancels the effect of the other. This option is ignored when the ‘--no-clobber’ or ‘-n’ option is also used. ‘-H’ If a command line argument specifies a symbolic link, then copy the file it points to rather than the symbolic link itself. However, copy (preserving its nature) any symbolic link that is encountered via recursive traversal. ‘-i’ ‘--interactive’ When copying a file other than a directory, prompt whether to overwrite an existing destination file, and fail if the response is not affirmative. The ‘-i’ option overrides a previous ‘-n’ option. ‘-l’ ‘--link’ Make hard links instead of copies of non-directories. ‘-L’ ‘--dereference’ Follow symbolic links when copying from them. With this option, ‘cp’ cannot create a symbolic link. For example, a symlink (to regular file) in the source tree will be copied to a regular file in the destination tree. ‘-n’ ‘--no-clobber’ Do not overwrite an existing file; silently fail instead. This option overrides a previous ‘-i’ option. This option is mutually exclusive with ‘-b’ or ‘--backup’ option. ‘-P’ ‘--no-dereference’ Copy symbolic links as symbolic links rather than copying the files that they point to. This option affects only symbolic links in the source; symbolic links in the destination are always followed if possible. ‘-p’ ‘--preserve[=ATTRIBUTE_LIST]’ Preserve the specified attributes of the original files. If specified, the ATTRIBUTE_LIST must be a comma-separated list of one or more of the following strings: ‘mode’ Preserve attributes relevant to access permissions, including file mode bits and (if possible) access control lists (ACLs). ACL preservation is system-dependent, and ACLs are not necessarily translated when the source and destination are on file systems with different ACL formats (e.g., NFSv4 versus POSIX formats). ‘ownership’ Preserve the owner and group. On most modern systems, only users with appropriate privileges may change the owner of a file, and ordinary users may preserve the group ownership of a file only if they happen to be a member of the desired group. ‘timestamps’ Preserve the times of last access and last modification, when possible. On older systems, it is not possible to preserve these attributes when the affected file is a symbolic link. However, many systems now provide the ‘utimensat’ function, which makes it possible even for symbolic links. ‘links’ Preserve in the destination files any links between corresponding source files. Note that with ‘-L’ or ‘-H’, this option can convert symbolic links to hard links. For example, $ mkdir c; : > a; ln -s a b; cp -aH a b c; ls -i1 c 74161745 a 74161745 b Note the inputs: ‘b’ is a symlink to regular file ‘a’, yet the files in destination directory, ‘c/’, are hard-linked. Since ‘-a’ implies ‘--no-dereference’ it would copy the symlink, but the later ‘-H’ tells ‘cp’ to dereference the command line arguments where it then sees two files with the same inode number. Then the ‘--preserve=links’ option also implied by ‘-a’ will preserve the perceived hard link. Here is a similar example that exercises ‘cp’’s ‘-L’ option: $ mkdir b c; (cd b; : > a; ln -s a b); cp -aL b c; ls -i1 c/b 74163295 a 74163295 b ‘context’ Preserve SELinux security context of the file, or fail with full diagnostics. ‘xattr’ Preserve extended attributes of the file, or fail with full diagnostics. If ‘cp’ is built without xattr support, ignore this option. If SELinux context, ACLs or Capabilities are implemented using xattrs, they are preserved implicitly by this option as well, i.e., even without specifying ‘--preserve=mode’ or ‘--preserve=context’. ‘all’ Preserve all file attributes. Equivalent to specifying all of the above, but with the difference that failure to preserve SELinux security context or extended attributes does not change ‘cp’’s exit status. In contrast to ‘-a’, all but ‘Operation not supported’ warnings are output. Using ‘--preserve’ with no ATTRIBUTE_LIST is equivalent to ‘--preserve=mode,ownership,timestamps’. In the absence of this option, the permissions of existing destination files are unchanged. Each new file is created with the mode of the corresponding source file minus the set-user-ID, set-group-ID, and sticky bits as the create mode; the operating system then applies either the umask or a default ACL, possibly resulting in a more restrictive file mode. *Note File permissions::. ‘--no-preserve=ATTRIBUTE_LIST’ Do not preserve the specified attributes. The ATTRIBUTE_LIST has the same form as for ‘--preserve’. ‘--parents’ Form the name of each destination file by appending to the target directory a slash and the specified name of the source file. The last argument given to ‘cp’ must be the name of an existing directory. For example, the command: cp --parents a/b/c existing_dir copies the file ‘a/b/c’ to ‘existing_dir/a/b/c’, creating any missing intermediate directories. ‘-R’ ‘-r’ ‘--recursive’ Copy directories recursively. By default, do not follow symbolic links in the source unless used together with the ‘--link’ (‘-l’) option; see the ‘--archive’ (‘-a’), ‘-d’, ‘--dereference’ (‘-L’), ‘--no-dereference’ (‘-P’), and ‘-H’ options. Special files are copied by creating a destination file of the same type as the source; see the ‘--copy-contents’ option. It is not portable to use ‘-r’ to copy symbolic links or special files. On some non-GNU systems, ‘-r’ implies the equivalent of ‘-L’ and ‘--copy-contents’ for historical reasons. Also, it is not portable to use ‘-R’ to copy symbolic links unless you also specify ‘-P’, as POSIX allows implementations that dereference symbolic links by default. ‘--reflink[=WHEN]’ Perform a lightweight, copy-on-write (COW) copy, if supported by the file system. Once it has succeeded, beware that the source and destination files share the same data blocks as long as they remain unmodified. Thus, if an I/O error affects data blocks of one of the files, the other suffers the same fate. The WHEN value can be one of the following: ‘always’ If the copy-on-write operation is not supported then report the failure for each file and exit with a failure status. Plain ‘--reflink’ is equivalent to ‘--reflink=always’. ‘auto’ If the copy-on-write operation is not supported then fall back to the standard copy behavior. This is the default if no ‘--reflink’ option is given. ‘never’ Disable copy-on-write operation and use the standard copy behavior. This option is overridden by the ‘--link’, ‘--symbolic-link’ and ‘--attributes-only’ options, thus allowing it to be used to configure the default data copying behavior for ‘cp’. ‘--remove-destination’ Remove each existing destination file before attempting to open it (contrast with ‘-f’ above). ‘--sparse=WHEN’ A “sparse file” contains “holes” – a sequence of zero bytes that does not occupy any file system blocks; the ‘read’ system call reads these as zeros. This can both save considerable space and increase speed, since many binary files contain lots of consecutive zero bytes. By default, ‘cp’ detects holes in input source files via a crude heuristic and makes the corresponding output file sparse as well. Only regular files may be sparse. The WHEN value can be one of the following: ‘auto’ The default behavior: if the input file is sparse, attempt to make the output file sparse, too. However, if an output file exists but refers to a non-regular file, then do not attempt to make it sparse. ‘always’ For each sufficiently long sequence of zero bytes in the input file, attempt to create a corresponding hole in the output file, even if the input file does not appear to be sparse. This is useful when the input file resides on a file system that does not support sparse files (for example, ‘efs’ file systems in SGI IRIX 5.3 and earlier), but the output file is on a type of file system that does support them. Holes may be created only in regular files, so if the destination file is of some other type, ‘cp’ does not even try to make it sparse. ‘never’ Never make the output file sparse. This is useful in creating a file for use with the ‘mkswap’ command, since such a file must not have any holes. For example, with the following alias, ‘cp’ will use the minimum amount of space supported by the file system. (Older versions of ‘cp’ can also benefit from ‘--reflink=auto’ here.) alias cp='cp --sparse=always' ‘--strip-trailing-slashes’ Remove any trailing slashes from each SOURCE argument. *Note Trailing slashes::. ‘-s’ ‘--symbolic-link’ Make symbolic links instead of copies of non-directories. All source file names must be absolute (starting with ‘/’) unless the destination files are in the current directory. This option merely results in an error message on systems that do not support symbolic links. ‘-S SUFFIX’ ‘--suffix=SUFFIX’ Append SUFFIX to each backup file made with ‘-b’. *Note Backup options::. ‘-t DIRECTORY’ ‘--target-directory=DIRECTORY’ Specify the destination DIRECTORY. *Note Target directory::. ‘-T’ ‘--no-target-directory’ Do not treat the last operand specially when it is a directory or a symbolic link to a directory. *Note Target directory::. ‘-u’ ‘--update[=WHICH]’ Do not copy a non-directory that has an existing destination with the same or newer modification timestamp; instead, silently skip the file without failing. If timestamps are being preserved, the comparison is to the source timestamp truncated to the resolutions of the destination file system and of the system calls used to update timestamps; this avoids duplicate work if several ‘cp -pu’ commands are executed with the same source and destination. This option is ignored if the ‘-n’ or ‘--no-clobber’ option is also specified. Also, if ‘--preserve=links’ is also specified (like with ‘cp -au’ for example), that will take precedence; consequently, depending on the order that files are processed from the source, newer files in the destination may be replaced, to mirror hard links in the source. WHICH gives more control over which existing files in the destination are replaced, and its value can be one of the following: ‘all’ This is the default operation when an ‘--update’ option is not specified, and results in all existing files in the destination being replaced. ‘none’ This is similar to the ‘--no-clobber’ option, in that no files in the destination are replaced, but also skipping a file does not induce a failure. ‘older’ This is the default operation when ‘--update’ is specified, and results in files being replaced if they’re older than the corresponding source file. ‘-v’ ‘--verbose’ Print the name of each file before copying it. ‘-x’ ‘--one-file-system’ Skip subdirectories that are on different file systems from the one that the copy started on. However, mount point directories _are_ copied. ‘-Z’ ‘--context[=CONTEXT]’ Without a specified CONTEXT, adjust the SELinux security context according to the system default type for destination files, similarly to the ‘restorecon’ command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued. This option is mutually exclusive with the ‘--preserve=context’ option, and overrides the ‘--preserve=all’ and ‘-a’ options. An exit status of zero indicates success, and a nonzero value indicates failure. 11.2 ‘dd’: Convert and copy a file ================================== ‘dd’ copies input to output with a changeable I/O block size, while optionally performing conversions on the data. Synopses: dd [OPERAND]... dd OPTION The only options are ‘--help’ and ‘--version’. *Note Common options::. By default, ‘dd’ copies standard input to standard output. To copy, ‘dd’ repeatedly does the following steps in order: 1. Read an input block. 2. If converting via ‘sync’, pad as needed to meet the input block size. Pad with spaces if converting via ‘block’ or ‘unblock’, NUL bytes otherwise. 3. If ‘bs=’ is given and no conversion mentioned in steps (4) or (5) is given, output the data as a single block and skip all remaining steps. 4. If the ‘swab’ conversion is given, swap each pair of input bytes. If the input data length is odd, preserve the last input byte (since there is nothing to swap it with). 5. If any of the conversions ‘swab’, ‘block’, ‘unblock’, ‘lcase’, ‘ucase’, ‘ascii’, ‘ebcdic’ and ‘ibm’ are given, do these conversions. These conversions operate independently of input blocking, and might deal with records that span block boundaries. 6. Aggregate the resulting data into output blocks of the specified size, and output each output block in turn. Do not pad the last output block; it can be shorter than usual. ‘dd’ accepts the following operands, whose syntax was inspired by the DD (data definition) statement of OS/360 JCL. ‘if=FILE’ Read from FILE instead of standard input. ‘of=FILE’ Write to FILE instead of standard output. Unless ‘conv=notrunc’ is given, truncate FILE before writing it. ‘ibs=BYTES’ Set the input block size to BYTES. This makes ‘dd’ read BYTES per block. The default is 512 bytes. ‘obs=BYTES’ Set the output block size to BYTES. This makes ‘dd’ write BYTES per block. The default is 512 bytes. ‘bs=BYTES’ Set both input and output block sizes to BYTES. This makes ‘dd’ read and write BYTES per block, overriding any ‘ibs’ and ‘obs’ settings. In addition, if no data-transforming ‘conv’ operand is specified, input is copied to the output as soon as it’s read, even if it is smaller than the block size. ‘cbs=BYTES’ Set the conversion block size to BYTES. When converting variable-length records to fixed-length ones (‘conv=block’) or the reverse (‘conv=unblock’), use BYTES as the fixed record length. ‘skip=N’ ‘iseek=N’ Skip N ‘ibs’-byte blocks in the input file before copying. If N ends in the letter ‘B’, interpret N as a byte count rather than a block count. (‘B’ and the ‘iseek=’ spelling are GNU extensions to POSIX.) ‘seek=N’ ‘oseek=N’ Skip N ‘obs’-byte blocks in the output file before truncating or copying. If N ends in the letter ‘B’, interpret N as a byte count rather than a block count. (‘B’ and the ‘oseek=’ spelling are GNU extensions to POSIX.) ‘count=N’ Copy N ‘ibs’-byte blocks from the input file, instead of everything until the end of the file. If N ends in the letter ‘B’, interpret N as a byte count rather than a block count; this is a GNU extension to POSIX. If short reads occur, as could be the case when reading from a pipe for example, ‘iflag=fullblock’ ensures that ‘count=’ counts complete input blocks rather than input read operations. As an extension to POSIX, ‘count=0’ copies zero blocks instead of copying all blocks. ‘status=LEVEL’ Specify the amount of information printed. If this operand is given multiple times, the last one takes precedence. The LEVEL value can be one of the following: ‘none’ Do not print any informational or warning messages to standard error. Error messages are output as normal. ‘noxfer’ Do not print the final transfer rate and volume statistics that normally make up the last status line. ‘progress’ Print the transfer rate and volume statistics on standard error, when processing each input block. Statistics are output on a single line at most once every second, but updates can be delayed when waiting on I/O. Transfer information is normally output to standard error upon receipt of the ‘INFO’ signal or when ‘dd’ exits, and defaults to the following form in the C locale: 7287+1 records in 116608+0 records out 59703296 bytes (60 MB, 57 MiB) copied, 0.0427974 s, 1.4 GB/s The notation ‘W+P’ stands for W whole blocks and P partial blocks. A partial block occurs when a read or write operation succeeds but transfers less data than the block size. An additional line like ‘1 truncated record’ or ‘10 truncated records’ is output after the ‘records out’ line if ‘conv=block’ processing truncated one or more input records. The ‘status=’ operand is a GNU extension to POSIX. ‘conv=CONVERSION[,CONVERSION]...’ Convert the file as specified by the CONVERSION argument(s). (No spaces around any comma(s).) Conversions: ‘ascii’ Convert EBCDIC to ASCII, using the conversion table specified by POSIX. This provides a 1:1 translation for all 256 bytes. This implies ‘conv=unblock’; input is converted to ASCII before trailing spaces are deleted. ‘ebcdic’ Convert ASCII to EBCDIC. This is the inverse of the ‘ascii’ conversion. This implies ‘conv=block’; trailing spaces are added before being converted to EBCDIC. ‘ibm’ This acts like ‘conv=ebcdic’, except it uses the alternate conversion table specified by POSIX. This is not a 1:1 translation, but reflects common historical practice for ‘~’, ‘[’, and ‘]’. The ‘ascii’, ‘ebcdic’, and ‘ibm’ conversions are mutually exclusive. If you use any of these conversions, you should also use the ‘cbs=’ operand. ‘block’ For each line in the input, output ‘cbs’ bytes, replacing the input newline with a space and truncating or padding input lines with spaces as necessary. ‘unblock’ Remove any trailing spaces in each ‘cbs’-sized input block, and append a newline. The ‘block’ and ‘unblock’ conversions are mutually exclusive. If you use either of these conversions, you should also use the ‘cbs=’ operand. ‘lcase’ Change uppercase letters to lowercase. ‘ucase’ Change lowercase letters to uppercase. The ‘lcase’ and ‘ucase’ conversions are mutually exclusive. ‘sparse’ Try to seek rather than write NUL output blocks. On a file system that supports sparse files, this will create sparse output when extending the output file. Be careful when using this conversion in conjunction with ‘conv=notrunc’ or ‘oflag=append’. With ‘conv=notrunc’, existing data in the output file corresponding to NUL blocks from the input, will be untouched. With ‘oflag=append’ the seeks performed will be ineffective. Similarly, when the output is a device rather than a file, NUL input blocks are not copied, and therefore this conversion is most useful with virtual or pre zeroed devices. The ‘sparse’ conversion is a GNU extension to POSIX. ‘swab’ Swap every pair of input bytes. ‘sync’ Pad every input block to size of ‘ibs’ with trailing zero bytes. When used with ‘block’ or ‘unblock’, pad with spaces instead of zero bytes. The following “conversions” are really file flags and don’t affect internal processing: ‘excl’ Fail if the output file already exists; ‘dd’ must create the output file itself. ‘nocreat’ Do not create the output file; the output file must already exist. The ‘excl’ and ‘nocreat’ conversions are mutually exclusive, and are GNU extensions to POSIX. ‘notrunc’ Do not truncate the output file. ‘noerror’ Continue after read errors. ‘fdatasync’ Synchronize output data just before finishing, even if there were write errors. This forces a physical write of output data, so that even if power is lost the output data will be preserved. If neither this nor ‘fsync’ are specified, output is treated as usual with file systems, i.e., output data and metadata may be cached in primary memory for some time before the operating system physically writes it, and thus output data and metadata may be lost if power is lost. *Note sync invocation::. This conversion is a GNU extension to POSIX. ‘fsync’ Synchronize output data and metadata just before finishing, even if there were write errors. This acts like ‘fdatasync’ except it also preserves output metadata, such as the last-modified time of the output file; for this reason it may be a bit slower than ‘fdatasync’ although the performance difference is typically insignificant for ‘dd’. This conversion is a GNU extension to POSIX. ‘iflag=FLAG[,FLAG]...’ Access the input file using the flags specified by the FLAG argument(s). (No spaces around any comma(s).) ‘oflag=FLAG[,FLAG]...’ Access the output file using the flags specified by the FLAG argument(s). (No spaces around any comma(s).) Here are the flags. ‘append’ Write in append mode, so that even if some other process is writing to this file, every ‘dd’ write will append to the current contents of the file. This flag makes sense only for output. If you combine this flag with the ‘of=FILE’ operand, you should also specify ‘conv=notrunc’ unless you want the output file to be truncated before being appended to. ‘cio’ Use concurrent I/O mode for data. This mode performs direct I/O and drops the POSIX requirement to serialize all I/O to the same file. A file cannot be opened in CIO mode and with a standard open at the same time. ‘direct’ Use direct I/O for data, avoiding the buffer cache. Note that the kernel may impose restrictions on read or write buffer sizes. For example, with an ext4 destination file system and a Linux-based kernel, using ‘oflag=direct’ will cause writes to fail with ‘EINVAL’ if the output buffer size is not a multiple of 512. ‘directory’ Fail unless the file is a directory. Most operating systems do not allow I/O to a directory, so this flag has limited utility. ‘dsync’ Use synchronized I/O for data. For the output file, this forces a physical write of output data on each write. For the input file, this flag can matter when reading from a remote file that has been written to synchronously by some other process. Metadata (e.g., last-access and last-modified time) is not necessarily synchronized. ‘sync’ Use synchronized I/O for both data and metadata. ‘nocache’ Request to discard the system data cache for a file. When count=0 all cached data for the file is specified, otherwise the cache is dropped for the processed portion of the file. Also when count=0, failure to discard the cache is diagnosed and reflected in the exit status. Note data that is not already persisted to storage will not be discarded from cache, so note the use of the ‘sync’ conversions in the examples below, which are used to maximize the effectiveness of the ‘nocache’ flag. Here are some usage examples: # Advise to drop cache for whole file dd if=ifile iflag=nocache count=0 # Ensure drop cache for the whole file dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0 # Advise to drop cache for part of file # Note the kernel will only consider complete and # already persisted pages. dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null # Stream data using just the read-ahead cache. # See also the ‘direct’ flag. dd if=ifile of=ofile iflag=nocache oflag=nocache,sync ‘nonblock’ Use non-blocking I/O. ‘noatime’ Do not update the file’s access timestamp. *Note File timestamps::. Some older file systems silently ignore this flag, so it is a good idea to test it on your files before relying on it. ‘noctty’ Do not assign the file to be a controlling terminal for ‘dd’. This has no effect when the file is not a terminal. On many hosts (e.g., GNU/Linux hosts), this flag has no effect at all. ‘nofollow’ Do not follow symbolic links. ‘nolinks’ Fail if the file has multiple hard links. ‘binary’ Use binary I/O. This flag has an effect only on nonstandard platforms that distinguish binary from text I/O. ‘text’ Use text I/O. Like ‘binary’, this flag has no effect on standard platforms. ‘fullblock’ Accumulate full blocks from input. The ‘read’ system call may return early if a full block is not available. When that happens, continue calling ‘read’ to fill the remainder of the block. This flag can be used only with ‘iflag’. This flag is useful with pipes for example as they may return short reads. In that case, this flag is needed to ensure that a ‘count=’ argument is interpreted as a block count rather than a count of read operations. These flags are all GNU extensions to POSIX. They are not supported on all systems, and ‘dd’ rejects attempts to use them when they are not supported. When reading from standard input or writing to standard output, the ‘nofollow’ and ‘noctty’ flags should not be specified, and the other flags (e.g., ‘nonblock’) can affect how other processes behave with the affected file descriptors, even after ‘dd’ exits. The behavior of ‘dd’ is unspecified if operands other than ‘conv=’, ‘iflag=’, ‘oflag=’, and ‘status=’ are specified more than once. The numeric-valued strings above (N and BYTES) are unsigned decimal integers that can be followed by a multiplier: ‘b’=512, ‘c’=1, ‘w’=2, ‘xM’=M, or any of the standard block size suffixes like ‘k’=1024 (*note Block size::). These multipliers are GNU extensions to POSIX, except that POSIX allows BYTES to be followed by ‘k’, ‘b’, and ‘xM’. Note ‘xM’ can be used more than once in a number. Block sizes (i.e., specified by BYTES strings) must be nonzero. Any block size you specify via ‘bs=’, ‘ibs=’, ‘obs=’, ‘cbs=’ should not be too large – values larger than a few megabytes are generally wasteful or (as in the gigabyte..exabyte case) downright counterproductive or error-inducing. To process data with offset or size that is not a multiple of the I/O block size, you can use a numeric string N that ends in the letter ‘B’. For example, the following shell commands copy data in 1 MiB blocks between a flash drive and a tape, but do not save or restore a 512-byte area at the start of the flash drive: flash=/dev/sda tape=/dev/st0 # Copy all but the initial 512 bytes from flash to tape. dd if=$flash iseek=512B bs=1MiB of=$tape # Copy from tape back to flash, leaving initial 512 bytes alone. dd if=$tape bs=1MiB of=$flash oseek=512B For failing storage devices, other tools come with a great variety of extra functionality to ease the saving of as much data as possible before the device finally dies, e.g. GNU ‘ddrescue’ (https://www.gnu.org/software/ddrescue/). However, in some cases such a tool is not available or the administrator feels more comfortable with the handling of ‘dd’. As a simple rescue method, call ‘dd’ as shown in the following example: the operand ‘conv=noerror,sync’ is used to continue after read errors and to pad out bad reads with NULs, while ‘iflag=fullblock’ caters for short reads (which traditionally never occur on flash or similar devices): # Rescue data from an (unmounted!) partition of a failing device. dd conv=noerror,sync iflag=fullblock /mnt/rescue.img Sending an ‘INFO’ signal (or ‘USR1’ signal where that is unavailable) to a running ‘dd’ process makes it print I/O statistics to standard error and then resume copying. In the example below, ‘dd’ is run in the background to copy 5GB of data. The ‘kill’ command makes it output intermediate I/O statistics, and when ‘dd’ completes normally or is killed by the ‘SIGINT’ signal, it outputs the final statistics. # Ignore the signal so we never inadvertently terminate the dd child. # Note this is not needed when SIGINFO is available. trap '' USR1 # Run dd with the fullblock iflag to avoid short reads # which can be triggered by reception of signals. dd iflag=fullblock if=/dev/zero of=/dev/null count=5000000 bs=1000 & pid=$! # Output stats every second. while kill -s USR1 $pid 2>/dev/null; do sleep 1; done The above script will output in the following format: 3441325+0 records in 3441325+0 records out 3441325000 bytes (3.4 GB, 3.2 GiB) copied, 1.00036 s, 3.4 GB/s 5000000+0 records in 5000000+0 records out 5000000000 bytes (5.0 GB, 4.7 GiB) copied, 1.44433 s, 3.5 GB/s The ‘status=progress’ operand periodically updates the last line of the transfer statistics above. On systems lacking the ‘INFO’ signal ‘dd’ responds to the ‘USR1’ signal instead, unless the ‘POSIXLY_CORRECT’ environment variable is set. An exit status of zero indicates success, and a nonzero value indicates failure. 11.3 ‘install’: Copy files and set attributes ============================================= ‘install’ copies files while setting their file mode bits and, if possible, their owner and group. Synopses: install [OPTION]... [-T] SOURCE DEST install [OPTION]... SOURCE... DIRECTORY install [OPTION]... -t DIRECTORY SOURCE... install [OPTION]... -d DIRECTORY... • If two file names are given, ‘install’ copies the first file to the second. • If the ‘--target-directory’ (‘-t’) option is given, or failing that if the last file is a directory and the ‘--no-target-directory’ (‘-T’) option is not given, ‘install’ copies each SOURCE file to the specified directory, using the SOURCEs’ names. • If the ‘--directory’ (‘-d’) option is given, ‘install’ creates each DIRECTORY and any missing parent directories. Parent directories are created with mode ‘u=rwx,go=rx’ (755), regardless of the ‘-m’ option or the current umask. *Note Directory Setuid and Setgid::, for how the set-user-ID and set-group-ID bits of parent directories are inherited. ‘install’ is similar to ‘cp’, but allows you to control the attributes of destination files. It is typically used in Makefiles to copy programs into their destination directories. It refuses to copy files onto themselves. ‘install’ never preserves extended attributes (xattr). The program accepts the following options. Also see *note Common options::. ‘-b’ ‘--backup[=METHOD]’ *Note Backup options::. Make a backup of each file that would otherwise be overwritten or removed. ‘-C’ ‘--compare’ Compare content of source and destination files, and if there would be no change to the destination content, owner, group, permissions, and possibly SELinux context, then do not modify the destination at all. Note this option is best used in conjunction with ‘--user’, ‘--group’ and ‘--mode’ options, lest ‘install’ incorrectly determines the default attributes that installed files would have (as it doesn’t consider setgid directories and POSIX default ACLs for example). This could result in redundant copies or attributes that are not reset to the correct defaults. ‘-c’ Ignored; for compatibility with old Unix versions of ‘install’. ‘-D’ Create any missing parent directories of DEST, then copy SOURCE to DEST. Explicitly specifying the ‘--target-directory=DIR’ will similarly ensure the presence of that hierarchy before copying SOURCE arguments. ‘-d’ ‘--directory’ Create any missing parent directories, giving them the default attributes. Then create each given directory, setting their owner, group and mode as given on the command line or to the defaults. ‘--debug’ Print extra information to stdout, explaining how files are copied. This option implies the ‘--verbose’ option. ‘-g GROUP’ ‘--group=GROUP’ Set the group ownership of installed files or directories to GROUP. The default is the process’s current group. GROUP may be either a group name or a numeric group ID. ‘-m MODE’ ‘--mode=MODE’ Set the file mode bits for the installed file or directory to MODE, which can be either an octal number, or a symbolic mode as in ‘chmod’, with ‘a=’ (no access allowed to anyone) as the point of departure (*note File permissions::). The default mode is ‘u=rwx,go=rx,a-s’ – read, write, and execute for the owner, read and execute for group and other, and with set-user-ID and set-group-ID disabled. This default is not quite the same as ‘755’, since it disables instead of preserving set-user-ID and set-group-ID on directories. *Note Directory Setuid and Setgid::. ‘-o OWNER’ ‘--owner=OWNER’ If ‘install’ has appropriate privileges (is run as root), set the ownership of installed files or directories to OWNER. The default is ‘root’. OWNER may be either a user name or a numeric user ID. ‘--preserve-context’ Preserve the SELinux security context of files and directories. Failure to preserve the context in all of the files or directories will result in an exit status of 1. If SELinux is disabled then print a warning and ignore the option. ‘-p’ ‘--preserve-timestamps’ Set the time of last access and the time of last modification of each installed file to match those of each corresponding original file. When a file is installed without this option, its last access and last modification timestamps are both set to the time of installation. This option is useful if you want to use the last modification timestamps of installed files to keep track of when they were last built as opposed to when they were last installed. ‘-s’ ‘--strip’ Strip the symbol tables from installed binary executables. ‘--strip-program=PROGRAM’ Program used to strip binaries. ‘-S SUFFIX’ ‘--suffix=SUFFIX’ Append SUFFIX to each backup file made with ‘-b’. *Note Backup options::. ‘-t DIRECTORY’ ‘--target-directory=DIRECTORY’ Specify the destination DIRECTORY. *Note Target directory::. Also specifying the ‘-D’ option will ensure the directory is present. ‘-T’ ‘--no-target-directory’ Do not treat the last operand specially when it is a directory or a symbolic link to a directory. *Note Target directory::. ‘-v’ ‘--verbose’ Print the name of each file before copying it. ‘-Z’ ‘--context[=CONTEXT]’ Without a specified CONTEXT, adjust the SELinux security context according to the system default type for destination files, similarly to the ‘restorecon’ command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued. This option is mutually exclusive with the ‘--preserve-context’ option. An exit status of zero indicates success, and a nonzero value indicates failure. 11.4 ‘mv’: Move (rename) files ============================== ‘mv’ moves or renames files (or directories). Synopses: mv [OPTION]... [-T] SOURCE DEST mv [OPTION]... SOURCE... DIRECTORY mv [OPTION]... -t DIRECTORY SOURCE... • If two file names are given, ‘mv’ moves the first file to the second. • If the ‘--target-directory’ (‘-t’) option is given, or failing that if the last file is a directory and the ‘--no-target-directory’ (‘-T’) option is not given, ‘mv’ moves each SOURCE file to the specified directory, using the SOURCEs’ names. To move a file, ‘mv’ ordinarily simply renames it. However, if renaming does not work because the destination’s file system differs, ‘mv’ falls back on copying as if by ‘cp -a’, then (assuming the copy succeeded) it removes the original. If the copy fails, then ‘mv’ removes any partially created copy in the destination. If you were to copy three directories from one file system to another and the copy of the first directory succeeded, but the second didn’t, the first would be left on the destination file system and the second and third would be left on the original file system. ‘mv’ always tries to copy extended attributes (xattr), which may include SELinux context, ACLs or Capabilities. Upon failure all but ‘Operation not supported’ warnings are output. If a destination file exists but is normally unwritable, standard input is a terminal, and the ‘-f’ or ‘--force’ option is not given, ‘mv’ prompts the user for whether to replace the file. (You might own the file, or have write permission on its directory.) If the response is not affirmative, the file is skipped. _Warning_: Avoid specifying a source name with a trailing slash, when it might be a symlink to a directory. Otherwise, ‘mv’ may do something very surprising, since its behavior depends on the underlying rename system call. On a system with a modern Linux-based kernel, it fails with ‘errno=ENOTDIR’. However, on other systems (at least FreeBSD 6.1 and Solaris 10) it silently renames not the symlink but rather the directory referenced by the symlink. *Note Trailing slashes::. _Note_: ‘mv’ will only replace empty directories in the destination. Conflicting populated directories are skipped with a diagnostic. The program accepts the following options. Also see *note Common options::. ‘-b’ ‘--backup[=METHOD]’ *Note Backup options::. Make a backup of each file that would otherwise be overwritten or removed. ‘--debug’ Print extra information to stdout, explaining how files are copied. This option implies the ‘--verbose’ option. ‘-f’ ‘--force’ Do not prompt the user before removing a destination file. If you specify more than one of the ‘-i’, ‘-f’, ‘-n’ options, only the final one takes effect. ‘-i’ ‘--interactive’ Prompt whether to overwrite each existing destination file, regardless of its permissions, and fail if the response is not affirmative. If you specify more than one of the ‘-i’, ‘-f’, ‘-n’ options, only the final one takes effect. ‘-n’ ‘--no-clobber’ Do not overwrite an existing file; silently fail instead. If you specify more than one of the ‘-i’, ‘-f’, ‘-n’ options, only the final one takes effect. This option is mutually exclusive with ‘-b’ or ‘--backup’ option. See also the ‘--update=none’ option which will skip existing files but not fail. ‘--no-copy’ If a file cannot be renamed because the destination file system differs, fail with a diagnostic instead of copying and then removing the file. ‘-u’ ‘--update’ Do not move a non-directory that has an existing destination with the same or newer modification timestamp; instead, silently skip the file without failing. If the move is across file system boundaries, the comparison is to the source timestamp truncated to the resolutions of the destination file system and of the system calls used to update timestamps; this avoids duplicate work if several ‘mv -u’ commands are executed with the same source and destination. This option is ignored if the ‘-n’ or ‘--no-clobber’ option is also specified. WHICH gives more control over which existing files in the destination are replaced, and its value can be one of the following: ‘all’ This is the default operation when an ‘--update’ option is not specified, and results in all existing files in the destination being replaced. ‘none’ This is similar to the ‘--no-clobber’ option, in that no files in the destination are replaced, but also skipping a file does not induce a failure. ‘older’ This is the default operation when ‘--update’ is specified, and results in files being replaced if they’re older than the corresponding source file. ‘-v’ ‘--verbose’ Print the name of each file before moving it. ‘--strip-trailing-slashes’ Remove any trailing slashes from each SOURCE argument. *Note Trailing slashes::. ‘-S SUFFIX’ ‘--suffix=SUFFIX’ Append SUFFIX to each backup file made with ‘-b’. *Note Backup options::. ‘-t DIRECTORY’ ‘--target-directory=DIRECTORY’ Specify the destination DIRECTORY. *Note Target directory::. ‘-T’ ‘--no-target-directory’ Do not treat the last operand specially when it is a directory or a symbolic link to a directory. *Note Target directory::. ‘-Z’ ‘--context’ This option functions similarly to the ‘restorecon’ command, by adjusting the SELinux security context according to the system default type for destination files and each created directory. An exit status of zero indicates success, and a nonzero value indicates failure. 11.5 ‘rm’: Remove files or directories ====================================== ‘rm’ removes each given FILE. By default, it does not remove directories. Synopsis: rm [OPTION]... [FILE]... If the ‘-I’ or ‘--interactive=once’ option is given, and there are more than three files or the ‘-r’, ‘-R’, or ‘--recursive’ are given, then ‘rm’ prompts the user for whether to proceed with the entire operation. If the response is not affirmative, the entire command is aborted. Otherwise, if a file is unwritable, standard input is a terminal, and the ‘-f’ or ‘--force’ option is not given, or the ‘-i’ or ‘--interactive=always’ option _is_ given, ‘rm’ prompts the user for whether to remove the file. If the response is not affirmative, the file is skipped. Any attempt to remove a file whose last file name component is ‘.’ or ‘..’ is rejected without any prompting, as mandated by POSIX. _Warning_: If you use ‘rm’ to remove a file, it is usually possible to recover the contents of that file. If you want more assurance that the contents are unrecoverable, consider using ‘shred’. The program accepts the following options. Also see *note Common options::. ‘-d’ ‘--dir’ Remove the listed directories if they are empty. ‘-f’ ‘--force’ Ignore nonexistent files and missing operands, and never prompt the user. Ignore any previous ‘--interactive’ (‘-i’) option. ‘-i’ Prompt whether to remove each file. If the response is not affirmative, silently skip the file without failing. Ignore any previous ‘--force’ (‘-f’) option. Equivalent to ‘--interactive=always’. ‘-I’ Prompt once whether to proceed with the command, if more than three files are named or if a recursive removal is requested. Ignore any previous ‘--force’ (‘-f’) option. Equivalent to ‘--interactive=once’. ‘--interactive [=WHEN]’ Specify when to issue an interactive prompt. WHEN may be omitted, or one of: • never - Do not prompt at all. • once - Prompt once if more than three files are named or if a recursive removal is requested. Equivalent to ‘-I’. • always - Prompt for every file being removed. Equivalent to ‘-i’. ‘--interactive’ with no WHEN is equivalent to ‘--interactive=always’. ‘--one-file-system’ When removing a hierarchy recursively, do not remove any directory that is on a file system different from that of the corresponding command line argument. This option is useful when removing a build “chroot” hierarchy, which normally contains no valuable data. However, it is not uncommon to bind-mount ‘/home’ into such a hierarchy, to make it easier to use one’s start-up file. The catch is that it’s easy to forget to unmount ‘/home’. Then, when you use ‘rm -rf’ to remove your normally throw-away chroot, that command will remove everything under ‘/home’, too. Use the ‘--one-file-system’ option, and it will diagnose and skip directories on other file systems. Of course, this will not save your ‘/home’ if it and your chroot happen to be on the same file system. See also ‘--preserve-root=all’ to protect command line arguments themselves. ‘--preserve-root [=all]’ Fail upon any attempt to remove the root directory, ‘/’, when used with the ‘--recursive’ option. This is the default behavior. *Note Treating / specially::. When ‘all’ is specified, reject any command line argument that is not on the same file system as its parent. ‘--no-preserve-root’ Do not treat ‘/’ specially when removing recursively. This option is not recommended unless you really want to remove all the files on your computer. *Note Treating / specially::. ‘-r’ ‘-R’ ‘--recursive’ Remove the listed directories and their contents recursively. ‘-v’ ‘--verbose’ Print the name of each file before removing it. One common question is how to remove files whose names begin with a ‘-’. GNU ‘rm’, like every program that uses the ‘getopt’ function to parse its arguments, lets you use the ‘--’ option to indicate that all following arguments are non-options. To remove a file called ‘-f’ in the current directory, you could type either: rm -- -f or: rm ./-f The Unix ‘rm’ program’s use of a single ‘-’ for this purpose predates the development of the ‘getopt’ standard syntax. An exit status of zero indicates success, and a nonzero value indicates failure. 11.6 ‘shred’: Remove files more securely ======================================== ‘shred’ overwrites devices or files, to help prevent even extensive forensics from recovering the data. Ordinarily when you remove a file (*note rm invocation::), its data and metadata are not actually destroyed. Only the file’s directory entry is removed, and the file’s storage is reclaimed only when no process has the file open and no other directory entry links to the file. And even if file’s data and metadata’s storage space is freed for further reuse, there are undelete utilities that will attempt to reconstruct the file from the data in freed storage, and that can bring the file back if the storage was not rewritten. On a busy system with a nearly-full device, space can get reused in a few seconds. But there is no way to know for sure. And although the undelete utilities and already-existing processes require insider or superuser access, you may be wary of the superuser, of processes running on your behalf, or of attackers that can physically access the storage device. So if you have sensitive data, you may want to be sure that recovery is not possible by plausible attacks like these. The best way to remove something irretrievably is to destroy the media it’s on with acid, melt it down, or the like. For cheap removable media this is often the preferred method. However, some storage devices are expensive or are harder to destroy, so the ‘shred’ utility tries to achieve a similar effect non-destructively, by overwriting the file with non-sensitive data. *Please note* that ‘shred’ relies on a crucial assumption: that the file system and hardware overwrite data in place. Although this is common and is the traditional way to do things, many modern file system designs do not satisfy this assumption. Exceptions include: • Log-structured or journaled file systems, such as ext3/ext4 (in ‘data=journal’ mode), Btrfs, NTFS, ReiserFS, XFS, ZFS, file systems supplied with AIX and Solaris, etc., when they are configured to journal data. • File systems that write redundant data and carry on even if some writes fail, such as RAID-based file systems. • File systems that make snapshots, such as Network Appliance’s NFS server. • File systems that cache in temporary locations, such as NFS version 3 clients. • Compressed file systems. For ext3 and ext4 file systems, ‘shred’ is less effective when the file system is in ‘data=journal’ mode, which journals file data in addition to just metadata. In both the ‘data=ordered’ (default) and ‘data=writeback’ modes, ‘shred’ works as usual. The ext3/ext4 journaling modes can be changed by adding the ‘data=something’ option to the mount options for a particular file system in the ‘/etc/fstab’ file, as documented in the ‘mount’ man page (‘man mount’). Alternatively, if you know how large the journal is, you can shred the journal by shredding enough file data so that the journal cycles around and fills up with shredded data. If you are not sure how your file system operates, then you should assume that it does not overwrite data in place, which means ‘shred’ cannot reliably operate on regular files in your file system. Generally speaking, it is more reliable to shred a device than a file, since this bypasses file system design issues mentioned above. However, devices are also problematic for shredding, for reasons such as the following: • Solid-state storage devices (SSDs) typically do wear leveling to prolong service life, and this means writes are distributed to other blocks by the hardware, so “overwritten” data blocks are still present in the underlying device. • Most storage devices map out bad blocks invisibly to the application; if the bad blocks contain sensitive data, ‘shred’ won’t be able to destroy it. • With some obsolete storage technologies, it may be possible to take (say) a floppy disk back to a laboratory and use a lot of sensitive (and expensive) equipment to look for the faint “echoes” of the original data underneath the overwritten data. With these older technologies, if the file has been overwritten only once, it’s reputedly not even that hard. Luckily, this kind of data recovery has become difficult, and there is no public evidence that today’s higher-density storage devices can be analyzed in this way. The ‘shred’ command can use many overwrite passes, with data patterns chosen to maximize the damage they do to the old data. By default the patterns are designed for best effect on hard drives using now-obsolete technology; for newer devices, a single pass should suffice. For more details, see the source code and Peter Gutmann’s paper ‘Secure Deletion of Data from Magnetic and Solid-State Memory’ (https://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html), from the proceedings of the Sixth USENIX Security Symposium (San Jose, California, July 22–25, 1996). ‘shred’ makes no attempt to detect or report these problems, just as it makes no attempt to do anything about backups. However, since it is more reliable to shred devices than files, ‘shred’ by default does not deallocate or remove the output file. This default is more suitable for devices, which typically cannot be deallocated and should not be removed. Finally, consider the risk of backups and mirrors. File system backups and remote mirrors may contain copies of the file that cannot be removed, and that will allow a shredded file to be recovered later. So if you keep any data you may later want to destroy using ‘shred’, be sure that it is not backed up or mirrored. shred [OPTION]... FILE[...] The program accepts the following options. Also see *note Common options::. ‘-f’ ‘--force’ Override file permissions if necessary to allow overwriting. ‘-n NUMBER’ ‘--iterations=NUMBER’ By default, ‘shred’ uses 3 passes of overwrite. You can reduce this to save time, or increase it if you think it’s appropriate. After 25 passes all of the internal overwrite patterns will have been used at least once. ‘--random-source=FILE’ Use FILE as a source of random data used to overwrite and to choose pass ordering. *Note Random sources::. ‘-s BYTES’ ‘--size=BYTES’ Shred the first BYTES bytes of the file. The default is to shred the whole file. BYTES can be followed by a size specification like ‘K’, ‘M’, or ‘G’ to specify a multiple. *Note Block size::. ‘-u’ ‘--remove[=HOW]’ After shredding a file, deallocate it (if possible) and then remove it. If a file has multiple links, only the named links will be removed. Often the file name is less sensitive than the file data, in which case the optional HOW parameter, supported with the long form option, gives control of how to more efficiently remove each directory entry. The ‘unlink’ parameter will just use a standard unlink call, ‘wipe’ will also first obfuscate bytes in the name, and ‘wipesync’ will also sync each obfuscated byte in the name to the file system. Note ‘wipesync’ is the default method, but can be expensive, requiring a sync for every character in every file. This can become significant with many files, or is redundant if your file system provides synchronous metadata updates. ‘-v’ ‘--verbose’ Display to standard error all status updates as sterilization proceeds. ‘-x’ ‘--exact’ By default, ‘shred’ rounds the size of a regular file up to the next multiple of the file system block size to fully erase the slack space in the last block of the file. This space may contain portions of the current system memory on some systems for example. Use ‘--exact’ to suppress that behavior. Thus, by default if you shred a 10-byte regular file on a system with 512-byte blocks, the resulting file will be 512 bytes long. With this option, shred does not increase the apparent size of the file. ‘-z’ ‘--zero’ Normally, the last pass that ‘shred’ writes is made up of random data. If this would be conspicuous on your storage device (for example, because it looks like encrypted data), or you just think it’s tidier, the ‘--zero’ option adds an additional overwrite pass with all zero bits. This is in addition to the number of passes specified by the ‘--iterations’ option. You might use the following command to erase the file system you created on a USB flash drive. This command typically takes several minutes, depending on the drive’s size and write speed. On modern storage devices a single pass should be adequate, and will take one third the time of the default three-pass approach. shred -v -n 1 /dev/sdd1 Similarly, to erase all data on a selected partition of your device, you could give a command like the following. # 1 pass, write pseudo-random data; 3x faster than the default shred -v -n1 /dev/sda5 To be on the safe side, use at least one pass that overwrites using pseudo-random data. I.e., don’t be tempted to use ‘-n0 --zero’, in case some device controller optimizes the process of writing blocks of all zeros, and thereby does not clear all bytes in a block. Some SSDs may do just that. A FILE of ‘-’ denotes standard output. The intended use of this is to shred a removed temporary file. For example: i=$(mktemp) exec 3<>"$i" rm -- "$i" echo "Hello, world" >&3 shred - >&3 exec 3>- However, the command ‘shred - >file’ does not shred the contents of FILE, since the shell truncates FILE before invoking ‘shred’. Use the command ‘shred file’ or (if using a Bourne-compatible shell) the command ‘shred - 1<>file’ instead. An exit status of zero indicates success, and a nonzero value indicates failure. 12 Special file types ********************* This chapter describes commands which create special types of files (and ‘rmdir’, which removes directories, one special file type). Although Unix-like operating systems have markedly fewer special file types than others, not _everything_ can be treated only as the undifferentiated byte stream of “normal files”. For example, when a file is created or removed, the system must record this information, which it does in a “directory” – a special type of file. Although you can read directories as normal files, if you’re curious, in order for the system to do its job it must impose a structure, a certain order, on the bytes of the file. Thus it is a “special” type of file. Besides directories, other special file types include named pipes (FIFOs), symbolic links, sockets, and so-called “special files”. 12.1 ‘link’: Make a hard link via the link syscall ================================================== ‘link’ creates a single hard link at a time. It is a minimalist interface to the system-provided ‘link’ function. *Note (libc)Hard Links::. It avoids the bells and whistles of the more commonly-used ‘ln’ command (*note ln invocation::). Synopsis: link FILENAME LINKNAME FILENAME must specify an existing file, and LINKNAME must specify a nonexistent entry in an existing directory. ‘link’ simply calls ‘link (FILENAME, LINKNAME)’ to create the link. On a GNU system, this command acts like ‘ln --directory --no-target-directory FILENAME LINKNAME’. However, the ‘--directory’ and ‘--no-target-directory’ options are not specified by POSIX, and the ‘link’ command is more portable in practice. If FILENAME is a symbolic link, it is unspecified whether LINKNAME will be a hard link to the symbolic link or to the target of the symbolic link. Use ‘ln -P’ or ‘ln -L’ to specify which behavior is desired. An exit status of zero indicates success, and a nonzero value indicates failure. 12.2 ‘ln’: Make links between files =================================== ‘ln’ makes links between files. By default, it makes hard links; with the ‘-s’ option, it makes symbolic (or “soft”) links. Synopses: ln [OPTION]... [-T] TARGET LINKNAME ln [OPTION]... TARGET ln [OPTION]... TARGET... DIRECTORY ln [OPTION]... -t DIRECTORY TARGET... • If two file names are given, ‘ln’ creates a link to the first file from the second. • If one TARGET is given, ‘ln’ creates a link to that file in the current directory. • If the ‘--target-directory’ (‘-t’) option is given, or failing that if the last file is a directory and the ‘--no-target-directory’ (‘-T’) option is not given, ‘ln’ creates a link to each TARGET file in the specified directory, using the TARGETs’ names. Normally ‘ln’ does not replace existing files. Use the ‘--force’ (‘-f’) option to replace them unconditionally, the ‘--interactive’ (‘-i’) option to replace them conditionally, and the ‘--backup’ (‘-b’) option to rename them. Unless the ‘--backup’ (‘-b’) option is used there is no brief moment when the destination does not exist; this is an extension to POSIX. A “hard link” is another name for an existing file; the link and the original are indistinguishable. Technically speaking, they share the same inode, and the inode contains all the information about a file – indeed, it is not incorrect to say that the inode _is_ the file. Most systems prohibit making a hard link to a directory; on those where it is allowed, only the super-user can do so (and with caution, since creating a cycle will cause problems to many other utilities). Hard links cannot cross file system boundaries. (These restrictions are not mandated by POSIX, however.) “Symbolic links” (“symlinks” for short), on the other hand, are a special file type (which not all kernels support: System V release 3 (and older) systems lack symlinks) in which the link file actually refers to a different file, by name. When most operations (opening, reading, writing, and so on) are passed the symbolic link file, the kernel automatically “dereferences” the link and operates on the target of the link. But some operations (e.g., removing) work on the link file itself, rather than on its target. The owner and group of a symlink are not significant to file access performed through the link, but do have implications on deleting a symbolic link from a directory with the restricted deletion bit set. On the GNU system, the mode of a symlink has no significance and cannot be changed, but on some BSD systems, the mode can be changed and will affect whether the symlink will be traversed in file name resolution. *Note (libc)Symbolic Links::. Symbolic links can contain arbitrary strings; a “dangling symlink” occurs when the string in the symlink does not resolve to a file. There are no restrictions against creating dangling symbolic links. There are trade-offs to using absolute or relative symlinks. An absolute symlink always points to the same file, even if the directory containing the link is moved. However, if the symlink is visible from more than one machine (such as on a networked file system), the file pointed to might not always be the same. A relative symbolic link is resolved in relation to the directory that contains the link, and is often useful in referring to files on the same device without regards to what name that device is mounted on when accessed via networked machines. When creating a relative symlink in a different location than the current directory, the resolution of the symlink will be different than the resolution of the same string from the current directory. Therefore, many users prefer to first change directories to the location where the relative symlink will be created, so that tab-completion or other file resolution will find the same target as what will be placed in the symlink. The program accepts the following options. Also see *note Common options::. ‘-b’ ‘--backup[=METHOD]’ *Note Backup options::. Make a backup of each file that would otherwise be overwritten or removed. ‘-d’ ‘-F’ ‘--directory’ Allow users with appropriate privileges to attempt to make hard links to directories. However, note that this will probably fail due to system restrictions, even for the super-user. ‘-f’ ‘--force’ Remove existing destination files. ‘-i’ ‘--interactive’ Prompt whether to remove existing destination files, and fail if the response is not affirmative. ‘-L’ ‘--logical’ If ‘-s’ is not in effect, and the source file is a symbolic link, create the hard link to the file referred to by the symbolic link, rather than the symbolic link itself. ‘-n’ ‘--no-dereference’ Do not treat the last operand specially when it is a symbolic link to a directory. Instead, treat it as if it were a normal file. When the destination is an actual directory (not a symlink to one), there is no ambiguity. The link is created in that directory. But when the specified destination is a symlink to a directory, there are two ways to treat the user’s request. ‘ln’ can treat the destination just as it would a normal directory and create the link in it. On the other hand, the destination can be viewed as a non-directory – as the symlink itself. In that case, ‘ln’ must delete or backup that symlink before creating the new link. The default is to treat a destination that is a symlink to a directory just like a directory. This option is weaker than the ‘--no-target-directory’ (‘-T’) option, so it has no effect if both options are given. ‘-P’ ‘--physical’ If ‘-s’ is not in effect, and the source file is a symbolic link, create the hard link to the symbolic link itself. On platforms where this is not supported by the kernel, this option creates a symbolic link with identical contents; since symbolic link contents cannot be edited, any file name resolution performed through either link will be the same as if a hard link had been created. ‘-r’ ‘--relative’ Make symbolic links relative to the link location. This option is only valid with the ‘--symbolic’ option. Example: ln -srv /a/file /tmp '/tmp/file' -> '../a/file' Relative symbolic links are generated based on their canonicalized containing directory, and canonicalized targets. I.e., all symbolic links in these file names will be resolved. *Note realpath invocation::, which gives greater control over relative file name generation, as demonstrated in the following example: ln--relative() { test "$1" = --no-symlinks && { nosym=$1; shift; } target="$1"; test -d "$2" && link="$2/." || link="$2" rtarget="$(realpath $nosym -m "$target" \ --relative-to "$(dirname "$link")")" ln -s -v "$rtarget" "$link" } ‘-s’ ‘--symbolic’ Make symbolic links instead of hard links. This option merely produces an error message on systems that do not support symbolic links. ‘-S SUFFIX’ ‘--suffix=SUFFIX’ Append SUFFIX to each backup file made with ‘-b’. *Note Backup options::. ‘-t DIRECTORY’ ‘--target-directory=DIRECTORY’ Specify the destination DIRECTORY. *Note Target directory::. ‘-T’ ‘--no-target-directory’ Do not treat the last operand specially when it is a directory or a symbolic link to a directory. *Note Target directory::. ‘-v’ ‘--verbose’ Print the name of each file after linking it successfully. If ‘-L’ and ‘-P’ are both given, the last one takes precedence. If ‘-s’ is also given, ‘-L’ and ‘-P’ are silently ignored. If neither option is given, then this implementation defaults to ‘-P’ if the system ‘link’ supports hard links to symbolic links (such as the GNU system), and ‘-L’ if ‘link’ follows symbolic links (such as on BSD). An exit status of zero indicates success, and a nonzero value indicates failure. Examples: Bad Example: # Create link ../a pointing to a in that directory. # Not really useful because it points to itself. ln -s a .. Better Example: # Change to the target before creating symlinks to avoid being confused. cd .. ln -s adir/a . Bad Example: # Hard coded file names don't move well. ln -s $(pwd)/a /some/dir/ Better Example: # Relative file names survive directory moves and also # work across networked file systems. ln -s afile anotherfile ln -s ../adir/afile yetanotherfile 12.3 ‘mkdir’: Make directories ============================== ‘mkdir’ creates directories with the specified names. Synopsis: mkdir [OPTION]... NAME... ‘mkdir’ creates each directory NAME in the order given. It reports an error if NAME already exists, unless the ‘-p’ option is given and NAME is a directory. The program accepts the following options. Also see *note Common options::. ‘-m MODE’ ‘--mode=MODE’ Set the file permission bits of created directories to MODE, which uses the same syntax as in ‘chmod’ and uses ‘a=rwx’ (read, write and execute allowed for everyone) for the point of the departure. *Note File permissions::. This option affects only directories given on the command line; it does not affect any parents that may be created via the ‘-p’ option. Normally the directory has the desired file mode bits at the moment it is created. As a GNU extension, MODE may also mention special mode bits, but in this case there may be a temporary window during which the directory exists but its special mode bits are incorrect. *Note Directory Setuid and Setgid::, for how the set-user-ID and set-group-ID bits of directories are inherited unless overridden in this way. ‘-p’ ‘--parents’ Make any missing parent directories for each argument, setting their file permission bits to ‘=rwx,u+wx’, that is, with the umask modified by ‘u+wx’. Ignore existing parent directories, and do not change their file permission bits. If the ‘-m’ option is also given, it does not affect file permission bits of any newly-created parent directories. To control these bits, set the umask before invoking ‘mkdir’. For example, if the shell command ‘(umask u=rwx,go=rx; mkdir -p P/Q)’ creates the parent ‘P’ it sets the parent’s file permission bits to ‘u=rwx,go=rx’. (The umask must include ‘u=wx’ for this method to work.) To set a parent’s special mode bits as well, you can invoke ‘chmod’ after ‘mkdir’. *Note Directory Setuid and Setgid::, for how the set-user-ID and set-group-ID bits of newly-created parent directories are inherited. ‘-v’ ‘--verbose’ Print a message for each created directory. This is most useful with ‘--parents’. ‘-Z’ ‘--context[=CONTEXT]’ Without a specified CONTEXT, adjust the SELinux security context according to the system default type for destination files, similarly to the ‘restorecon’ command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued. An exit status of zero indicates success, and a nonzero value indicates failure. 12.4 ‘mkfifo’: Make FIFOs (named pipes) ======================================= ‘mkfifo’ creates FIFOs (also called “named pipes”) with the specified names. Synopsis: mkfifo [OPTION] NAME... A “FIFO” is a special file type that permits independent processes to communicate. One process opens the FIFO file for writing, and another for reading, after which data can flow as with the usual anonymous pipe in shells or elsewhere. The program accepts the following options. Also see *note Common options::. ‘-m MODE’ ‘--mode=MODE’ Set the mode of created FIFOs to MODE, which is symbolic as in ‘chmod’ and uses ‘a=rw’ (read and write allowed for everyone) for the point of departure. MODE should specify only file permission bits. *Note File permissions::. ‘-Z’ ‘--context[=CONTEXT]’ Without a specified CONTEXT, adjust the SELinux security context according to the system default type for destination files, similarly to the ‘restorecon’ command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued. An exit status of zero indicates success, and a nonzero value indicates failure. 12.5 ‘mknod’: Make block or character special files =================================================== ‘mknod’ creates a FIFO, character special file, or block special file with the specified name. Synopsis: mknod [OPTION]... NAME TYPE [MAJOR MINOR] Unlike the phrase “special file type” above, the term “special file” has a technical meaning on Unix: something that can generate or receive data. Usually this corresponds to a physical piece of hardware, e.g., a printer or a flash drive. (These files are typically created at system-configuration time.) The ‘mknod’ command is what creates files of this type. Such devices can be read either a character at a time or a “block” (many characters) at a time, hence we say there are “block special” files and “character special” files. Due to shell aliases and built-in ‘mknod’ functions, using an unadorned ‘mknod’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env mknod ...’) to avoid interference from the shell. The arguments after NAME specify the type of file to make: ‘p’ for a FIFO ‘b’ for a block special file ‘c’ for a character special file When making a block or character special file, the major and minor device numbers must be given after the file type. If a major or minor device number begins with ‘0x’ or ‘0X’, it is interpreted as hexadecimal; otherwise, if it begins with ‘0’, as octal; otherwise, as decimal. The program accepts the following options. Also see *note Common options::. ‘-m MODE’ ‘--mode=MODE’ Set the mode of created files to MODE, which is symbolic as in ‘chmod’ and uses ‘a=rw’ as the point of departure. MODE should specify only file permission bits. *Note File permissions::. ‘-Z’ ‘--context[=CONTEXT]’ Without a specified CONTEXT, adjust the SELinux security context according to the system default type for destination files, similarly to the ‘restorecon’ command. The long form of this option with a specific context specified, will set the context for newly created files only. With a specified context, if both SELinux and SMACK are disabled, a warning is issued. An exit status of zero indicates success, and a nonzero value indicates failure. 12.6 ‘readlink’: Print value of a symlink or canonical file name ================================================================ ‘readlink’ may work in one of two supported modes: ‘Readlink mode’ ‘readlink’ outputs the value of the given symbolic links. If ‘readlink’ is invoked with an argument other than the name of a symbolic link, it produces no output and exits with a nonzero exit code. ‘Canonicalize mode’ ‘readlink’ outputs the absolute name of the given files which contain no ‘.’, ‘..’ components nor any repeated separators (‘/’) or symbolic links. Note the ‘realpath’ command is the preferred command to use for canonicalization. *Note realpath invocation::. readlink [OPTION]... FILE... By default, ‘readlink’ operates in readlink mode. The program accepts the following options. Also see *note Common options::. ‘-f’ ‘--canonicalize’ Activate canonicalize mode. If any component of the file name except the last one is missing or unavailable, ‘readlink’ produces no output and exits with a nonzero exit code. A trailing slash is ignored. ‘-e’ ‘--canonicalize-existing’ Activate canonicalize mode. If any component is missing or unavailable, ‘readlink’ produces no output and exits with a nonzero exit code. A trailing slash requires that the name resolve to a directory. ‘-m’ ‘--canonicalize-missing’ Activate canonicalize mode. If any component is missing or unavailable, ‘readlink’ treats it as a directory. ‘-n’ ‘--no-newline’ Do not print the output delimiter, when a single FILE is specified. Print a warning if specified along with multiple FILEs. ‘-s’ ‘-q’ ‘--silent’ ‘--quiet’ Suppress most error messages. On by default. ‘-v’ ‘--verbose’ Report error messages. ‘-z’ ‘--zero’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. The ‘readlink’ utility first appeared in OpenBSD 2.1. The ‘realpath’ command without options, operates like ‘readlink’ in canonicalize mode. An exit status of zero indicates success, and a nonzero value indicates failure. 12.7 ‘rmdir’: Remove empty directories ====================================== ‘rmdir’ removes empty directories. Synopsis: rmdir [OPTION]... DIRECTORY... If any DIRECTORY argument does not refer to an existing empty directory, it is an error. The program accepts the following options. Also see *note Common options::. ‘--ignore-fail-on-non-empty’ Ignore each failure to remove a directory that is non-empty. ‘-p’ ‘--parents’ Remove DIRECTORY, then try to remove each component of DIRECTORY. So, for example, ‘rmdir -p a/b/c’ is similar to ‘rmdir a/b/c a/b a’. As such, it fails if any of those directories turns out not to be empty. Use the ‘--ignore-fail-on-non-empty’ option to make it so such a failure does not evoke a diagnostic and does not cause ‘rmdir’ to exit unsuccessfully. ‘-v’ ‘--verbose’ Give a diagnostic for each successful removal. DIRECTORY is removed. *Note rm invocation::, for how to remove non-empty directories recursively. To remove all empty directories under DIRNAME, including directories that become empty because other directories are removed, you can use either of the following commands: # This uses GNU extensions. find DIRNAME -type d -empty -delete # This runs on any POSIX platform. find DIRNAME -depth -type d -exec rmdir {} + An exit status of zero indicates success, and a nonzero value indicates failure. 12.8 ‘unlink’: Remove files via the unlink syscall ================================================== ‘unlink’ deletes a single specified file name. It is a minimalist interface to the system-provided ‘unlink’ function. *Note (libc)Deleting Files::. Synopsis: It avoids the bells and whistles of the more commonly-used ‘rm’ command (*note rm invocation::). unlink FILENAME On some systems ‘unlink’ can be used to delete the name of a directory. On others, it can be used that way only by a privileged user. In the GNU system ‘unlink’ can never delete the name of a directory. The ‘unlink’ command honors the ‘--help’ and ‘--version’ options. To remove a file whose name begins with ‘-’, prefix the name with ‘./’, e.g., ‘unlink ./--help’. An exit status of zero indicates success, and a nonzero value indicates failure. 13 Changing file attributes *************************** A file is not merely its contents, a name, and a file type (*note Special file types::). A file also has an owner (a user ID), a group (a group ID), permissions (what the owner can do with the file, what people in the group can do, and what everyone else can do), various timestamps, and other information. Collectively, we call these a file’s “attributes”. These commands change file attributes. 13.1 ‘chown’: Change file owner and group ========================================= ‘chown’ changes the user and/or group ownership of each given FILE to NEW-OWNER or to the user and group of an existing reference file. Synopsis: chown [OPTION]... {NEW-OWNER | --reference=REF_FILE} FILE... If used, NEW-OWNER specifies the new owner and/or group as follows (with no embedded white space): [OWNER] [ : [GROUP] ] Specifically: OWNER If only an OWNER (a user name or numeric user ID) is given, that user is made the owner of each given file, and the files’ group is not changed. OWNER‘:’GROUP If the OWNER is followed by a colon and a GROUP (a group name or numeric group ID), with no spaces between them, the group ownership of the files is changed as well (to GROUP). OWNER‘:’ If a colon but no group name follows OWNER, that user is made the owner of the files and the group of the files is changed to OWNER’s login group. ‘:’GROUP If the colon and following GROUP are given, but the owner is omitted, only the group of the files is changed; in this case, ‘chown’ performs the same function as ‘chgrp’. ‘:’ If only a colon is given, or if NEW-OWNER is empty, neither the owner nor the group is changed. If OWNER or GROUP is intended to represent a numeric user or group ID, then you may specify it with a leading ‘+’. *Note Disambiguating names and IDs::. Some older scripts may still use ‘.’ in place of the ‘:’ separator. POSIX 1003.1-2001 (*note Standards conformance::) does not require support for that, but for backward compatibility GNU ‘chown’ supports ‘.’ so long as no ambiguity results, although it issues a warning and support may be removed in future versions. New scripts should avoid the use of ‘.’ because it is not portable, and because it has undesirable results if the entire OWNER‘.’GROUP happens to identify a user whose name contains ‘.’. It is system dependent whether a user can change the group to an arbitrary one, or the more portable behavior of being restricted to setting a group of which the user is a member. The ‘chown’ command sometimes clears the set-user-ID or set-group-ID permission bits. This behavior depends on the policy and functionality of the underlying ‘chown’ system call, which may make system-dependent file mode modifications outside the control of the ‘chown’ command. For example, the ‘chown’ command might not affect those bits when invoked by a user with appropriate privileges, or when the bits signify some function other than executable permission (e.g., mandatory locking). When in doubt, check the underlying system behavior. The program accepts the following options. Also see *note Common options::. ‘-c’ ‘--changes’ Verbosely describe the action for each FILE whose ownership actually changes. ‘-f’ ‘--silent’ ‘--quiet’ Do not print error messages about files whose ownership cannot be changed. ‘--from=OLD-OWNER’ Change a FILE’s ownership only if it has current attributes specified by OLD-OWNER. OLD-OWNER has the same form as NEW-OWNER described above. This option is useful primarily from a security standpoint in that it narrows considerably the window of potential abuse. For example, to reflect a user ID numbering change for one user’s files without an option like this, ‘root’ might run find / -owner OLDUSER -print0 | xargs -0 chown -h NEWUSER But that is dangerous because the interval between when the ‘find’ tests the existing file’s owner and when the ‘chown’ is actually run may be quite large. One way to narrow the gap would be to invoke chown for each file as it is found: find / -owner OLDUSER -exec chown -h NEWUSER {} \; But that is very slow if there are many affected files. With this option, it is safer (the gap is narrower still) though still not perfect: chown -h -R --from=OLDUSER NEWUSER / ‘--dereference’ Do not act on symbolic links themselves but rather on what they point to. This is the default when not operating recursively. Combining this dereferencing option with the ‘--recursive’ option may create a security risk: During the traversal of the directory tree, an attacker may be able to introduce a symlink to an arbitrary target; when the tool reaches that, the operation will be performed on the target of that symlink, possibly allowing the attacker to escalate privileges. ‘-h’ ‘--no-dereference’ Act on symbolic links themselves instead of what they point to. This mode relies on the ‘lchown’ system call. On systems that do not provide the ‘lchown’ system call, ‘chown’ fails when a file specified on the command line is a symbolic link. By default, no diagnostic is issued for symbolic links encountered during a recursive traversal, but see ‘--verbose’. ‘--preserve-root’ Fail upon any attempt to recursively change the root directory, ‘/’. Without ‘--recursive’, this option has no effect. *Note Treating / specially::. ‘--no-preserve-root’ Cancel the effect of any preceding ‘--preserve-root’ option. *Note Treating / specially::. ‘--reference=REF_FILE’ Change the user and group of each FILE to be the same as those of REF_FILE. If REF_FILE is a symbolic link, do not use the user and group of the symbolic link, but rather those of the file it refers to. ‘-v’ ‘--verbose’ Output a diagnostic for every file processed. If a symbolic link is encountered during a recursive traversal on a system without the ‘lchown’ system call, and ‘--no-dereference’ is in effect, then issue a diagnostic saying neither the symbolic link nor its referent is being changed. ‘-R’ ‘--recursive’ Recursively change ownership of directories and their contents. ‘-H’ If ‘--recursive’ (‘-R’) is specified and a command line argument is a symbolic link to a directory, traverse it. *Note Traversing symlinks::. ‘-L’ In a recursive traversal, traverse every symbolic link to a directory that is encountered. Combining this dereferencing option with the ‘--recursive’ option may create a security risk: During the traversal of the directory tree, an attacker may be able to introduce a symlink to an arbitrary target; when the tool reaches that, the operation will be performed on the target of that symlink, possibly allowing the attacker to escalate privileges. *Note Traversing symlinks::. ‘-P’ Do not traverse any symbolic links. This is the default if none of ‘-H’, ‘-L’, or ‘-P’ is specified. *Note Traversing symlinks::. An exit status of zero indicates success, and a nonzero value indicates failure. Examples: # Change the owner of /u to "root". chown root /u # Likewise, but also change its group to "staff". chown root:staff /u # Change the owner of /u and subfiles to "root". chown -hR root /u 13.2 ‘chgrp’: Change group ownership ==================================== ‘chgrp’ changes the group ownership of each given FILE to GROUP (which can be either a group name or a numeric group ID) or to the group of an existing reference file. *Note chown invocation::. Synopsis: chgrp [OPTION]... {GROUP | --reference=REF_FILE} FILE... If GROUP is intended to represent a numeric group ID, then you may specify it with a leading ‘+’. *Note Disambiguating names and IDs::. It is system dependent whether a user can change the group to an arbitrary one, or the more portable behavior of being restricted to setting a group of which the user is a member. The program accepts the following options. Also see *note Common options::. ‘-c’ ‘--changes’ Verbosely describe the action for each FILE whose group actually changes. ‘-f’ ‘--silent’ ‘--quiet’ Do not print error messages about files whose group cannot be changed. ‘--dereference’ Do not act on symbolic links themselves but rather on what they point to. This is the default when not operating recursively. Combining this dereferencing option with the ‘--recursive’ option may create a security risk: During the traversal of the directory tree, an attacker may be able to introduce a symlink to an arbitrary target; when the tool reaches that, the operation will be performed on the target of that symlink, possibly allowing the attacker to escalate privileges. ‘-h’ ‘--no-dereference’ Act on symbolic links themselves instead of what they point to. This mode relies on the ‘lchown’ system call. On systems that do not provide the ‘lchown’ system call, ‘chgrp’ fails when a file specified on the command line is a symbolic link. By default, no diagnostic is issued for symbolic links encountered during a recursive traversal, but see ‘--verbose’. ‘--preserve-root’ Fail upon any attempt to recursively change the root directory, ‘/’. Without ‘--recursive’, this option has no effect. *Note Treating / specially::. ‘--no-preserve-root’ Cancel the effect of any preceding ‘--preserve-root’ option. *Note Treating / specially::. ‘--reference=REF_FILE’ Change the group of each FILE to be the same as that of REF_FILE. If REF_FILE is a symbolic link, do not use the group of the symbolic link, but rather that of the file it refers to. ‘-v’ ‘--verbose’ Output a diagnostic for every file processed. If a symbolic link is encountered during a recursive traversal on a system without the ‘lchown’ system call, and ‘--no-dereference’ is in effect, then issue a diagnostic saying neither the symbolic link nor its referent is being changed. ‘-R’ ‘--recursive’ Recursively change the group ownership of directories and their contents. ‘-H’ If ‘--recursive’ (‘-R’) is specified and a command line argument is a symbolic link to a directory, traverse it. *Note Traversing symlinks::. ‘-L’ In a recursive traversal, traverse every symbolic link to a directory that is encountered. Combining this dereferencing option with the ‘--recursive’ option may create a security risk: During the traversal of the directory tree, an attacker may be able to introduce a symlink to an arbitrary target; when the tool reaches that, the operation will be performed on the target of that symlink, possibly allowing the attacker to escalate privileges. *Note Traversing symlinks::. ‘-P’ Do not traverse any symbolic links. This is the default if none of ‘-H’, ‘-L’, or ‘-P’ is specified. *Note Traversing symlinks::. An exit status of zero indicates success, and a nonzero value indicates failure. Examples: # Change the group of /u to "staff". chgrp staff /u # Change the group of /u and subfiles to "staff". chgrp -hR staff /u 13.3 ‘chmod’: Change access permissions ======================================= ‘chmod’ changes the access permissions of the named files. Synopsis: chmod [OPTION]... {MODE | --reference=REF_FILE} FILE... ‘chmod’ never changes the permissions of symbolic links, since the ‘chmod’ system call cannot change their permissions. This is not a problem since the permissions of symbolic links are never used. However, for each symbolic link listed on the command line, ‘chmod’ changes the permissions of the pointed-to file. In contrast, ‘chmod’ ignores symbolic links encountered during recursive directory traversals. Only a process whose effective user ID matches the user ID of the file, or a process with appropriate privileges, is permitted to change the file mode bits of a file. A successful use of ‘chmod’ clears the set-group-ID bit of a regular file if the file’s group ID does not match the user’s effective group ID or one of the user’s supplementary group IDs, unless the user has appropriate privileges. Additional restrictions may cause the set-user-ID and set-group-ID bits of MODE or REF_FILE to be ignored. This behavior depends on the policy and functionality of the underlying ‘chmod’ system call. When in doubt, check the underlying system behavior. If used, MODE specifies the new file mode bits. For details, see the section on *note File permissions::. If you really want MODE to have a leading ‘-’, you should use ‘--’ first, e.g., ‘chmod -- -w file’. Typically, though, ‘chmod a-w file’ is preferable, and ‘chmod -w file’ (without the ‘--’) complains if it behaves differently from what ‘chmod a-w file’ would do. The program accepts the following options. Also see *note Common options::. ‘-c’ ‘--changes’ Verbosely describe the action for each FILE whose permissions actually change. ‘-f’ ‘--silent’ ‘--quiet’ Do not print error messages about files whose permissions cannot be changed. ‘--preserve-root’ Fail upon any attempt to recursively change the root directory, ‘/’. Without ‘--recursive’, this option has no effect. *Note Treating / specially::. ‘--no-preserve-root’ Cancel the effect of any preceding ‘--preserve-root’ option. *Note Treating / specially::. ‘-v’ ‘--verbose’ Verbosely describe the action or non-action taken for every FILE. ‘--reference=REF_FILE’ Change the mode of each FILE to be the same as that of REF_FILE. *Note File permissions::. If REF_FILE is a symbolic link, do not use the mode of the symbolic link, but rather that of the file it refers to. ‘-R’ ‘--recursive’ Recursively change permissions of directories and their contents. An exit status of zero indicates success, and a nonzero value indicates failure. Examples: # Change file permissions of FOO to be world readable # and user writable, with no other permissions. chmod 644 foo chmod a=r,u+w foo # Add user and group execute permissions to FOO. chmod +110 file chmod ug+x file # Set file permissions of DIR and subsidiary files to # be the umask default, assuming execute permissions for # directories and for files already executable. chmod -R a=,+rwX dir 13.4 ‘touch’: Change file timestamps ==================================== ‘touch’ changes the access and/or modification timestamps of the specified files. Synopsis: touch [OPTION]... FILE... Any FILE argument that does not exist is created empty, unless option ‘--no-create’ (‘-c’) or ‘--no-dereference’ (‘-h’) was in effect. A FILE argument string of ‘-’ is handled specially and causes ‘touch’ to change the times of the file associated with standard output. By default, ‘touch’ sets file timestamps to the current time. Because ‘touch’ acts on its operands left to right, the resulting timestamps of earlier and later operands may disagree. When setting file timestamps to the current time, ‘touch’ can change the timestamps for files that the user does not own but has write permission for. Otherwise, the user must own the files. Some older systems have a further restriction: the user must own the files unless both the access and modification timestamps are being set to the current time. The ‘touch’ command cannot set a file’s status change timestamp to a user-specified value, and cannot change the file’s birth time (if supported) at all. Also, ‘touch’ has issues similar to those affecting all programs that update file timestamps. For example, ‘touch’ may set a file’s timestamp to a value that differs slightly from the requested time. *Note File timestamps::. Timestamps assume the time zone rules specified by the ‘TZ’ environment variable, or by the system default rules if ‘TZ’ is not set. *Note Specifying the Time Zone with ‘TZ’: (libc)TZ Variable. You can avoid ambiguities during daylight saving transitions by using UTC timestamps. The program accepts the following options. Also see *note Common options::. ‘-a’ ‘--time=atime’ ‘--time=access’ ‘--time=use’ Change the access timestamp only. *Note File timestamps::. ‘-c’ ‘--no-create’ Do not warn about or create files that do not exist. ‘-d TIME’ ‘--date=TIME’ Use TIME instead of the current time. It can contain month names, time zones, ‘am’ and ‘pm’, ‘yesterday’, etc. For example, ‘--date="2020-07-21 14:19:13.489392193 +0530"’ specifies the instant of time that is 489,392,193 nanoseconds after July 21, 2020 at 2:19:13 PM in a time zone that is 5 hours and 30 minutes east of UTC. *Note Date input formats::. File systems that do not support high-resolution timestamps silently ignore any excess precision here. ‘-f’ Ignored; for compatibility with BSD versions of ‘touch’. ‘-h’ ‘--no-dereference’ Attempt to change the timestamps of a symbolic link, rather than what the link refers to. When using this option, empty files are not created, but option ‘-c’ must also be used to avoid warning about files that do not exist. Not all systems support changing the timestamps of symlinks, since underlying system support for this action was not required until POSIX 2008. Also, on some systems, the mere act of examining a symbolic link changes the access timestamp, such that only changes to the modification timestamp will persist long enough to be observable. When coupled with option ‘-r’, a reference timestamp is taken from a symbolic link rather than the file it refers to. ‘-m’ ‘--time=mtime’ ‘--time=modify’ Change the modification timestamp only. ‘-r FILE’ ‘--reference=FILE’ Use the times of the reference FILE instead of the current time. If this option is combined with the ‘--date=TIME’ (‘-d TIME’) option, the reference FILE’s time is the origin for any relative TIMEs given, but is otherwise ignored. For example, ‘-r foo -d '-5 seconds'’ specifies a timestamp equal to five seconds before the corresponding timestamp for ‘foo’. If FILE is a symbolic link, the reference timestamp is taken from the target of the symlink, unless ‘-h’ was also in effect. ‘-t [[CC]YY]MMDDHHMM[.SS]’ Use the argument (optional four-digit or two-digit years, months, days, hours, minutes, optional seconds) instead of the current time. If the year is specified with only two digits, then CC is 20 for years in the range 0 ... 68, and 19 for years in 69 ... 99. If no digits of the year are specified, the argument is interpreted as a date in the current year. On the atypical systems that support leap seconds, SS may be ‘60’. On systems predating POSIX 1003.1-2001, ‘touch’ supports an obsolete syntax, as follows. If no timestamp is given with any of the ‘-d’, ‘-r’, or ‘-t’ options, and if there are two or more FILEs and the first FILE is of the form ‘MMDDHHMM[YY]’ and this would be a valid argument to the ‘-t’ option (if the YY, if any, were moved to the front), and if the represented year is in the range 1969–1999, that argument is interpreted as the time for the other files instead of as a file name. Although this obsolete behavior can be controlled with the ‘_POSIX2_VERSION’ environment variable (*note Standards conformance::), portable scripts should avoid commands whose behavior depends on this variable. For example, use ‘touch ./12312359 main.c’ or ‘touch -t 12312359 main.c’ rather than the ambiguous ‘touch 12312359 main.c’. An exit status of zero indicates success, and a nonzero value indicates failure. 14 File space usage ******************* No file system can hold an infinite amount of data. These commands report how much storage is in use or available, report other file and file status information, and write buffers to file systems. 14.1 ‘df’: Report file system space usage ========================================= ‘df’ reports the amount of space used and available on file systems. Synopsis: df [OPTION]... [FILE]... With no arguments, ‘df’ reports the space used and available on all currently mounted file systems (of all types). Otherwise, ‘df’ reports on the file system containing each argument FILE. Normally the space is printed in units of 1024 bytes, but this can be overridden (*note Block size::). Non-integer quantities are rounded up to the next higher unit. For bind mounts and without arguments, ‘df’ only outputs the statistics for that device with the shortest mount point name in the list of file systems (MTAB), i.e., it hides duplicate entries, unless the ‘-a’ option is specified. With the same logic, ‘df’ elides a mount entry of a dummy pseudo device if there is another mount entry of a real block device for that mount point with the same device number, e.g. the early-boot pseudo file system ‘rootfs’ is not shown per default when already the real root device has been mounted. If an argument FILE resolves to a special file containing a mounted file system, ‘df’ shows the space available on that file system rather than on the file system containing the device node. GNU ‘df’ does not attempt to determine the usage on unmounted file systems, because on most kinds of systems doing so requires extremely non-portable intimate knowledge of file system structures. The program accepts the following options. Also see *note Common options::. ‘-a’ ‘--all’ Include in the listing dummy, duplicate, or inaccessible file systems, which are omitted by default. Dummy file systems are typically special purpose pseudo file systems such as ‘/proc’, with no associated storage. Duplicate file systems are local or remote file systems that are mounted at separate locations in the local file hierarchy, or bind mounted locations. Inaccessible file systems are those which are mounted but subsequently over-mounted by another file system at that point, or otherwise inaccessible due to permissions of the mount point etc. ‘-B SIZE’ ‘--block-size=SIZE’ Scale sizes by SIZE before printing them (*note Block size::). For example, ‘-BG’ prints sizes in units of 1,073,741,824 bytes. ‘-h’ ‘--human-readable’ Append a size letter to each size, such as ‘M’ for mebibytes. Powers of 1024 are used, not 1000; ‘M’ stands for 1,048,576 bytes. This option is equivalent to ‘--block-size=human-readable’. Use the ‘--si’ option if you prefer powers of 1000. ‘-H’ Equivalent to ‘--si’. ‘-i’ ‘--inodes’ List inode usage information instead of block usage. An inode (short for index node) contains information about a file such as its owner, permissions, timestamps, and location on the file system. ‘-k’ Print sizes in 1024-byte blocks, overriding the default block size (*note Block size::). This option is equivalent to ‘--block-size=1K’. ‘-l’ ‘--local’ Limit the listing to local file systems. By default, remote file systems are also listed. ‘--no-sync’ Do not invoke the ‘sync’ system call before getting any usage data. This may make ‘df’ run significantly faster on systems with many file systems, but on some systems (notably Solaris) the results may be slightly out of date. This is the default. ‘--output’ ‘--output[=FIELD_LIST]’ Use the output format defined by FIELD_LIST, or print all fields if FIELD_LIST is omitted. In the latter case, the order of the columns conforms to the order of the field descriptions below. The use of the ‘--output’ together with each of the options ‘-i’, ‘-P’, and ‘-T’ is mutually exclusive. FIELD_LIST is a comma-separated list of columns to be included in ‘df’’s output and therefore effectively controls the order of output columns. Each field can thus be used at the place of choice, but yet must only be used once. Valid field names in the FIELD_LIST are: ‘source’ The source of the mount point, usually a device. ‘fstype’ File system type. ‘itotal’ Total number of inodes. ‘iused’ Number of used inodes. ‘iavail’ Number of available inodes. ‘ipcent’ Percentage of IUSED divided by ITOTAL. ‘size’ Total number of blocks. ‘used’ Number of used blocks. ‘avail’ Number of available blocks. ‘pcent’ Percentage of USED divided by SIZE. ‘file’ The file name if specified on the command line. ‘target’ The mount point. The fields for block and inodes statistics are affected by the scaling options like ‘-h’ as usual. The definition of the FIELD_LIST can even be split among several ‘--output’ uses. #!/bin/sh # Print the TARGET (i.e., the mount point) along with their percentage # statistic regarding the blocks and the inodes. df --out=target --output=pcent,ipcent # Print all available fields. df --o ‘-P’ ‘--portability’ Use the POSIX output format. This is like the default format except for the following: 1. The information about each file system is always printed on exactly one line; a mount device is never put on a line by itself. This means that if the mount device name is more than 20 characters long (e.g., for some network mounts), the columns are misaligned. 2. The labels in the header output line are changed to conform to POSIX. 3. The default block size and output format are unaffected by the ‘DF_BLOCK_SIZE’, ‘BLOCK_SIZE’ and ‘BLOCKSIZE’ environment variables. However, the default block size is still affected by ‘POSIXLY_CORRECT’: it is 512 if ‘POSIXLY_CORRECT’ is set, 1024 otherwise. *Note Block size::. ‘--si’ Append an SI-style abbreviation to each size, such as ‘M’ for megabytes. Powers of 1000 are used, not 1024; ‘M’ stands for 1,000,000 bytes. This option is equivalent to ‘--block-size=si’. Use the ‘-h’ or ‘--human-readable’ option if you prefer powers of 1024. ‘--sync’ Invoke the ‘sync’ system call before getting any usage data. On some systems (notably Solaris), doing this yields more up to date results, but in general this option makes ‘df’ much slower, especially when there are many or very busy file systems. ‘--total’ Print a grand total of all arguments after all arguments have been processed. This can be used to find out the total size, usage and available space of all listed devices. If no arguments are specified df will try harder to elide file systems insignificant to the total available space, by suppressing duplicate remote file systems. For the grand total line, ‘df’ prints ‘"total"’ into the SOURCE column, and ‘"-"’ into the TARGET column. If there is no SOURCE column (see ‘--output’), then ‘df’ prints ‘"total"’ into the TARGET column, if present. ‘-t FSTYPE’ ‘--type=FSTYPE’ Limit the listing to file systems of type FSTYPE. Multiple file system types can be specified by giving multiple ‘-t’ options. By default, nothing is omitted. ‘-T’ ‘--print-type’ Print each file system’s type. The types printed here are the same ones you can include or exclude with ‘-t’ and ‘-x’. The particular types printed are whatever is supported by the system. Here are some of the common names (this list is certainly not exhaustive): ‘nfs’ An NFS file system, i.e., one mounted over a network from another machine. This is the one type name which seems to be used uniformly by all systems. ‘ext2, ext3, ext4, xfs, btrfs...’ A file system on a locally-mounted device. (The system might even support more than one type here; GNU/Linux does.) ‘iso9660, cdfs’ A file system on a CD or DVD drive. HP-UX uses ‘cdfs’, most other systems use ‘iso9660’. ‘ntfs,fat’ File systems used by MS-Windows / MS-DOS. ‘-x FSTYPE’ ‘--exclude-type=FSTYPE’ Limit the listing to file systems not of type FSTYPE. Multiple file system types can be eliminated by giving multiple ‘-x’ options. By default, no file system types are omitted. ‘-v’ Ignored; for compatibility with System V versions of ‘df’. ‘df’ is installed only on systems that have usable mount tables, so portable scripts should not rely on its existence. An exit status of zero indicates success, and a nonzero value indicates failure. Failure includes the case where no output is generated, so you can inspect the exit status of a command like ‘df -t ext3 -t reiserfs DIR’ to test whether DIR is on a file system of type ‘ext3’ or ‘reiserfs’. Since the list of file systems (MTAB) is needed to determine the file system type, failure includes the cases when that list cannot be read and one or more of the options ‘-a’, ‘-l’, ‘-t’ or ‘-x’ is used together with a file name argument. 14.2 ‘du’: Estimate file space usage ==================================== ‘du’ reports the space needed to represent a set of files. Synopsis: du [OPTION]... [FILE]... With no arguments, ‘du’ reports the space needed to represent the files at or under the current directory. Normally the space is printed in units of 1024 bytes, but this can be overridden (*note Block size::). Non-integer quantities are rounded up to the next higher unit. If two or more hard links point to the same file, only one of the hard links is counted. The FILE argument order affects which links are counted, and changing the argument order may change the numbers and entries that ‘du’ outputs. The program accepts the following options. Also see *note Common options::. ‘-0’ ‘--null’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. ‘-a’ ‘--all’ Show counts for all files, not just directories. ‘--apparent-size’ Print apparent sizes, rather than file system usage. The apparent size of a file is the number of bytes reported by ‘wc -c’ on regular files, or more generally, ‘ls -l --block-size=1’ or ‘stat --format=%s’. For example, a file containing the word ‘zoo’ with no newline would, of course, have an apparent size of 3. Such a small file may require anywhere from 0 to 16 KiB or more of file system space, depending on the type and configuration of the file system on which the file resides. However, a sparse file created with this command: dd bs=1 seek=2GiB if=/dev/null of=big has an apparent size of 2 GiB, yet on most modern file systems, it actually uses almost no space. Apparent sizes are meaningful only for regular files and symbolic links. Other file types do not contribute to apparent size. ‘-B SIZE’ ‘--block-size=SIZE’ Scale sizes by SIZE before printing them (*note Block size::). For example, ‘-BG’ prints sizes in units of 1,073,741,824 bytes. ‘-b’ ‘--bytes’ Equivalent to ‘--apparent-size --block-size=1’. ‘-c’ ‘--total’ Print a grand total of all arguments after all arguments have been processed. This can be used to find out the total file system usage of a given set of files or directories. ‘-D’ ‘--dereference-args’ Dereference symbolic links that are command line arguments. Does not affect other symbolic links. This is helpful for finding out the file system usage of directories, such as ‘/usr/tmp’, which are often symbolic links. ‘-d DEPTH’ ‘--max-depth=DEPTH’ Show the total for each directory (and file if ‘--all’) that is at most MAX_DEPTH levels down from the root of the hierarchy. The root is at level 0, so ‘du --max-depth=0’ is equivalent to ‘du -s’. ‘--files0-from=FILE’ Disallow processing files named on the command line, and instead process those named in file FILE; each name being terminated by a zero byte (ASCII NUL). This is useful when the list of file names is so long that it may exceed a command line length limitation. In such cases, running ‘du’ via ‘xargs’ is undesirable because it splits the list into pieces and makes ‘du’ print with the ‘--total’ (‘-c’) option for each sublist rather than for the entire list. One way to produce a list of ASCII NUL terminated file names is with GNU ‘find’, using its ‘-print0’ predicate. If FILE is ‘-’ then the ASCII NUL terminated file names are read from standard input. ‘-H’ Equivalent to ‘--dereference-args’ (‘-D’). ‘-h’ ‘--human-readable’ Append a size letter to each size, such as ‘M’ for mebibytes. Powers of 1024 are used, not 1000; ‘M’ stands for 1,048,576 bytes. This option is equivalent to ‘--block-size=human-readable’. Use the ‘--si’ option if you prefer powers of 1000. ‘--inodes’ List inode usage information instead of block usage. This option is useful for finding directories which contain many files, and therefore eat up most of the inodes space of a file system (see ‘df’, option ‘--inodes’). It can well be combined with the options ‘-a’, ‘-c’, ‘-h’, ‘-l’, ‘-s’, ‘-S’, ‘-t’ and ‘-x’; however, passing other options regarding the block size, for example ‘-b’, ‘-m’ and ‘--apparent-size’, is ignored. ‘-k’ Print sizes in 1024-byte blocks, overriding the default block size (*note Block size::). This option is equivalent to ‘--block-size=1K’. ‘-L’ ‘--dereference’ Dereference symbolic links (show the file system space used by the file or directory that the link points to instead of the space used by the link). ‘-l’ ‘--count-links’ Count the size of all files, even if they have appeared already (as a hard link). ‘-m’ Print sizes in 1,048,576-byte blocks, overriding the default block size (*note Block size::). This option is equivalent to ‘--block-size=1M’. ‘-P’ ‘--no-dereference’ For each symbolic link encountered by ‘du’, consider the file system space used by the symbolic link itself. ‘-S’ ‘--separate-dirs’ Normally, in the output of ‘du’ (when not using ‘--summarize’), the size listed next to a directory name, D, represents the sum of sizes of all entries beneath D as well as the size of D itself. With ‘--separate-dirs’, the size reported for a directory name, D, will exclude the size of any subdirectories. ‘--si’ Append an SI-style abbreviation to each size, such as ‘M’ for megabytes. Powers of 1000 are used, not 1024; ‘M’ stands for 1,000,000 bytes. This option is equivalent to ‘--block-size=si’. Use the ‘-h’ or ‘--human-readable’ option if you prefer powers of 1024. ‘-s’ ‘--summarize’ Display only a total for each argument. ‘-t SIZE’ ‘--threshold=SIZE’ Exclude entries based on a given SIZE. The SIZE refers to used blocks in normal mode (*note Block size::), or inodes count in conjunction with the ‘--inodes’ option. If SIZE is positive, then ‘du’ will only print entries with a size greater than or equal to that. If SIZE is negative, then ‘du’ will only print entries with a size smaller than or equal to that. Although GNU ‘find’ can be used to find files of a certain size, ‘du’’s ‘--threshold’ option can be used to also filter directories based on a given size. When combined with the ‘--apparent-size’ option, the ‘--threshold’ option elides entries based on apparent size. When combined with the ‘--inodes’ option, it elides entries based on inode counts. Here’s how you would use ‘--threshold’ to find directories with a size greater than or equal to 200 megabytes: du --threshold=200MB Here’s how you would use ‘--threshold’ to find directories and files – note the ‘-a’ – with an apparent size smaller than or equal to 500 bytes: du -a -t -500 --apparent-size Here’s how you would use ‘--threshold’ to find directories on the root file system with more than 20000 inodes used in the directory tree below: du --inodes -x --threshold=20000 / ‘--time’ Show the most recent modification timestamp (mtime) of any file in the directory, or any of its subdirectories. *Note File timestamps::. ‘--time=ctime’ ‘--time=status’ ‘--time=use’ Show the most recent status change timestamp (ctime) of any file in the directory, or any of its subdirectories. *Note File timestamps::. ‘--time=atime’ ‘--time=access’ Show the most recent access timestamp (atime) of any file in the directory, or any of its subdirectories. *Note File timestamps::. ‘--time-style=STYLE’ List timestamps in style STYLE. This option has an effect only if the ‘--time’ option is also specified. The STYLE should be one of the following: ‘+FORMAT’ List timestamps using FORMAT, where FORMAT is interpreted like the format argument of ‘date’ (*note date invocation::). For example, ‘--time-style="+%Y-%m-%d %H:%M:%S"’ causes ‘du’ to list timestamps like ‘2020-07-21 23:45:56’. As with ‘date’, FORMAT’s interpretation is affected by the ‘LC_TIME’ locale category. ‘full-iso’ List timestamps in full using ISO 8601-like date, time, and time zone components with nanosecond precision, e.g., ‘2020-07-21 23:45:56.477817180 -0400’. This style is equivalent to ‘+%Y-%m-%d %H:%M:%S.%N %z’. ‘long-iso’ List ISO 8601 date and time components with minute precision, e.g., ‘2020-07-21 23:45’. These timestamps are shorter than ‘full-iso’ timestamps, and are usually good enough for everyday work. This style is equivalent to ‘+%Y-%m-%d %H:%M’. ‘iso’ List ISO 8601 dates for timestamps, e.g., ‘2020-07-21’. This style is equivalent to ‘+%Y-%m-%d’. You can specify the default value of the ‘--time-style’ option with the environment variable ‘TIME_STYLE’; if ‘TIME_STYLE’ is not set the default style is ‘long-iso’. For compatibility with ‘ls’, if ‘TIME_STYLE’ begins with ‘+’ and contains a newline, the newline and any later characters are ignored; if ‘TIME_STYLE’ begins with ‘posix-’ the ‘posix-’ is ignored; and if ‘TIME_STYLE’ is ‘locale’ it is ignored. ‘-X FILE’ ‘--exclude-from=FILE’ Like ‘--exclude’, except take the patterns to exclude from FILE, one per line. If FILE is ‘-’, take the patterns from standard input. ‘--exclude=PATTERN’ When recursing, skip subdirectories or files matching PATTERN. For example, ‘du --exclude='*.o'’ excludes files whose names end in ‘.o’. ‘-x’ ‘--one-file-system’ Skip directories that are on different file systems from the one that the argument being processed is on. Since ‘du’ relies on information reported by the operating system, its output might not reflect the space consumed in the underlying devices. For example; • Operating systems normally do not report device space consumed by duplicate or backup blocks, error correction bits, and so forth. This causes ‘du’ to underestimate the device space actually used. • In file systems that use copy-on-write, if two distinct files share space the output of ‘du’ typically counts the space that would be consumed if all files’ non-holes were rewritten, not the space currently consumed. • In file systems that use compression, the operating system might report the uncompressed space. (If it does report the compressed space, that report might change after one merely overwrites existing file data.) • Networked file systems historically have had difficulty communicating accurate file system information from server to client. For these reasons ‘du’ might better be thought of as an estimate of the size of a ‘tar’ or other conventional backup for a set of files, rather than as a measure of space consumed in the underlying devices. An exit status of zero indicates success, and a nonzero value indicates failure. 14.3 ‘stat’: Report file or file system status ============================================== ‘stat’ displays information about the specified file(s). Synopsis: stat [OPTION]... [FILE]... With no option, ‘stat’ reports all information about the given files. But it also can be used to report the information of the file systems the given files are located on. If the files are links, ‘stat’ can also give information about the files the links point to. Due to shell aliases and built-in ‘stat’ functions, using an unadorned ‘stat’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env stat ...’) to avoid interference from the shell. ‘-L’ ‘--dereference’ Change how ‘stat’ treats symbolic links. With this option, ‘stat’ acts on the file referenced by each symbolic link argument. Without it, ‘stat’ acts on any symbolic link argument directly. ‘-f’ ‘--file-system’ Report information about the file systems where the given files are located instead of information about the files themselves. This option implies the ‘-L’ option. ‘--cached=MODE’ Control how attributes are read from the file system; if supported by the system. This allows one to control the trade-off between freshness and efficiency of attribute access, especially useful with remote file systems. MODE can be: ‘always’ Always read the already cached attributes if available. ‘never’ Always synchronize with the latest file system attributes. This also mounts automounted files. ‘default’ Leave the caching behavior to the underlying file system. ‘-c’ ‘--format=FORMAT’ Use FORMAT rather than the default format. FORMAT is automatically newline-terminated, so running a command like the following with two or more FILE operands produces a line of output for each operand: $ stat --format=%d:%i / /usr 2050:2 2057:2 ‘--printf=FORMAT’ Use FORMAT rather than the default format. Like ‘--format’, but interpret backslash escapes, and do not output a mandatory trailing newline. If you want a newline, include ‘\n’ in the FORMAT. Here’s how you would use ‘--printf’ to print the device and inode numbers of ‘/’ and ‘/usr’: $ stat --printf='%d:%i\n' / /usr 2050:2 2057:2 ‘-t’ ‘--terse’ Print the information in terse form, suitable for parsing by other programs. The output of the following commands are identical and the ‘--format’ also identifies the items printed (in fuller form) in the default format. Note the format string would include another ‘%C’ at the end with an active SELinux security context. $ stat --format="%n %s %b %f %u %g %D %i %h %t %T %X %Y %Z %W %o" ... $ stat --terse ... The same illustrating terse output in ‘--file-system’ mode: $ stat -f --format="%n %i %l %t %s %S %b %f %a %c %d" ... $ stat -f --terse ... The valid FORMAT directives for files with ‘--format’ and ‘--printf’ are: • %a – Permission bits in octal (note ‘#’ and ‘0’ printf flags) • %A – Permission bits in symbolic form (similar to ‘ls -ld’) • %b – Number of blocks allocated (see ‘%B’) • %B – The size in bytes of each block reported by ‘%b’ • %C – The SELinux security context of a file, if available • %d – Device number in decimal (st_dev) • %D – Device number in hex (st_dev) • %Hd – Major device number in decimal • %Ld – Minor device number in decimal • %f – Raw mode in hex • %F – File type • %g – Group ID of owner • %G – Group name of owner • %h – Number of hard links • %i – Inode number • %m – Mount point (See note below) • %n – File name • %N – Quoted file name with dereference if symbolic link (see below) • %o – Optimal I/O transfer size hint • %s – Total size, in bytes • %r – Device type in decimal (st_rdev) • %R – Device type in hex (st_rdev) • %Hr – Major device type in decimal (see below) • %Lr – Minor device type in decimal (see below) • %t – Major device type in hex (see below) • %T – Minor device type in hex (see below) • %u – User ID of owner • %U – User name of owner • %w – Time of file birth, or ‘-’ if unknown • %W – Time of file birth as seconds since Epoch, or ‘0’ • %x – Time of last access • %X – Time of last access as seconds since Epoch • %y – Time of last data modification • %Y – Time of last data modification as seconds since Epoch • %z – Time of last status change • %Z – Time of last status change as seconds since Epoch The ‘%a’ format prints the octal mode, and so it is useful to control the zero padding of the output with the ‘#’ and ‘0’ printf flags. For example to pad to at least 3 wide while making larger numbers unambiguously octal, you can use ‘%#03a’. The ‘%N’ format can be set with the environment variable ‘QUOTING_STYLE’. If that environment variable is not set, the default value is ‘shell-escape-always’. Valid quoting styles are: ‘literal’ Output strings as-is; this is the same as the ‘--literal’ (‘-N’) option. ‘shell’ Quote strings for the shell if they contain shell metacharacters or would cause ambiguous output. The quoting is suitable for POSIX-compatible shells like ‘bash’, but it does not always work for incompatible shells like ‘csh’. ‘shell-always’ Quote strings for the shell, even if they would normally not require quoting. ‘shell-escape’ Like ‘shell’, but also quoting non-printable characters using the POSIX proposed ‘$''’ syntax suitable for most shells. ‘shell-escape-always’ Like ‘shell-escape’, but quote strings even if they would normally not require quoting. ‘c’ Quote strings as for C character string literals, including the surrounding double-quote characters; this is the same as the ‘--quote-name’ (‘-Q’) option. ‘escape’ Quote strings as for C character string literals, except omit the surrounding double-quote characters; this is the same as the ‘--escape’ (‘-b’) option. ‘clocale’ Quote strings as for C character string literals, except use surrounding quotation marks appropriate for the locale. ‘locale’ Quote strings as for C character string literals, except use surrounding quotation marks appropriate for the locale, and quote 'like this' instead of "like this" in the default C locale. This looks nicer on many displays. The ‘r’, ‘R’, ‘%t’, and ‘%T’ formats operate on the st_rdev member of the stat(2) structure, i.e., the represented device rather than the containing device, and so are only defined for character and block special files. On some systems or file types, st_rdev may be used to represent other quantities. The ‘%W’, ‘%X’, ‘%Y’, and ‘%Z’ formats accept a precision preceded by a period to specify the number of digits to print after the decimal point. For example, ‘%.3X’ outputs the access timestamp to millisecond precision. If a period is given but no precision, ‘stat’ uses 9 digits, so ‘%.X’ is equivalent to ‘%.9X’. When discarding excess precision, timestamps are truncated toward minus infinity. zero pad: $ stat -c '[%015Y]' /usr [000001288929712] space align: $ stat -c '[%15Y]' /usr [ 1288929712] $ stat -c '[%-15Y]' /usr [1288929712 ] precision: $ stat -c '[%.3Y]' /usr [1288929712.114] $ stat -c '[%.Y]' /usr [1288929712.114951834] The mount point printed by ‘%m’ is similar to that output by ‘df’, except that: • stat does not dereference symlinks by default (unless ‘-L’ is specified) • stat does not search for specified device nodes in the file system list, instead operating on them directly • stat outputs the alias for a bind mounted file, rather than the initial mount point of its backing device. One can recursively call stat until there is no change in output, to get the current base mount point When listing file system information (‘--file-system’ (‘-f’)), you must use a different set of FORMAT directives: • %a – Free blocks available to non-super-user • %b – Total data blocks in file system • %c – Total file nodes in file system • %d – Free file nodes in file system • %f – Free blocks in file system • %i – File System ID in hex • %l – Maximum length of file names • %n – File name • %s – Block size (for faster transfers) • %S – Fundamental block size (for block counts) • %t – Type in hex • %T – Type in human readable form Timestamps are listed according to the time zone rules specified by the ‘TZ’ environment variable, or by the system default rules if ‘TZ’ is not set. *Note Specifying the Time Zone with ‘TZ’: (libc)TZ Variable. An exit status of zero indicates success, and a nonzero value indicates failure. 14.4 ‘sync’: Synchronize cached writes to persistent storage ============================================================ ‘sync’ synchronizes in memory files or file systems to persistent storage. Synopsis: sync [OPTION] [FILE]... ‘sync’ writes any data buffered in memory out to the storage device. This can include (but is not limited to) modified superblocks, modified inodes, and delayed reads and writes. This must be implemented by the kernel; The ‘sync’ program does nothing but exercise the ‘sync’, ‘syncfs’, ‘fsync’, and ‘fdatasync’ system calls. The kernel keeps data in memory to avoid doing (relatively slow) device reads and writes. This improves performance, but if the computer crashes, data may be lost or the file system corrupted as a result. The ‘sync’ command instructs the kernel to write data in memory to persistent storage. If any argument is specified then only those files will be synchronized using the fsync(2) syscall by default. If at least one file is specified, it is possible to change the synchronization method with the following options. Also see *note Common options::. ‘-d’ ‘--data’ Use fdatasync(2) to sync only the data for the file, and any metadata required to maintain file system consistency. ‘-f’ ‘--file-system’ Synchronize all the I/O waiting for the file systems that contain the file, using the syscall syncfs(2). Note you would usually _not_ specify this option if passing a device node like ‘/dev/sda’ for example, as that would sync the containing file system rather than the referenced one. Note also that depending on the system, passing individual device nodes or files may have different sync characteristics than using no arguments. I.e., arguments passed to fsync(2) may provide greater guarantees through write barriers, than a global sync(2) used when no arguments are provided. An exit status of zero indicates success, and a nonzero value indicates failure. 14.5 ‘truncate’: Shrink or extend the size of a file ==================================================== ‘truncate’ shrinks or extends the size of each FILE to the specified size. Synopsis: truncate OPTION... FILE... Any FILE that does not exist is created. If a FILE is larger than the specified size, the extra data is lost. If a FILE is shorter, it is extended and the sparse extended part (or hole) reads as zero bytes. The program accepts the following options. Also see *note Common options::. ‘-c’ ‘--no-create’ Do not create files that do not exist. ‘-o’ ‘--io-blocks’ Treat SIZE as number of I/O blocks of the FILE rather than bytes. ‘-r RFILE’ ‘--reference=RFILE’ Base the size of each FILE on the size of RFILE. ‘-s SIZE’ ‘--size=SIZE’ Set or adjust the size of each FILE according to SIZE. SIZE is in bytes unless ‘--io-blocks’ is specified. SIZE may be, or may be an integer optionally followed by, one of the following multiplicative suffixes: ‘KB’ => 1000 (KiloBytes) ‘K’ => 1024 (KibiBytes) ‘MB’ => 1000*1000 (MegaBytes) ‘M’ => 1024*1024 (MebiBytes) ‘GB’ => 1000*1000*1000 (GigaBytes) ‘G’ => 1024*1024*1024 (GibiBytes) and so on for ‘T’, ‘P’, ‘E’, ‘Z’, ‘Y’, ‘R’, and ‘Q’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on. SIZE may also be prefixed by one of the following to adjust the size of each FILE based on its current size: ‘+’ => extend by ‘-’ => reduce by ‘<’ => at most ‘>’ => at least ‘/’ => round down to multiple of ‘%’ => round up to multiple of An exit status of zero indicates success, and a nonzero value indicates failure. 15 Printing text **************** This section describes commands that display text strings. 15.1 ‘echo’: Print a line of text ================================= ‘echo’ writes each given STRING to standard output, with a space between each and a newline after the last one. Synopsis: echo [OPTION]... [STRING]... Due to shell aliases and built-in ‘echo’ functions, using an unadorned ‘echo’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env echo ...’) to avoid interference from the shell. Due to historical and backwards compatibility reasons, certain bare option-like strings cannot be passed to ‘echo’ as non-option arguments. It is therefore not advisable to use ‘echo’ for printing unknown or variable arguments. The ‘printf’ command is recommended as a more portable and flexible replacement for tasks historically performed by ‘echo’. *Note printf invocation::. The program accepts the following options. Also see *note Common options::. Options must precede operands, and the normally-special argument ‘--’ has no special meaning and is treated like any other STRING. ‘-n’ Do not output the trailing newline. ‘-e’ Enable interpretation of the following backslash-escaped characters in each STRING: ‘\a’ alert (bell) ‘\b’ backspace ‘\c’ produce no further output ‘\e’ escape ‘\f’ form feed ‘\n’ newline ‘\r’ carriage return ‘\t’ horizontal tab ‘\v’ vertical tab ‘\\’ backslash ‘\0NNN’ the eight-bit value that is the octal number NNN (zero to three octal digits), if NNN is a nine-bit value, the ninth bit is ignored ‘\NNN’ the eight-bit value that is the octal number NNN (one to three octal digits), if NNN is a nine-bit value, the ninth bit is ignored ‘\xHH’ the eight-bit value that is the hexadecimal number HH (one or two hexadecimal digits) ‘-E’ Disable interpretation of backslash escapes in each STRING. This is the default. If ‘-e’ and ‘-E’ are both specified, the last one given takes effect. If the ‘POSIXLY_CORRECT’ environment variable is set, then when ‘echo’’s first argument is not ‘-n’ it outputs option-like arguments instead of treating them as options. For example, ‘echo -ne hello’ outputs ‘-ne hello’ instead of plain ‘hello’. Also backslash escapes are always enabled. Note to echo the string ‘-n’, one of the characters can be escaped in either octal or hexadecimal representation. For example, ‘echo -e '\x2dn'’. POSIX does not require support for any options, and says that the behavior of ‘echo’ is implementation-defined if any STRING contains a backslash or if the first argument is ‘-n’. Portable programs should use the ‘printf’ command instead. *Note printf invocation::. An exit status of zero indicates success, and a nonzero value indicates failure. 15.2 ‘printf’: Format and print data ==================================== ‘printf’ does formatted printing of text. Synopsis: printf FORMAT [ARGUMENT]... ‘printf’ prints the FORMAT string, interpreting ‘%’ directives and ‘\’ escapes to format numeric and string arguments in a way that is mostly similar to the C ‘printf’ function. *Note ‘printf’ format directives: (libc)Output Conversion Syntax, for details. The differences are listed below. Due to shell aliases and built-in ‘printf’ functions, using an unadorned ‘printf’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env printf ...’) to avoid interference from the shell. • The FORMAT argument is reused as necessary to convert all the given ARGUMENTs. For example, the command ‘printf %s a b’ outputs ‘ab’. • Missing ARGUMENTs are treated as null strings or as zeros, depending on whether the context expects a string or a number. For example, the command ‘printf %sx%d’ prints ‘x0’. • An additional escape, ‘\c’, causes ‘printf’ to produce no further output. For example, the command ‘printf 'A%sC\cD%sF' B E’ prints ‘ABC’. • The hexadecimal escape sequence ‘\xHH’ has at most two digits, as opposed to C where it can have an unlimited number of digits. For example, the command ‘printf '\x07e'’ prints two bytes, whereas the C statement ‘printf ("\x07e")’ prints just one. • An additional directive ‘%b’, prints its argument string with ‘\’ escapes interpreted in the same way as in the FORMAT string, except that octal escapes are of the form ‘\0OOO’ where OOO is 0 to 3 octal digits. If ‘\OOO’ is nine-bit value, ignore the ninth bit. If a precision is also given, it limits the number of bytes printed from the converted string. • An additional directive ‘%q’, prints its argument string in a format that can be reused as input by most shells. Non-printable characters are escaped with the POSIX proposed ‘$''’ syntax, and shell metacharacters are quoted appropriately. This is an equivalent format to ‘ls --quoting=shell-escape’ output. • Numeric arguments must be single C constants, possibly with leading ‘+’ or ‘-’. For example, ‘printf %.4d -3’ outputs ‘-0003’. • If the leading character of a numeric argument is ‘"’ or ‘'’ then its value is the numeric value of the immediately following character. Any remaining characters are silently ignored if the ‘POSIXLY_CORRECT’ environment variable is set; otherwise, a warning is printed. For example, ‘printf "%d" "'a"’ outputs ‘97’ on hosts that use the ASCII character set, since ‘a’ has the numeric value 97 in ASCII. A floating point argument is interpreted according to the ‘LC_NUMERIC’ category of either the current or the C locale, and is printed according to the current locale. For example, in a locale whose decimal point character is a comma, the command ‘printf '%g %g' 2,5 2.5’ outputs ‘2,5 2,5’. *Note Floating point::. ‘printf’ interprets ‘\OOO’ in FORMAT as an octal number (if OOO is 1 to 3 octal digits) specifying a byte to print, and ‘\xHH’ as a hexadecimal number (if HH is 1 to 2 hex digits) specifying a character to print. Note however that when ‘\OOO’ specifies a number larger than 255, ‘printf’ ignores the ninth bit. For example, ‘printf '\400'’ is equivalent to ‘printf '\0'’. ‘printf’ interprets two syntax forms for specifying Unicode (ISO/IEC 10646) characters. ‘\u’ for 16-bit Unicode characters, specified as four hexadecimal digits HHHH, and ‘\U’ for 32-bit Unicode characters, specified as eight hexadecimal digits HHHHHHHH. ‘printf’ outputs the Unicode characters according to the ‘LC_CTYPE’ locale. Unicode characters in the range U+D800...U+DFFF cannot be specified by this syntax. This syntax fully supports the universal character subset introduced in ISO C 99. The processing of ‘\u’ and ‘\U’ requires a full-featured ‘iconv’ facility. It is activated on systems with glibc 2.2 (or newer), or when ‘libiconv’ is installed prior to this package. Otherwise ‘\u’ and ‘\U’ will print as-is. Unicode character syntax is useful for writing strings in a locale independent way. For example, a string containing the Euro currency symbol $ env printf '\u20AC 14.95' will be output correctly in all locales supporting the Euro symbol (ISO-8859-15, UTF-8, and others). Similarly, a Chinese string $ env printf '\u4e2d\u6587' will be output correctly in all Chinese locales (GB2312, BIG5, UTF-8, etc). Note that in these examples, the ‘printf’ command has been invoked via ‘env’ to ensure that we run the program found via your shell’s search path, and not a shell alias or a built-in function. For larger strings, you don’t need to look up the hexadecimal code values of each character one by one. ASCII characters mixed with \u escape sequences is also known as the JAVA source file encoding. You can use GNU recode 3.5c (or newer) to convert strings to this encoding. Here is how to convert a piece of text into a shell script which will output this text in a locale-independent way: $ LC_CTYPE=zh_TW.big5 env printf \ '\u4e2d\u6587\n' > sample.txt $ recode BIG5..JAVA < sample.txt \ | sed -e "s|^|env printf '|" -e "s|%|%%|g" -e "s|$|\\\\n'|" \ > sample.sh The only options are a lone ‘--help’ or ‘--version’. *Note Common options::. Options must precede operands. An exit status of zero indicates success, and a nonzero value indicates failure. 15.3 ‘yes’: Print a string until interrupted ============================================ ‘yes’ prints the command line arguments, separated by spaces and followed by a newline, forever until it is killed. If no arguments are given, it prints ‘y’ followed by a newline forever until killed. Upon a write error, ‘yes’ exits with status ‘1’. The only options are a lone ‘--help’ or ‘--version’. To output an argument that begins with ‘-’, precede it with ‘--’, e.g., ‘yes -- --help’. *Note Common options::. 16 Conditions ************* This section describes commands that are primarily useful for their exit status, rather than their output. Thus, they are often used as the condition of shell ‘if’ statements, or as the last command in a pipeline. 16.1 ‘false’: Do nothing, unsuccessfully ======================================== ‘false’ does nothing except return an exit status of 1, meaning “failure”. It can be used as a place holder in shell scripts where an unsuccessful command is needed. In most modern shells, ‘false’ is a built-in command, so when you use ‘false’ in a script, you’re probably using the built-in command, not the one documented here. ‘false’ honors the ‘--help’ and ‘--version’ options. This version of ‘false’ is implemented as a C program, and is thus more secure and faster than a shell script implementation, and may safely be used as a dummy shell for the purpose of disabling accounts. Note that ‘false’ (unlike all other programs documented herein) exits unsuccessfully, even when invoked with ‘--help’ or ‘--version’. Portable programs should not assume that the exit status of ‘false’ is 1, as it is greater than 1 on some non-GNU hosts. 16.2 ‘true’: Do nothing, successfully ===================================== ‘true’ does nothing except return an exit status of 0, meaning “success”. It can be used as a place holder in shell scripts where a successful command is needed, although the shell built-in command ‘:’ (colon) may do the same thing faster. In most modern shells, ‘true’ is a built-in command, so when you use ‘true’ in a script, you’re probably using the built-in command, not the one documented here. ‘true’ honors the ‘--help’ and ‘--version’ options. Note, however, that it is possible to cause ‘true’ to exit with nonzero status: with the ‘--help’ or ‘--version’ option, and with standard output already closed or redirected to a file that evokes an I/O error. For example, using a Bourne-compatible shell: $ ./true --version >&- ./true: write error: Bad file number $ ./true --version > /dev/full ./true: write error: No space left on device This version of ‘true’ is implemented as a C program, and is thus more secure and faster than a shell script implementation, and may safely be used as a dummy shell for the purpose of disabling accounts. 16.3 ‘test’: Check file types and compare values ================================================ ‘test’ returns a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression EXPR. Each part of the expression must be a separate argument. ‘test’ has file status checks, string operators, and numeric comparison operators. ‘test’ has an alternate form that uses opening and closing square brackets instead a leading ‘test’. For example, instead of ‘test -d /’, you can write ‘[ -d / ]’. The square brackets must be separate arguments; for example, ‘[-d /]’ does not have the desired effect. Since ‘test EXPR’ and ‘[ EXPR ]’ have the same meaning, only the former form is discussed below. Synopses: test EXPRESSION test [ EXPRESSION ] [ ] [ OPTION Due to shell aliases and built-in ‘test’ functions, using an unadorned ‘test’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env test ...’) to avoid interference from the shell. If EXPRESSION is omitted, ‘test’ returns false. If EXPRESSION is a single argument, ‘test’ returns false if the argument is null and true otherwise. The argument can be any string, including strings like ‘-d’, ‘-1’, ‘--’, ‘--help’, and ‘--version’ that most other programs would treat as options. To get help and version information, invoke the commands ‘[ --help’ and ‘[ --version’, without the usual closing brackets. *Note Common options::. Exit status: 0 if the expression is true, 1 if the expression is false, 2 if an error occurred. 16.3.1 File type tests ---------------------- These options test for particular types of files. (Everything’s a file, but not all files are the same!) ‘-b FILE’ True if FILE exists and is a block special device. ‘-c FILE’ True if FILE exists and is a character special device. ‘-d FILE’ True if FILE exists and is a directory. ‘-f FILE’ True if FILE exists and is a regular file. ‘-h FILE’ ‘-L FILE’ True if FILE exists and is a symbolic link. Unlike all other file-related tests, this test does not dereference FILE if it is a symbolic link. ‘-p FILE’ True if FILE exists and is a named pipe. ‘-S FILE’ True if FILE exists and is a socket. ‘-t FD’ True if FD is a file descriptor that is associated with a terminal. 16.3.2 Access permission tests ------------------------------ These options test for particular access permissions. ‘-g FILE’ True if FILE exists and has its set-group-ID bit set. ‘-k FILE’ True if FILE exists and has its “sticky” bit set. ‘-r FILE’ True if FILE exists and the user has read access. ‘-u FILE’ True if FILE exists and has its set-user-ID bit set. ‘-w FILE’ True if FILE exists and the user has write access. ‘-x FILE’ True if FILE exists and the user has execute access (or search permission, if it is a directory). ‘-O FILE’ True if FILE exists and is owned by the current effective user ID. ‘-G FILE’ True if FILE exists and is owned by the current effective group ID. 16.3.3 File characteristic tests -------------------------------- These options test other file characteristics. ‘-e FILE’ True if FILE exists. ‘-s FILE’ True if FILE exists and has a size greater than zero. ‘FILE1 -nt FILE2’ True if FILE1 is newer (according to modification date) than FILE2, or if FILE1 exists and FILE2 does not. ‘FILE1 -ot FILE2’ True if FILE1 is older (according to modification date) than FILE2, or if FILE2 exists and FILE1 does not. ‘FILE1 -ef FILE2’ True if FILE1 and FILE2 have the same device and inode numbers, i.e., if they are hard links to each other. ‘-N FILE’ True if FILE exists and has been modified (mtime) since it was last read (atime). 16.3.4 String tests ------------------- These options test string characteristics. You may need to quote STRING arguments for the shell. For example: test -n "$V" The quotes here prevent the wrong arguments from being passed to ‘test’ if ‘$V’ is empty or contains special characters. ‘-z STRING’ True if the length of STRING is zero. ‘-n STRING’ ‘STRING’ True if the length of STRING is nonzero. ‘STRING1 = STRING2’ True if the strings are equal. ‘STRING1 == STRING2’ True if the strings are equal (synonym for =). Note this form is not as portable to other shells and systems. ‘STRING1 != STRING2’ True if the strings are not equal. 16.3.5 Numeric tests -------------------- Numeric relational operators. The arguments must be entirely numeric (possibly negative), or the special expression ‘-l STRING’, which evaluates to the length of STRING. ‘ARG1 -eq ARG2’ ‘ARG1 -ne ARG2’ ‘ARG1 -lt ARG2’ ‘ARG1 -le ARG2’ ‘ARG1 -gt ARG2’ ‘ARG1 -ge ARG2’ These arithmetic binary operators return true if ARG1 is equal, not-equal, less-than, less-than-or-equal, greater-than, or greater-than-or-equal than ARG2, respectively. For example: test -1 -gt -2 && echo yes ⇒ yes test -l abc -gt 1 && echo yes ⇒ yes test 0x100 -eq 1 error→ test: integer expression expected before -eq 16.3.6 Connectives for ‘test’ ----------------------------- Note it’s preferred to use shell logical primitives rather than these logical connectives internal to ‘test’, because an expression may become ambiguous depending on the expansion of its parameters. For example, this becomes ambiguous when ‘$1’ is set to ‘'!'’ and ‘$2’ to the empty string ‘''’: test "$1" -a "$2" and should be written as: test "$1" && test "$2" Note the shell logical primitives also benefit from short circuit operation, which can be significant for file attribute tests. ‘! EXPR’ True if EXPR is false. ‘!’ has lower precedence than all parts of EXPR. Note ‘!’ needs to be specified to the left of a binary expression, I.e., ‘'!' 1 -gt 2’ rather than ‘1 '!' -gt 2’. Also ‘!’ is often a shell special character and is best used quoted. ‘EXPR1 -a EXPR2’ True if both EXPR1 and EXPR2 are true. ‘-a’ is left associative, and has a higher precedence than ‘-o’. ‘EXPR1 -o EXPR2’ True if either EXPR1 or EXPR2 is true. ‘-o’ is left associative. 16.4 ‘expr’: Evaluate expressions ================================= ‘expr’ evaluates an expression and writes the result on standard output. Each token of the expression must be a separate argument. Operands are either integers or strings. Integers consist of one or more decimal digits, with an optional leading ‘-’. ‘expr’ converts anything appearing in an operand position to an integer or a string depending on the operation being applied to it. Strings are not quoted for ‘expr’ itself, though you may need to quote them to protect characters with special meaning to the shell, e.g., spaces. However, regardless of whether it is quoted, a string operand should not be a parenthesis or any of ‘expr’’s operators like ‘+’, so you cannot safely pass an arbitrary string ‘$str’ to expr merely by quoting it to the shell. One way to work around this is to use the GNU extension ‘+’, (e.g., ‘+ "$str" = foo’); a more portable way is to use ‘" $str"’ and to adjust the rest of the expression to take the leading space into account (e.g., ‘" $str" = " foo"’). You should not pass a negative integer or a string with leading ‘-’ as ‘expr’’s first argument, as it might be misinterpreted as an option; this can be avoided by parenthesization. Also, portable scripts should not use a string operand that happens to take the form of an integer; this can be worked around by inserting leading spaces as mentioned above. Operators may be given as infix symbols or prefix keywords. Parentheses may be used for grouping in the usual manner. You must quote parentheses and many operators to avoid the shell evaluating them, however. Because ‘expr’ uses multiple-precision arithmetic, it works with integers wider than those of machine registers. The only options are ‘--help’ and ‘--version’. *Note Common options::. Options must precede operands. Exit status: 0 if the expression is neither null nor 0, 1 if the expression is null or 0, 2 if the expression is invalid, 3 if an internal error occurred (e.g., arithmetic overflow). 16.4.1 String expressions ------------------------- ‘expr’ supports pattern matching and other string operators. These have higher precedence than both the numeric and relational operators (in the next sections). ‘STRING : REGEX’ Perform pattern matching. The arguments are converted to strings and the second is considered to be a (basic, a la GNU ‘grep’) regular expression, with a ‘^’ implicitly prepended. The first argument is then matched against this regular expression. If REGEX does not use ‘\(’ and ‘\)’, the ‘:’ expression returns the number of characters matched, or 0 if the match fails. If REGEX uses ‘\(’ and ‘\)’, the ‘:’ expression returns the part of STRING that matched the subexpression, or the null string if the match failed or the subexpression did not contribute to the match. Only the first ‘\( ... \)’ pair is relevant to the return value; additional pairs are meaningful only for grouping the regular expression operators. In the regular expression, ‘\+’, ‘\?’, and ‘\|’ are operators which respectively match one or more, zero or one, or separate alternatives. These operators are GNU extensions. *Note Regular Expressions: (grep)Regular Expressions, for details of regular expression syntax. Some examples are in *note Examples of expr::. ‘match STRING REGEX’ An alternative way to do pattern matching. This is the same as ‘STRING : REGEX’. ‘substr STRING POSITION LENGTH’ Returns the substring of STRING beginning at POSITION with length at most LENGTH. If either POSITION or LENGTH is negative, zero, or non-numeric, returns the null string. ‘index STRING CHARSET’ Returns the first position in STRING where the first character in CHARSET was found. If no character in CHARSET is found in STRING, return 0. ‘length STRING’ Returns the length of STRING. ‘+ TOKEN’ Interpret TOKEN as a string, even if it is a keyword like MATCH or an operator like ‘/’. This makes it possible to test ‘expr length + "$x"’ or ‘expr + "$x" : '.*/\(.\)'’ and have it do the right thing even if the value of $X happens to be (for example) ‘/’ or ‘index’. This operator is a GNU extension. Portable shell scripts should use ‘" $token" : ' \(.*\)'’ instead of ‘+ "$token"’. To make ‘expr’ interpret keywords as strings, you must use the ‘quote’ operator. 16.4.2 Numeric expressions -------------------------- ‘expr’ supports the usual numeric operators, in order of increasing precedence. These numeric operators have lower precedence than the string operators described in the previous section, and higher precedence than the connectives (next section). ‘+ -’ Addition and subtraction. Both arguments are converted to integers; an error occurs if this cannot be done. ‘* / %’ Multiplication, division, remainder. Both arguments are converted to integers; an error occurs if this cannot be done. 16.4.3 Relations for ‘expr’ --------------------------- ‘expr’ supports the usual logical connectives and relations. These have lower precedence than the string and numeric operators (previous sections). Here is the list, lowest-precedence operator first. ‘|’ Returns its first argument if that is neither null nor zero, otherwise its second argument if it is neither null nor zero, otherwise 0. It does not evaluate its second argument if its first argument is neither null nor zero. ‘&’ Return its first argument if neither argument is null or zero, otherwise 0. It does not evaluate its second argument if its first argument is null or zero. ‘< <= = == != >= >’ Compare the arguments and return 1 if the relation is true, 0 otherwise. ‘==’ is a synonym for ‘=’. ‘expr’ first tries to convert both arguments to integers and do a numeric comparison; if either conversion fails, it does a lexicographic comparison using the character collating sequence specified by the ‘LC_COLLATE’ locale. 16.4.4 Examples of using ‘expr’ ------------------------------- Here are a few examples, including quoting for shell metacharacters. To add 1 to the shell variable ‘foo’, in Bourne-compatible shells: foo=$(expr $foo + 1) To print the non-directory part of the file name stored in ‘$fname’, which need not contain a ‘/’: expr $fname : '.*/\(.*\)' '|' $fname An example showing that ‘\+’ is an operator: expr aaa : 'a\+' ⇒ 3 expr abc : 'a\(.\)c' ⇒ b expr index abcdef cz ⇒ 3 expr index index a error→ expr: syntax error expr index + index a ⇒ 0 17 Redirection ************** Unix shells commonly provide several forms of “redirection” – ways to change the input source or output destination of a command. But one useful redirection is performed by a separate command, not by the shell; it’s described here. 17.1 ‘tee’: Redirect output to multiple files or processes ========================================================== The ‘tee’ command copies standard input to standard output and also to any files given as arguments. This is useful when you want not only to send some data down a pipe, but also to save a copy. Synopsis: tee [OPTION]... [FILE]... If a file being written to does not already exist, it is created. If a file being written to already exists, the data it previously contained is overwritten unless the ‘-a’ option is used. In previous versions of GNU Coreutils (5.3.0–8.23), a FILE of ‘-’ caused ‘tee’ to send another copy of input to standard output. However, as the interleaved output was not very useful, ‘tee’ now conforms to POSIX and treats ‘-’ as a file name. The program accepts the following options. Also see *note Common options::. ‘-a’ ‘--append’ Append standard input to the given files rather than overwriting them. ‘-i’ ‘--ignore-interrupts’ Ignore interrupt signals. ‘-p’ ‘--output-error[=MODE]’ Adjust the behavior with errors on the outputs. In summary ‘-p’ allows ‘tee’ to operate in a more appropriate manner with pipes, and to continue to process data to any remaining outputs, if any pipe outputs exit early. The default operation when ‘--output-error’ is _not_ specified is to exit immediately on error writing to a pipe, and diagnose errors writing to a non-pipe. The long form ‘--output-error’ option supports selection between the following MODEs: ‘warn’ Warn on error opening or writing any output, including pipes. Writing is continued to still open files/pipes. Exit status indicates failure if any output has an error. ‘warn-nopipe’ This is the default MODE when not specified, or when the short form ‘-p’ is used. Warn on error opening or writing any output, except pipes. Writing is continued to still open files/pipes. Exit immediately if all remaining outputs become broken pipes. Exit status indicates failure if any non pipe output had an error. ‘exit’ Exit on error opening or writing any output, including pipes. ‘exit-nopipe’ Exit on error opening or writing any output, except pipes. Exit immediately if all remaining outputs become broken pipes. The ‘tee’ command is useful when you happen to be transferring a large amount of data and also want to summarize that data without reading it a second time. For example, when you are downloading a DVD image, you often want to verify its signature or checksum right away. The inefficient way to do it is simply: wget https://example.com/some.iso && sha1sum some.iso One problem with the above is that it makes you wait for the download to complete before starting the time-consuming SHA1 computation. Perhaps even more importantly, the above requires reading the DVD image a second time (the first was from the network). The efficient way to do it is to interleave the download and SHA1 computation. Then, you’ll get the checksum for free, because the entire process parallelizes so well: # slightly contrived, to demonstrate process substitution wget -O - https://example.com/dvd.iso \ | tee >(sha1sum > dvd.sha1) > dvd.iso That makes ‘tee’ write not just to the expected output file, but also to a pipe running ‘sha1sum’ and saving the final checksum in a file named ‘dvd.sha1’. Note, however, that this example relies on a feature of modern shells called “process substitution” (the ‘>(command)’ syntax, above; *Note Process Substitution: (bash)Process Substitution.), so it works with ‘zsh’, ‘bash’, and ‘ksh’, but not with ‘/bin/sh’. So if you write code like this in a shell script, be sure to start the script with ‘#!/bin/bash’. Note also that if any of the process substitutions (or piped standard output) might exit early without consuming all the data, the ‘-p’ option is needed to allow ‘tee’ to continue to process the input to any remaining outputs. Since the above example writes to one file and one process, a more conventional and portable use of ‘tee’ is even better: wget -O - https://example.com/dvd.iso \ | tee dvd.iso | sha1sum > dvd.sha1 You can extend this example to make ‘tee’ write to two processes, computing MD5 and SHA1 checksums in parallel. In this case, process substitution is required: wget -O - https://example.com/dvd.iso \ | tee >(sha1sum > dvd.sha1) \ >(md5sum > dvd.md5) \ > dvd.iso This technique is also useful when you want to make a _compressed_ copy of the contents of a pipe. Consider a tool to graphically summarize file system usage data from ‘du -ak’. For a large hierarchy, ‘du -ak’ can run for a long time, and can easily produce terabytes of data, so you won’t want to rerun the command unnecessarily. Nor will you want to save the uncompressed output. Doing it the inefficient way, you can’t even start the GUI until after you’ve compressed all of the ‘du’ output: du -ak | gzip -9 > /tmp/du.gz gzip -d /tmp/du.gz | checkspace -a With ‘tee’ and process substitution, you start the GUI right away and eliminate the decompression completely: du -ak | tee >(gzip -9 > /tmp/du.gz) | checkspace -a Finally, if you regularly create more than one type of compressed tarball at once, for example when ‘make dist’ creates both ‘gzip’-compressed and ‘bzip2’-compressed tarballs, there may be a better way. Typical ‘automake’-generated ‘Makefile’ rules create the two compressed tar archives with commands in sequence, like this (slightly simplified): tardir=your-pkg-M.N tar chof - "$tardir" | gzip -9 -c > your-pkg-M.N.tar.gz tar chof - "$tardir" | bzip2 -9 -c > your-pkg-M.N.tar.bz2 However, if the hierarchy you are archiving and compressing is larger than a couple megabytes, and especially if you are using a multi-processor system with plenty of memory, then you can do much better by reading the directory contents only once and running the compression programs in parallel: tardir=your-pkg-M.N tar chof - "$tardir" \ | tee >(gzip -9 -c > your-pkg-M.N.tar.gz) \ | bzip2 -9 -c > your-pkg-M.N.tar.bz2 If you want to further process the output from process substitutions, and those processes write atomically (i.e., write less than the system’s PIPE_BUF size at a time), that’s possible with a construct like: tardir=your-pkg-M.N tar chof - "$tardir" \ | tee >(md5sum --tag) > >(sha256sum --tag) \ | sort | gpg --clearsign > your-pkg-M.N.tar.sig An exit status of zero indicates success, and a nonzero value indicates failure. 18 File name manipulation ************************* This section describes commands that manipulate file names. 18.1 ‘basename’: Strip directory and suffix from a file name ============================================================ ‘basename’ removes any leading directory components from NAME. Synopsis: basename NAME [SUFFIX] basename OPTION... NAME... If SUFFIX is specified and is identical to the end of NAME, it is removed from NAME as well. Note that since trailing slashes are removed prior to suffix matching, SUFFIX will do nothing if it contains slashes. ‘basename’ prints the result on standard output. Together, ‘basename’ and ‘dirname’ are designed such that if ‘ls "$name"’ succeeds, then the command sequence ‘cd "$(dirname "$name")"; ls "$(basename "$name")"’ will, too. This works for everything except file names containing a trailing newline. POSIX allows the implementation to define the results if NAME is empty or ‘//’. In the former case, GNU ‘basename’ returns the empty string. In the latter case, the result is ‘//’ on platforms where // is distinct from /, and ‘/’ on platforms where there is no difference. The program accepts the following options. Also see *note Common options::. Options must precede operands. ‘-a’ ‘--multiple’ Support more than one argument. Treat every argument as a NAME. With this, an optional SUFFIX must be specified using the ‘-s’ option. ‘-s SUFFIX’ ‘--suffix=SUFFIX’ Remove a trailing SUFFIX. This option implies the ‘-a’ option. ‘-z’ ‘--zero’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. An exit status of zero indicates success, and a nonzero value indicates failure. Examples: # Output "sort". basename /usr/bin/sort # Output "stdio". basename include/stdio.h .h # Output "stdio". basename -s .h include/stdio.h # Output "stdio" followed by "stdlib" basename -a -s .h include/stdio.h include/stdlib.h 18.2 ‘dirname’: Strip last file name component ============================================== ‘dirname’ prints all but the final slash-delimited component of each NAME. Slashes on either side of the final component are also removed. If the string contains no slash, ‘dirname’ prints ‘.’ (meaning the current directory). Synopsis: dirname [OPTION] NAME... NAME need not be a file name, but if it is, this operation effectively lists the directory that contains the final component, including the case when the final component is itself a directory. Together, ‘basename’ and ‘dirname’ are designed such that if ‘ls "$name"’ succeeds, then the command sequence ‘cd "$(dirname "$name")"; ls "$(basename "$name")"’ will, too. This works for everything except file names containing a trailing newline. POSIX allows the implementation to define the results if NAME is ‘//’. With GNU ‘dirname’, the result is ‘//’ on platforms where // is distinct from /, and ‘/’ on platforms where there is no difference. The program accepts the following option. Also see *note Common options::. ‘-z’ ‘--zero’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. An exit status of zero indicates success, and a nonzero value indicates failure. Examples: # Output "/usr/bin". dirname /usr/bin/sort dirname /usr/bin//.// # Output "dir1" followed by "dir2" dirname dir1/str dir2/str # Output ".". dirname stdio.h 18.3 ‘pathchk’: Check file name validity and portability ======================================================== ‘pathchk’ checks validity and portability of file names. Synopsis: pathchk [OPTION]... NAME... For each NAME, ‘pathchk’ prints an error message if any of these conditions is true: 1. One of the existing directories in NAME does not have search (execute) permission, 2. The length of NAME is larger than the maximum supported by the operating system. 3. The length of one component of NAME is longer than its file system’s maximum. A nonexistent NAME is not an error, so long as a file with that name could be created under the above conditions. The program accepts the following options. Also see *note Common options::. Options must precede operands. ‘-p’ Instead of performing checks based on the underlying file system, print an error message if any of these conditions is true: 1. A file name is empty. 2. A file name contains a character outside the POSIX portable file name character set, namely, the ASCII letters and digits, ‘.’, ‘_’, ‘-’, and ‘/’. 3. The length of a file name or one of its components exceeds the POSIX minimum limits for portability. ‘-P’ Print an error message if a file name is empty, or if it contains a component that begins with ‘-’. ‘--portability’ Print an error message if a file name is not portable to all POSIX hosts. This option is equivalent to ‘-p -P’. Exit status: 0 if all specified file names passed all checks, 1 otherwise. 18.4 ‘mktemp’: Create temporary file or directory ================================================= ‘mktemp’ manages the creation of temporary files and directories. Synopsis: mktemp [OPTION]... [TEMPLATE] Safely create a temporary file or directory based on TEMPLATE, and print its name. If given, TEMPLATE must include at least three consecutive ‘X’s in the last component. If omitted, the template ‘tmp.XXXXXXXXXX’ is used, and option ‘--tmpdir’ is implied. The final run of ‘X’s in the TEMPLATE will be replaced by alpha-numeric characters; thus, on a case-sensitive file system, and with a TEMPLATE including a run of N instances of ‘X’, there are ‘62**N’ potential file names. Older scripts used to create temporary files by simply joining the name of the program with the process id (‘$$’) as a suffix. However, that naming scheme is easily predictable, and suffers from a race condition where the attacker can create an appropriately named symbolic link, such that when the script then opens a handle to what it thought was an unused file, it is instead modifying an existing file. Using the same scheme to create a directory is slightly safer, since the ‘mkdir’ will fail if the target already exists, but it is still inferior because it allows for denial of service attacks. Therefore, modern scripts should use the ‘mktemp’ command to guarantee that the generated name will be unpredictable, and that knowledge of the temporary file name implies that the file was created by the current script and cannot be modified by other users. When creating a file, the resulting file has read and write permissions for the current user, but no permissions for the group or others; these permissions are reduced if the current umask is more restrictive. Here are some examples (although note that if you repeat them, you will most likely get different file names): • Create a temporary file in the current directory. $ mktemp file.XXXX file.H47c • Create a temporary file with a known suffix. $ mktemp --suffix=.txt file-XXXX file-H08W.txt $ mktemp file-XXXX-XXXX.txt file-XXXX-eI9L.txt • Create a secure fifo relative to the user’s choice of ‘TMPDIR’, but falling back to the current directory rather than ‘/tmp’. Note that ‘mktemp’ does not create fifos, but can create a secure directory in which the fifo can live. Exit the shell if the directory or fifo could not be created. $ dir=$(mktemp -p "${TMPDIR:-.}" -d dir-XXXX) || exit 1 $ fifo=$dir/fifo $ mkfifo "$fifo" || { rmdir "$dir"; exit 1; } • Create and use a temporary file if possible, but ignore failure. The file will reside in the directory named by ‘TMPDIR’, if specified, or else in ‘/tmp’. $ file=$(mktemp -q) && { > # Safe to use $file only within this block. Use quotes, > # since $TMPDIR, and thus $file, may contain whitespace. > echo ... > "$file" > rm "$file" > } • Act as a semi-random character generator (it is not fully random, since it is impacted by the contents of the current directory). To avoid security holes, do not use the resulting names to create a file. $ mktemp -u XXX Gb9 $ mktemp -u XXX nzC The program accepts the following options. Also see *note Common options::. ‘-d’ ‘--directory’ Create a directory rather than a file. The directory will have read, write, and search permissions for the current user, but no permissions for the group or others; these permissions are reduced if the current umask is more restrictive. ‘-q’ ‘--quiet’ Suppress diagnostics about failure to create a file or directory. The exit status will still reflect whether a file was created. ‘-u’ ‘--dry-run’ Generate a temporary name that does not name an existing file, without changing the file system contents. Using the output of this command to create a new file is inherently unsafe, as there is a window of time between generating the name and using it where another process can create an object by the same name. ‘-p DIR’ ‘--tmpdir[=DIR]’ Treat TEMPLATE relative to the directory DIR. If DIR is not specified (only possible with the long option ‘--tmpdir’) or is the empty string, use the value of ‘TMPDIR’ if available, otherwise use ‘/tmp’. If this is specified, TEMPLATE must not be absolute. However, TEMPLATE can still contain slashes, although intermediate directories must already exist. ‘--suffix=SUFFIX’ Append SUFFIX to the TEMPLATE. SUFFIX must not contain slash. If ‘--suffix’ is specified, TEMPLATE must end in ‘X’; if it is not specified, then an appropriate ‘--suffix’ is inferred by finding the last ‘X’ in TEMPLATE. This option exists for use with the default TEMPLATE and for the creation of a SUFFIX that starts with ‘X’. ‘-t’ Treat TEMPLATE as a single file relative to the value of ‘TMPDIR’ if available, or to the directory specified by ‘-p’, otherwise to ‘/tmp’. TEMPLATE must not contain slashes. This option is deprecated; the use of ‘-p’ without ‘-t’ offers better defaults (by favoring the command line over ‘TMPDIR’) and more flexibility (by allowing intermediate directories). Exit status: 0 if the file was created, 1 otherwise. 18.5 ‘realpath’: Print the resolved file name. ============================================== ‘realpath’ expands all symbolic links and resolves references to ‘/./’, ‘/../’ and extra ‘/’ characters. By default, all but the last component of the specified files must exist. Synopsis: realpath [OPTION]... FILE... The file name canonicalization functionality overlaps with that of the ‘readlink’ command. This is the preferred command for canonicalization as it’s a more suitable and standard name. In addition this command supports relative file name processing functionality. The program accepts the following options. Also see *note Common options::. ‘-e’ ‘--canonicalize-existing’ Ensure that all components of the specified file names exist. If any component is missing or unavailable, ‘realpath’ will output a diagnostic unless the ‘-q’ option is specified, and exit with a nonzero exit code. A trailing slash requires that the name resolve to a directory. ‘-m’ ‘--canonicalize-missing’ If any component of a specified file name is missing or unavailable, treat it as a directory. ‘-L’ ‘--logical’ Symbolic links are resolved in the specified file names, but they are resolved after any subsequent ‘..’ components are processed. ‘-P’ ‘--physical’ Symbolic links are resolved in the specified file names, and they are resolved before any subsequent ‘..’ components are processed. This is the default mode of operation. ‘-q’ ‘--quiet’ Suppress diagnostic messages for specified file names. ‘--relative-to=DIR’ Print the resolved file names relative to the specified directory. Note this option honors the ‘-m’ and ‘-e’ options pertaining to file existence. ‘--relative-base=DIR’ Print the resolved file names as relative _if_ the files are descendants of DIR. Otherwise, print the resolved file names as absolute. Note this option honors the ‘-m’ and ‘-e’ options pertaining to file existence. For details about combining ‘--relative-to’ and ‘--relative-base’, *note Realpath usage examples::. ‘-s’ ‘--strip’ ‘--no-symlinks’ Do not resolve symbolic links. Only resolve references to ‘/./’, ‘/../’ and remove extra ‘/’ characters. When combined with the ‘-m’ option, realpath operates only on the file name, and does not touch any actual file. ‘-z’ ‘--zero’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. Exit status: 0 if all file names were printed without issue. 1 otherwise. 18.5.1 Realpath usage examples ------------------------------ By default, ‘realpath’ prints the absolute file name of given files (symlinks are resolved, ‘words’ is resolved to ‘american-english’): cd /home/user realpath /usr/bin/sort /tmp/foo /usr/share/dict/words 1.txt ⇒ /usr/bin/sort ⇒ /tmp/foo ⇒ /usr/share/dict/american-english ⇒ /home/user/1.txt With ‘--relative-to’, file names are printed relative to the given directory: realpath --relative-to=/usr/bin \ /usr/bin/sort /tmp/foo /usr/share/dict/words 1.txt ⇒ sort ⇒ ../../tmp/foo ⇒ ../share/dict/american-english ⇒ ../../home/user/1.txt With ‘--relative-base’, relative file names are printed _if_ the resolved file name is below the given base directory. For files outside the base directory absolute file names are printed: realpath --relative-base=/usr \ /usr/bin/sort /tmp/foo /usr/share/dict/words 1.txt ⇒ bin/sort ⇒ /tmp/foo ⇒ share/dict/american-english ⇒ /home/user/1.txt When both ‘--relative-to=DIR1’ and ‘--relative-base=DIR2’ are used, file names are printed relative to DIR1 _if_ they are located below DIR2. If the files are not below DIR2, they are printed as absolute file names: realpath --relative-to=/usr/bin --relative-base=/usr \ /usr/bin/sort /tmp/foo /usr/share/dict/words 1.txt ⇒ sort ⇒ /tmp/foo ⇒ ../share/dict/american-english ⇒ /home/user/1.txt When both ‘--relative-to=DIR1’ and ‘--relative-base=DIR2’ are used, DIR1 _must_ be a subdirectory of DIR2. Otherwise, ‘realpath’ prints absolutes file names. 19 Working context ****************** This section describes commands that display or alter the context in which you are working: the current directory, the terminal settings, and so forth. See also the user-related commands in the next section. 19.1 ‘pwd’: Print working directory =================================== ‘pwd’ prints the name of the current directory. Synopsis: pwd [OPTION]... The program accepts the following options. Also see *note Common options::. ‘-L’ ‘--logical’ If the contents of the environment variable ‘PWD’ provide an absolute name of the current directory with no ‘.’ or ‘..’ components, but possibly with symbolic links, then output those contents. Otherwise, fall back to default ‘-P’ handling. ‘-P’ ‘--physical’ Print a fully resolved name for the current directory. That is, all components of the printed name will be actual directory names – none will be symbolic links. If ‘-L’ and ‘-P’ are both given, the last one takes precedence. If neither option is given, then this implementation uses ‘-P’ as the default unless the ‘POSIXLY_CORRECT’ environment variable is set. Due to shell aliases and built-in ‘pwd’ functions, using an unadorned ‘pwd’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env pwd ...’) to avoid interference from the shell. An exit status of zero indicates success, and a nonzero value indicates failure. 19.2 ‘stty’: Print or change terminal characteristics ===================================================== ‘stty’ prints or changes terminal characteristics, such as baud rate. Synopses: stty [OPTION] [SETTING]... stty [OPTION] If given no line settings, ‘stty’ prints the baud rate, line discipline number (on systems that support it), and line settings that have been changed from the values set by ‘stty sane’. By default, mode reading and setting are performed on the tty line connected to standard input, although this can be modified by the ‘--file’ option. ‘stty’ accepts many non-option arguments that change aspects of the terminal line operation, as described below. The program accepts the following options. Also see *note Common options::. ‘-a’ ‘--all’ Print all current settings in human-readable form. This option may not be used in combination with any line settings. ‘-F DEVICE’ ‘--file=DEVICE’ Set the line opened by the file name specified in DEVICE instead of the tty line connected to standard input. This option is necessary because opening a POSIX tty requires use of the ‘O_NONDELAY’ flag to prevent a POSIX tty from blocking until the carrier detect line is high if the ‘clocal’ flag is not set. Hence, it is not always possible to allow the shell to open the device in the traditional manner. ‘-g’ ‘--save’ Print all current settings in a form that can be used as an argument to another ‘stty’ command to restore the current settings. This option may not be used in combination with any line settings. Many settings can be turned off by preceding them with a ‘-’. Such arguments are marked below with “May be negated” in their description. The descriptions themselves refer to the positive case, that is, when _not_ negated (unless stated otherwise, of course). Some settings are not available on all POSIX systems, since they use extensions. Such arguments are marked below with “Non-POSIX” in their description. On non-POSIX systems, those or other settings also may not be available, but it’s not feasible to document all the variations: just try it and see. ‘stty’ is installed only on platforms with the POSIX terminal interface, so portable scripts should not rely on its existence on non-POSIX platforms. An exit status of zero indicates success, and a nonzero value indicates failure. 19.2.1 Control settings ----------------------- Control settings: ‘parenb’ Generate parity bit in output and expect parity bit in input. May be negated. ‘parodd’ Set odd parity (even if negated). May be negated. ‘cmspar’ Use "stick" (mark/space) parity. If parodd is set, the parity bit is always 1; if parodd is not set, the parity bit is always zero. Non-POSIX. May be negated. ‘cs5’ ‘cs6’ ‘cs7’ ‘cs8’ Set character size to 5, 6, 7, or 8 bits. ‘hup’ ‘hupcl’ Send a hangup signal when the last process closes the tty. May be negated. ‘cstopb’ Use two stop bits per character (one if negated). May be negated. ‘cread’ Allow input to be received. May be negated. ‘clocal’ Disable modem control signals. May be negated. ‘crtscts’ Enable RTS/CTS flow control. Non-POSIX. May be negated. ‘cdtrdsr’ Enable DTR/DSR flow control. Non-POSIX. May be negated. 19.2.2 Input settings --------------------- These settings control operations on data received from the terminal. ‘ignbrk’ Ignore break characters. May be negated. ‘brkint’ Make breaks cause an interrupt signal. May be negated. ‘ignpar’ Ignore characters with parity errors. May be negated. ‘parmrk’ Mark parity errors (with a 255-0-character sequence). May be negated. ‘inpck’ Enable input parity checking. May be negated. ‘istrip’ Clear high (8th) bit of input characters. May be negated. ‘inlcr’ Translate newline to carriage return. May be negated. ‘igncr’ Ignore carriage return. May be negated. ‘icrnl’ Translate carriage return to newline. May be negated. ‘iutf8’ Assume input characters are UTF-8 encoded. May be negated. ‘ixon’ Enable XON/XOFF flow control (that is, ‘Ctrl-S’/‘Ctrl-Q’). May be negated. ‘ixoff’ ‘tandem’ Enable sending of ‘stop’ character when the system input buffer is almost full, and ‘start’ character when it becomes almost empty again. May be negated. ‘iuclc’ Translate uppercase characters to lowercase. Non-POSIX. May be negated. Note ilcuc is not implemented, as one would not be able to issue almost any (lowercase) Unix command, after invoking it. ‘ixany’ Allow any character to restart output (only the start character if negated). Non-POSIX. May be negated. ‘imaxbel’ Enable beeping and not flushing input buffer if a character arrives when the input buffer is full. Non-POSIX. May be negated. 19.2.3 Output settings ---------------------- These settings control operations on data sent to the terminal. ‘opost’ Postprocess output. May be negated. ‘olcuc’ Translate lowercase characters to uppercase. Non-POSIX. May be negated. (Note ouclc is not currently implemented.) ‘ocrnl’ Translate carriage return to newline. Non-POSIX. May be negated. ‘onlcr’ Translate newline to carriage return-newline. Non-POSIX. May be negated. ‘onocr’ Do not print carriage returns in the first column. Non-POSIX. May be negated. ‘onlret’ Newline performs a carriage return. Non-POSIX. May be negated. ‘ofill’ Use fill (padding) characters instead of timing for delays. Non-POSIX. May be negated. ‘ofdel’ Use ASCII DEL characters for fill instead of ASCII NUL characters. Non-POSIX. May be negated. ‘nl1’ ‘nl0’ Newline delay style. Non-POSIX. ‘cr3’ ‘cr2’ ‘cr1’ ‘cr0’ Carriage return delay style. Non-POSIX. ‘tab3’ ‘tab2’ ‘tab1’ ‘tab0’ Horizontal tab delay style. Non-POSIX. ‘bs1’ ‘bs0’ Backspace delay style. Non-POSIX. ‘vt1’ ‘vt0’ Vertical tab delay style. Non-POSIX. ‘ff1’ ‘ff0’ Form feed delay style. Non-POSIX. 19.2.4 Local settings --------------------- ‘isig’ Enable ‘interrupt’, ‘quit’, and ‘suspend’ special characters. May be negated. ‘icanon’ Enable ‘erase’, ‘kill’, ‘werase’, and ‘rprnt’ special characters. May be negated. ‘iexten’ Enable non-POSIX special characters. May be negated. ‘echo’ Echo input characters. May be negated. ‘echoe’ ‘crterase’ Echo ‘erase’ characters as backspace-space-backspace. May be negated. ‘echok’ Echo a newline after a ‘kill’ character. May be negated. ‘echonl’ Echo newline even if not echoing other characters. May be negated. ‘noflsh’ Disable flushing after ‘interrupt’ and ‘quit’ special characters. May be negated. ‘xcase’ Enable input and output of uppercase characters by preceding their lowercase equivalents with ‘\’, when ‘icanon’ is set. Non-POSIX. May be negated. ‘tostop’ Stop background jobs that try to write to the terminal. Non-POSIX. May be negated. ‘echoprt’ ‘prterase’ Echo erased characters backward, between ‘\’ and ‘/’. Non-POSIX. May be negated. ‘echoctl’ ‘ctlecho’ Echo control characters in hat notation (‘^C’) instead of literally. Non-POSIX. May be negated. ‘echoke’ ‘crtkill’ Echo the ‘kill’ special character by erasing each character on the line as indicated by the ‘echoprt’ and ‘echoe’ settings, instead of by the ‘echoctl’ and ‘echok’ settings. Non-POSIX. May be negated. ‘extproc’ Enable ‘LINEMODE’, which is used to avoid echoing each character over high latency links. See also Internet RFC 1116 (https://tools.ietf.org/search/rfc1116). Non-POSIX. May be negated. ‘flusho’ Discard output. Note this setting is currently ignored on GNU/Linux systems. Non-POSIX. May be negated. 19.2.5 Combination settings --------------------------- Combination settings: ‘evenp’ ‘parity’ Same as ‘parenb -parodd cs7’. May be negated. If negated, same as ‘-parenb cs8’. ‘oddp’ Same as ‘parenb parodd cs7’. May be negated. If negated, same as ‘-parenb cs8’. ‘nl’ Same as ‘-icrnl -onlcr’. May be negated. If negated, same as ‘icrnl -inlcr -igncr onlcr -ocrnl -onlret’. ‘ek’ Reset the ‘erase’ and ‘kill’ special characters to their default values. ‘sane’ Same as: cread -ignbrk brkint -inlcr -igncr icrnl icanon iexten echo echoe echok -echonl -noflsh -ixoff -iutf8 -iuclc -ixany imaxbel -xcase -olcuc -ocrnl opost -ofill onlcr -onocr -onlret nl0 cr0 tab0 bs0 vt0 ff0 isig -tostop -ofdel -echoprt echoctl echoke -extproc and also sets all special characters to their default values. ‘cooked’ Same as ‘brkint ignpar istrip icrnl ixon opost isig icanon’, plus sets the ‘eof’ and ‘eol’ characters to their default values if they are the same as the ‘min’ and ‘time’ characters. May be negated. If negated, same as ‘raw’. ‘raw’ Same as: -ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -icanon -opost -isig -iuclc -ixany -imaxbel -xcase min 1 time 0 May be negated. If negated, same as ‘cooked’. ‘cbreak’ Same as ‘-icanon’. May be negated. If negated, same as ‘icanon’. ‘pass8’ Same as ‘-parenb -istrip cs8’. May be negated. If negated, same as ‘parenb istrip cs7’. ‘litout’ Same as ‘-parenb -istrip -opost cs8’. May be negated. If negated, same as ‘parenb istrip opost cs7’. ‘decctlq’ Same as ‘-ixany’. Non-POSIX. May be negated. ‘tabs’ Same as ‘tab0’. Non-POSIX. May be negated. If negated, same as ‘tab3’. ‘lcase’ ‘LCASE’ Same as ‘xcase iuclc olcuc’. Non-POSIX. May be negated. (Used for terminals with uppercase characters only.) ‘crt’ Same as ‘echoe echoctl echoke’. ‘dec’ Same as ‘echoe echoctl echoke -ixany intr ^C erase ^? kill C-u’. 19.2.6 Special characters ------------------------- The special characters’ default values vary from system to system. They are set with the syntax ‘name value’, where the names are listed below and the value can be given either literally, in hat notation (‘^C’), or as an integer which may start with ‘0x’ to indicate hexadecimal, ‘0’ to indicate octal, or any other digit to indicate decimal. For GNU stty, giving a value of ‘^-’ or ‘undef’ disables that special character. (This is incompatible with Ultrix ‘stty’, which uses a value of ‘u’ to disable a special character. GNU ‘stty’ treats a value ‘u’ like any other, namely to set that special character to .) ‘intr’ Send an interrupt signal. ‘quit’ Send a quit signal. ‘erase’ Erase the last character typed. ‘kill’ Erase the current line. ‘eof’ Send an end of file (terminate the input). ‘eol’ End the line. ‘eol2’ Alternate character to end the line. Non-POSIX. ‘discard’ Alternate character to toggle discarding of output. Non-POSIX. ‘swtch’ Switch to a different shell layer. Non-POSIX. ‘status’ Send an info signal. Not currently supported on GNU/Linux. Non-POSIX. ‘start’ Restart the output after stopping it. ‘stop’ Stop the output. ‘susp’ Send a terminal stop signal. ‘dsusp’ Send a terminal stop signal after flushing the input. Non-POSIX. ‘rprnt’ Redraw the current line. Non-POSIX. ‘werase’ Erase the last word typed. Non-POSIX. ‘lnext’ Enter the next character typed literally, even if it is a special character. Non-POSIX. 19.2.7 Special settings ----------------------- ‘min N’ Set the minimum number of characters that will satisfy a read until the time value has expired, when ‘-icanon’ is set. ‘time N’ Set the number of tenths of a second before reads time out if the minimum number of characters have not been read, when ‘-icanon’ is set. ‘ispeed N’ Set the input speed to N. ‘ospeed N’ Set the output speed to N. ‘rows N’ Tell the tty kernel driver that the terminal has N rows. Non-POSIX. ‘cols N’ ‘columns N’ Tell the kernel that the terminal has N columns. Non-POSIX. ‘drain’ Apply settings after first waiting for pending output to be transmitted. This is enabled by default for GNU ‘stty’. Note this is treated as an option rather than a line setting, and will follow the option processing rules described in the summary above. It is useful to disable this option in cases where the system may be in a state where serial transmission is not possible. For example, if the system has received the ‘DC3’ character with ‘ixon’ (software flow control) enabled, then ‘stty’ would block without ‘-drain’ being specified. May be negated. Non-POSIX. ‘size’ Print the number of rows and columns that the kernel thinks the terminal has. (Systems that don’t support rows and columns in the kernel typically use the environment variables ‘LINES’ and ‘COLUMNS’ instead; however, GNU ‘stty’ does not know anything about them.) Non-POSIX. ‘line N’ Use line discipline N. Non-POSIX. ‘speed’ Print the terminal speed. ‘N’ Set the input and output speeds to N. N can be one of: 0 50 75 110 134 134.5 150 200 300 600 1200 1800 2400 4800 9600 19200 38400 ‘exta’ ‘extb’. ‘exta’ is the same as 19200; ‘extb’ is the same as 38400. Many systems, including GNU/Linux, support higher speeds. The ‘stty’ command includes support for speeds of 57600, 115200, 230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000, or 4000000 where the system supports these. 0 hangs up the line if ‘-clocal’ is set. 19.3 ‘printenv’: Print all or some environment variables ======================================================== ‘printenv’ prints environment variable values. Synopsis: printenv [OPTION] [VARIABLE]... If no VARIABLEs are specified, ‘printenv’ prints the value of every environment variable. Otherwise, it prints the value of each VARIABLE that is set, and nothing for those that are not set. The program accepts the following option. Also see *note Common options::. ‘-0’ ‘--null’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. Exit status: 0 if all variables specified were found 1 if at least one specified variable was not found 2 if a write error occurred 19.4 ‘tty’: Print file name of terminal on standard input ========================================================= ‘tty’ prints the file name of the terminal connected to its standard input. It prints ‘not a tty’ if standard input is not a terminal. Synopsis: tty [OPTION]... The program accepts the following option. Also see *note Common options::. ‘-s’ ‘--silent’ ‘--quiet’ Print nothing; only return an exit status. Exit status: 0 if standard input is a terminal 1 if standard input is a non-terminal file 2 if given incorrect arguments 3 if a write error occurs 20 User information ******************* This section describes commands that print user-related information: logins, groups, and so forth. 20.1 ‘id’: Print user identity ============================== ‘id’ prints information about the given user, or the process running it if no user is specified. Synopsis: id [OPTION]... [USER]... USER can be either a user ID or a name, with name look-up taking precedence unless the ID is specified with a leading ‘+’. *Note Disambiguating names and IDs::. By default, it prints the real user ID, real group ID, effective user ID if different from the real user ID, effective group ID if different from the real group ID, and supplemental group IDs. In addition, if SELinux is enabled and the ‘POSIXLY_CORRECT’ environment variable is not set, then print ‘context=C’, where C is the security context. Each of these numeric values is preceded by an identifying string and followed by the corresponding user or group name in parentheses. The options cause ‘id’ to print only part of the above information. Also see *note Common options::. ‘-g’ ‘--group’ Print only the group ID. ‘-G’ ‘--groups’ Print only the group ID and the supplementary groups. ‘-n’ ‘--name’ Print the user or group name instead of the ID number. Requires ‘-u’, ‘-g’, or ‘-G’. ‘-r’ ‘--real’ Print the real, instead of effective, user or group ID. Requires ‘-u’, ‘-g’, or ‘-G’. ‘-u’ ‘--user’ Print only the user ID. ‘-Z’ ‘--context’ Print only the security context of the process, which is generally the user’s security context inherited from the parent process. If neither SELinux or SMACK is enabled then print a warning and set the exit status to 1. ‘-z’ ‘--zero’ Delimit output items with ASCII NUL characters. This option is not permitted when using the default format. When multiple users are specified, and the ‘--groups’ option is also in effect, groups are delimited with a single NUL character, while users are delimited with two NUL characters. Example: $ id -Gn --zero users devs Primary and supplementary groups for a process are normally inherited from its parent and are usually unchanged since login. This means that if you change the group database after logging in, ‘id’ will not reflect your changes within your existing login session. Running ‘id’ with a user argument causes the user and group database to be consulted afresh, and so will give a different result. An exit status of zero indicates success, and a nonzero value indicates failure. 20.2 ‘logname’: Print current login name ======================================== ‘logname’ prints the calling user’s name, as found in a system-maintained file (often ‘/var/run/utmp’ or ‘/etc/utmp’), and exits with a status of 0. If there is no entry for the calling process, ‘logname’ prints an error message and exits with a status of 1. The only options are ‘--help’ and ‘--version’. *Note Common options::. An exit status of zero indicates success, and a nonzero value indicates failure. 20.3 ‘whoami’: Print effective user name ======================================== ‘whoami’ prints the user name associated with the current effective user ID. It is equivalent to the command ‘id -un’. The only options are ‘--help’ and ‘--version’. *Note Common options::. An exit status of zero indicates success, and a nonzero value indicates failure. 20.4 ‘groups’: Print group names a user is in ============================================= ‘groups’ prints the names of the primary and any supplementary groups for each given USERNAME, or the current process if no names are given. If more than one name is given, the name of each user is printed before the list of that user’s groups and the user name is separated from the group list by a colon. Synopsis: groups [USERNAME]... The group lists are equivalent to the output of the command ‘id -Gn’. The only options are ‘--help’ and ‘--version’. *Note Common options::. Primary and supplementary groups for a process are normally inherited from its parent and are usually unchanged since login. This means that if you change the group database after logging in, ‘groups’ will not reflect your changes within your existing login session. Running ‘groups’ with a list of users causes the user and group database to be consulted afresh, and so will give a different result. An exit status of zero indicates success, and a nonzero value indicates failure. 20.5 ‘users’: Print login names of users currently logged in ============================================================ ‘users’ prints on a single line a blank-separated list of user names of users currently logged in to the current host. Each user name corresponds to a login session, so if a user has more than one login session, that user’s name will appear the same number of times in the output. Synopsis: users [FILE] With no FILE argument, ‘users’ extracts its information from a system-maintained file (often ‘/var/run/utmp’ or ‘/etc/utmp’). If a file argument is given, ‘users’ uses that file instead. A common choice is ‘/var/log/wtmp’. The only options are ‘--help’ and ‘--version’. *Note Common options::. The ‘users’ command is installed only on platforms with the POSIX ‘’ include file or equivalent, so portable scripts should not rely on its existence on non-POSIX platforms. An exit status of zero indicates success, and a nonzero value indicates failure. 20.6 ‘who’: Print who is currently logged in ============================================ ‘who’ prints information about users who are currently logged on. Synopsis: who [OPTION] [FILE] [am i] If given no non-option arguments, ‘who’ prints the following information for each user currently logged on: login name, terminal line, login time, and remote hostname or X display. If given one non-option argument, ‘who’ uses that instead of a default system-maintained file (often ‘/var/run/utmp’ or ‘/etc/utmp’) as the name of the file containing the record of users logged on. ‘/var/log/wtmp’ is commonly given as an argument to ‘who’ to look at who has previously logged on. If given two non-option arguments, ‘who’ prints only the entry for the user running it (determined from its standard input), preceded by the hostname. Traditionally, the two arguments given are ‘am i’, as in ‘who am i’. Timestamps are listed according to the time zone rules specified by the ‘TZ’ environment variable, or by the system default rules if ‘TZ’ is not set. *Note Specifying the Time Zone with ‘TZ’: (libc)TZ Variable. The program accepts the following options. Also see *note Common options::. ‘-a’ ‘--all’ Same as ‘-b -d --login -p -r -t -T -u’. ‘-b’ ‘--boot’ Print the date and time of last system boot. ‘-d’ ‘--dead’ Print information corresponding to dead processes. ‘-H’ ‘--heading’ Print a line of column headings. ‘-l’ ‘--login’ List only the entries that correspond to processes via which the system is waiting for a user to login. The user name is always ‘LOGIN’. ‘--lookup’ Attempt to canonicalize hostnames found in utmp through a DNS lookup. This is not the default because it can cause significant delays on systems with automatic dial-up internet access. ‘-m’ Same as ‘who am i’. ‘-p’ ‘--process’ List active processes spawned by init. ‘-q’ ‘--count’ Print only the login names and the number of users logged on. Overrides all other options. ‘-r’ ‘--runlevel’ Print the current (and maybe previous) run-level of the init process. ‘-s’ Ignored; for compatibility with other versions of ‘who’. ‘-t’ ‘--time’ Print last system clock change. ‘-u’ After the login time, print the number of hours and minutes that the user has been idle. ‘.’ means the user was active in the last minute. ‘old’ means the user has been idle for more than 24 hours. ‘-w’ ‘-T’ ‘--mesg’ ‘--message’ ‘--writable’ After each login name print a character indicating the user’s message status: ‘+’ allowing ‘write’ messages ‘-’ disallowing ‘write’ messages ‘?’ cannot find terminal device The ‘who’ command is installed only on platforms with the POSIX ‘’ include file or equivalent, so portable scripts should not rely on its existence on non-POSIX platforms. An exit status of zero indicates success, and a nonzero value indicates failure. 21 System context ***************** This section describes commands that print or change system-wide information. 21.1 ‘date’: Print or set system date and time ============================================== Synopses: date [OPTION]... [+FORMAT] date [-u|--utc|--universal] [ MMDDhhmm[[CC]YY][.ss] ] The ‘date’ command displays the date and time. With the ‘--set’ (‘-s’) option, or with ‘MMDDhhmm[[CC]YY][.ss]’, it sets the date and time. Invoking ‘date’ with no FORMAT argument is equivalent to invoking it with a default format that depends on the ‘LC_TIME’ locale category. In the default C locale, this format is ‘'+%a %b %e %H:%M:%S %Z %Y'’, so the output looks like ‘Thu Jul 9 17:00:00 EDT 2020’. Normally, ‘date’ uses the time zone rules indicated by the ‘TZ’ environment variable, or the system default rules if ‘TZ’ is not set. *Note Specifying the Time Zone with ‘TZ’: (libc)TZ Variable. An exit status of zero indicates success, and a nonzero value indicates failure. 21.1.1 Specifying the format of ‘date’ output --------------------------------------------- If given an argument that starts with a ‘+’, ‘date’ prints the current date and time (or the date and time specified by the ‘--date’ option, see below) in the format defined by that argument, which is similar to that of the ‘strftime’ function. Except for conversion specifiers, which start with ‘%’, characters in the format string are printed unchanged. The conversion specifiers are described below. 21.1.1.1 Time conversion specifiers ................................... ‘date’ conversion specifiers related to times. ‘%H’ hour (‘00’...‘23’) ‘%I’ hour (‘01’...‘12’) ‘%k’ hour, space padded (‘ 0’...‘23’); equivalent to ‘%_H’. This is a GNU extension. ‘%l’ hour, space padded (‘ 1’...‘12’); equivalent to ‘%_I’. This is a GNU extension. ‘%M’ minute (‘00’...‘59’) ‘%N’ nanoseconds (‘000000000’...‘999999999’). This is a GNU extension. ‘%p’ locale’s equivalent of either ‘AM’ or ‘PM’; blank in many locales. Noon is treated as ‘PM’ and midnight as ‘AM’. ‘%P’ like ‘%p’, except lower case. This is a GNU extension. ‘%r’ locale’s 12-hour clock time (e.g., ‘11:11:04 PM’) ‘%R’ 24-hour hour and minute. Same as ‘%H:%M’. ‘%s’ seconds since the Epoch, i.e., since 1970-01-01 00:00 UTC. Leap seconds are not counted unless leap second support is available. *Note %s-examples::, for examples. This is a GNU extension. ‘%S’ second (‘00’...‘60’). This may be ‘60’ if leap seconds are supported. ‘%T’ 24-hour hour, minute, and second. Same as ‘%H:%M:%S’. ‘%X’ locale’s time representation (e.g., ‘23:13:48’) ‘%z’ Four-digit numeric time zone, e.g., ‘-0600’ or ‘+0530’, or ‘-0000’ if no time zone is determinable. This value reflects the numeric time zone appropriate for the current time, using the time zone rules specified by the ‘TZ’ environment variable. A time zone is not determinable if its numeric offset is zero and its abbreviation begins with ‘-’. The time (and optionally, the time zone rules) can be overridden by the ‘--date’ option. ‘%:z’ Numeric time zone with ‘:’, e.g., ‘-06:00’ or ‘+05:30’), or ‘-00:00’ if no time zone is determinable. This is a GNU extension. ‘%::z’ Numeric time zone to the nearest second with ‘:’ (e.g., ‘-06:00:00’ or ‘+05:30:00’), or ‘-00:00:00’ if no time zone is determinable. This is a GNU extension. ‘%:::z’ Numeric time zone with ‘:’ using the minimum necessary precision (e.g., ‘-06’, ‘+05:30’, or ‘-04:56:02’), or ‘-00’ if no time zone is determinable. This is a GNU extension. ‘%Z’ alphabetic time zone abbreviation (e.g., ‘EDT’), or nothing if no time zone is determinable. See ‘%z’ for how it is determined. 21.1.1.2 Date conversion specifiers ................................... ‘date’ conversion specifiers related to dates. ‘%a’ locale’s abbreviated weekday name (e.g., ‘Sun’) ‘%A’ locale’s full weekday name, variable length (e.g., ‘Sunday’) ‘%b’ locale’s abbreviated month name (e.g., ‘Jan’) ‘%B’ locale’s full month name, variable length (e.g., ‘January’) ‘%c’ locale’s date and time (e.g., ‘Thu Mar 3 23:05:25 2020’) ‘%C’ century. This is like ‘%Y’, except the last two digits are omitted. For example, it is ‘20’ if ‘%Y’ is ‘2019’, and is ‘-0’ if ‘%Y’ is ‘-001’. It is normally at least two characters, but it may be more. ‘%d’ day of month (e.g., ‘01’) ‘%D’ date; same as ‘%m/%d/%y’ ‘%e’ day of month, space padded; same as ‘%_d’ ‘%F’ full date in ISO 8601 format; like ‘%+4Y-%m-%d’ except that any flags or field width override the ‘+’ and (after subtracting 6) the ‘4’. This is a good choice for a date format, as it is standard and is easy to sort in the usual case where years are in the range 0000...9999. ‘%g’ year corresponding to the ISO week number, but without the century (range ‘00’ through ‘99’). This has the same format and value as ‘%y’, except that if the ISO week number (see ‘%V’) belongs to the previous or next year, that year is used instead. ‘%G’ year corresponding to the ISO week number. This has the same format and value as ‘%Y’, except that if the ISO week number (see ‘%V’) belongs to the previous or next year, that year is used instead. It is normally useful only if ‘%V’ is also used; for example, the format ‘%G-%m-%d’ is probably a mistake, since it combines the ISO week number year with the conventional month and day. ‘%h’ same as ‘%b’ ‘%j’ day of year (‘001’...‘366’) ‘%m’ month (‘01’...‘12’) ‘%q’ quarter of year (‘1’...‘4’) ‘%u’ day of week (‘1’...‘7’) with ‘1’ corresponding to Monday ‘%U’ week number of year, with Sunday as the first day of the week (‘00’...‘53’). Days in a new year preceding the first Sunday are in week zero. ‘%V’ ISO week number, that is, the week number of year, with Monday as the first day of the week (‘01’...‘53’). If the week containing January 1 has four or more days in the new year, then it is considered week 1; otherwise, it is week 53 of the previous year, and the next week is week 1. (See the ISO 8601 standard.) ‘%w’ day of week (‘0’...‘6’) with 0 corresponding to Sunday ‘%W’ week number of year, with Monday as first day of week (‘00’...‘53’). Days in a new year preceding the first Monday are in week zero. ‘%x’ locale’s date representation (e.g., ‘12/31/99’) ‘%y’ last two digits of year (‘00’...‘99’) ‘%Y’ year. This is normally at least four characters, but it may be more. Year ‘0000’ precedes year ‘0001’, and year ‘-001’ precedes year ‘0000’. 21.1.1.3 Literal conversion specifiers ...................................... ‘date’ conversion specifiers that produce literal strings. ‘%%’ a literal % ‘%n’ a newline ‘%t’ a horizontal tab 21.1.1.4 Padding and other flags ................................ Unless otherwise specified, ‘date’ normally pads numeric fields with zeros, so that, for example, numeric months are always output as two digits. Most numeric fields are padded on the left. However, nanoseconds are padded on the right since they are commonly used after decimal points in formats like ‘%s.%-N’. Also, seconds since the Epoch are not padded since there is no natural width for them. The following optional flags can appear after the ‘%’: ‘-’ (hyphen) Do not pad the field; useful if the output is intended for human consumption. This is a GNU extension. As a special case, ‘%-N’ outputs only enough trailing digits to not lose information, assuming that the timestamp’s resolution is the same as the current hardware clock. For example, if the hardware clock resolution is 1 microsecond, ‘%s.%-N’ outputs something like ‘1640890100.395710’. ‘_’ (underscore) Pad with spaces; useful if you need a fixed number of characters in the output, but zeros are too distracting. This is a GNU extension. ‘0’ (zero) Pad with zeros even if the conversion specifier would normally pad with spaces. ‘+’ Pad with zeros, like ‘0’. In addition, precede any year number with ‘+’ if it exceeds 9999 or if its field width exceeds 4; similarly, precede any century number with ‘+’ if it exceeds 99 or if its field width exceeds 2. This supports ISO 8601 formats for dates far in the future; for example, the command ‘date --date=12019-02-25 +%+13F’ outputs the string ‘+012019-02-25’. ‘^’ Use upper case characters if possible. This is a GNU extension. ‘#’ Use opposite case characters if possible. A field that is normally upper case becomes lower case, and vice versa. This is a GNU extension. Here are some examples of padding: date +%d/%m -d "Feb 1" ⇒ 01/02 date +%-d/%-m -d "Feb 1" ⇒ 1/2 date +%_d/%_m -d "Feb 1" ⇒ 1/ 2 You can optionally specify the field width (after any flag, if present) as a decimal number. If the natural size of the output of the field has less than the specified number of characters, the result is normally written right adjusted and padded to the given size. For example, ‘%9B’ prints the right adjusted month name in a field of width 9. Nanoseconds are left adjusted, and are truncated or padded to the field width. An optional modifier can follow the optional flag and width specification. The modifiers are: ‘E’ Use the locale’s alternate representation for date and time. This modifier applies to the ‘%c’, ‘%C’, ‘%x’, ‘%X’, ‘%y’ and ‘%Y’ conversion specifiers. In a Japanese locale, for example, ‘%Ex’ might yield a date format based on the Japanese Emperors’ reigns. ‘O’ Use the locale’s alternate numeric symbols for numbers. This modifier applies only to numeric conversion specifiers. If the format supports the modifier but no alternate representation is available, it is ignored. POSIX specifies the behavior of flags and field widths only for ‘%C’, ‘%F’, ‘%G’, and ‘%Y’ (all without modifiers), and requires a flag to be present if and only if a field width is also present. Other combinations of flags, field widths and modifiers are GNU extensions. 21.1.2 Setting the time ----------------------- You must have appropriate privileges to set the system clock. For changes to persist across a reboot, the hardware clock may need to be updated from the system clock, which might not happen automatically on your system. To set the clock, you can use the ‘--set’ (‘-s’) option (*note Options for date::). To set the clock without using GNU extensions, you can give ‘date’ an argument of the form ‘MMDDhhmm[[CC]YY][.ss]’ where each two-letter component stands for two digits with the following meanings: MM month DD day within month HH hour MM minute CC first two digits of year (optional) YY last two digits of year (optional) SS second (optional) Note, the ‘--date’ and ‘--set’ options may not be used with an argument in the above format. The ‘--universal’ option may be used with such an argument to indicate that the specified date and time are relative to Universal Time rather than to the local time zone. 21.1.3 Options for ‘date’ ------------------------- The program accepts the following options. Also see *note Common options::. Except for ‘-u’, these options are all GNU extensions to POSIX. All options that specify the date to display are mutually exclusive. I.e.: ‘--date’, ‘--file’, ‘--reference’, ‘--resolution’. ‘-d DATESTR’ ‘--date=DATESTR’ Display the date and time specified in DATESTR instead of the current date and time. DATESTR can be in almost any common format. It can contain month names, time zones, ‘am’ and ‘pm’, ‘yesterday’, etc. For example, ‘--date="2020-07-21 14:19:13.489392193 +0530"’ specifies the instant of time that is 489,392,193 nanoseconds after July 21, 2020 at 2:19:13 PM in a time zone that is 5 hours and 30 minutes east of UTC. Note: input currently must be in locale independent format. E.g., the LC_TIME=C below is needed to print back the correct date in many locales: date -d "$(LC_TIME=C date)" *Note Date input formats::. ‘--debug’ Annotate the parsed date, display the effective time zone, and warn about potential misuse. ‘-f DATEFILE’ ‘--file=DATEFILE’ Parse each line in DATEFILE as with ‘-d’ and display the resulting date and time. If DATEFILE is ‘-’, use standard input. This is useful when you have many dates to process, because the system overhead of starting up the ‘date’ executable many times can be considerable. ‘-I[TIMESPEC]’ ‘--iso-8601[=TIMESPEC]’ Display the date using an ISO 8601 format, ‘%Y-%m-%d’. The argument TIMESPEC specifies the number of additional terms of the time to include. It can be one of the following: ‘auto’ Print just the date. This is the default if TIMESPEC is omitted. This is like the format ‘%Y-%m-%d’. ‘hours’ Also print hours and time zone. This is like the format ‘%Y-%m-%dT%H%:z’. ‘minutes’ Also print minutes. This is like the format ‘%Y-%m-%dT%H:%M%:z’. ‘seconds’ Also print seconds. This is like the format ‘%Y-%m-%dT%H:%M:%S%:z’. ‘ns’ Also print nanoseconds. This is like the format ‘%Y-%m-%dT%H:%M:%S,%N%:z’. This format is always suitable as input for the ‘--date’ (‘-d’) and ‘--file’ (‘-f’) options, regardless of the current locale. ‘-r FILE’ ‘--reference=FILE’ Display the date and time of the last modification of FILE, instead of the current date and time. ‘--resolution’ Display the timestamp resolution instead of the time. Current clock timestamps that are output by ‘date’ are integer multiples of the timestamp resolution. With this option, the format defaults to ‘%s.%N’. For example, if the clock resolution is 1 millisecond, the output is: 0.001000000 ‘-R’ ‘--rfc-email’ Display the date and time using the format ‘%a, %d %b %Y %H:%M:%S %z’, evaluated in the C locale so abbreviations are always in English. For example: Mon, 09 Jul 2020 17:00:00 -0400 This format conforms to Internet RFCs 5322 (https://tools.ietf.org/search/rfc5322), 2822 (https://tools.ietf.org/search/rfc2822) and 822 (https://tools.ietf.org/search/rfc822), the current and previous standards for Internet email. For compatibility with older versions of ‘date’, ‘--rfc-2822’ and ‘--rfc-822’ are aliases for ‘--rfc-email’. ‘--rfc-3339=TIMESPEC’ Display the date using a format specified by Internet RFC 3339 (https://tools.ietf.org/search/rfc3339). This is like ‘--iso-8601’, except that a space rather than a ‘T’ separates dates from times, and a period rather than a comma separates seconds from subseconds. This format is always suitable as input for the ‘--date’ (‘-d’) and ‘--file’ (‘-f’) options, regardless of the current locale. The argument TIMESPEC specifies how much of the time to include. It can be one of the following: ‘date’ Print just the full-date, e.g., ‘2020-07-21’. This is like the format ‘%Y-%m-%d’. ‘seconds’ Print the full-date and full-time separated by a space, e.g., ‘2020-07-21 04:30:37+05:30’. The output ends with a numeric time-offset; here the ‘+05:30’ means that local time is five hours and thirty minutes east of UTC. This is like the format ‘%Y-%m-%d %H:%M:%S%:z’. ‘ns’ Like ‘seconds’, but also print nanoseconds, e.g., ‘2020-07-21 04:30:37.998458565+05:30’. This is like the format ‘%Y-%m-%d %H:%M:%S.%N%:z’. ‘-s DATESTR’ ‘--set=DATESTR’ Set the date and time to DATESTR. See ‘-d’ above. See also *note Setting the time::. ‘-u’ ‘--utc’ ‘--universal’ Use Universal Time by operating as if the ‘TZ’ environment variable were set to the string ‘UTC0’. UTC stands for Coordinated Universal Time, established in 1960. Universal Time is often called “Greenwich Mean Time” (GMT) for historical reasons. Typically, systems ignore leap seconds and thus implement an approximation to UTC rather than true UTC. 21.1.4 Examples of ‘date’ ------------------------- Here are a few examples. Also see the documentation for the ‘-d’ option in the previous section. • To print the date of the day before yesterday: date --date='2 days ago' • To print the date of the day three months and one day hence: date --date='3 months 1 day' • To print the day of year of Christmas in the current year: date --date='25 Dec' +%j • To print the current full month name and the day of the month: date '+%B %d' But this may not be what you want because for the first nine days of the month, the ‘%d’ expands to a zero-padded two-digit field, for example ‘date -d 1may '+%B %d'’ will print ‘May 01’. • To print a date without the leading zero for one-digit days of the month, you can use the (GNU extension) ‘-’ flag to suppress the padding altogether: date -d 1may '+%B %-d' • To print the current date and time in the format required by many non-GNU versions of ‘date’ when setting the system clock: date +%m%d%H%M%Y.%S • To set the system clock forward by two minutes: date --set='+2 minutes' • To print the date in Internet RFC 5322 format, use ‘date --rfc-email’. Here is some example output: Tue, 09 Jul 2020 19:00:37 -0400 • To convert a date string to the number of seconds since the Epoch (which is 1970-01-01 00:00 UTC), use the ‘--date’ option with the ‘%s’ format. That can be useful in sorting and/or graphing and/or comparing data by date. The following command outputs the number of the seconds since the Epoch for the time two minutes after the Epoch: date --date='1970-01-01 00:02:00 +0000' +%s 120 To convert a date string from one time zone FROM to another TO, specify ‘TZ="FROM"’ in the environment and ‘TZ="TO"’ in the ‘--date’ option. *Note Specifying time zone rules::. For example: TZ="Asia/Tokyo" date --date='TZ="America/New_York" 2023-05-07 12:23' Mon May 8 01:23:00 JST 2023 If you do not specify time zone information in the date string, ‘date’ uses your computer’s idea of the time zone when interpreting the string. For example, if your computer’s time zone is that of Cambridge, Massachusetts, which was then 5 hours (i.e., 18,000 seconds) behind UTC: # local time zone used date --date='1970-01-01 00:02:00' +%s 18120 • If you’re sorting or graphing dated data, your raw date values may be represented as seconds since the Epoch. But few people can look at the date ‘1577836800’ and casually note “Oh, that’s the first second of the year 2020 in Greenwich, England.” date --date='2020-01-01 UTC' +%s 1577836800 An alternative is to use the ‘--utc’ (‘-u’) option. Then you may omit ‘UTC’ from the date string. Although this produces the same result for ‘%s’ and many other format sequences, with a time zone offset different from zero, it would give a different result for zone-dependent formats like ‘%z’. date -u --date=2020-07-21 +%s 1595289600 To convert such an unwieldy number of seconds back to a more readable form, use a command like this: date -d @1595289600 +"%F %T %z" 2020-07-20 20:00:00 -0400 Often it is better to output UTC-relative date and time: date -u -d @1595289600 +"%F %T %z" 2020-07-21 00:00:00 +0000 • Typically the seconds count omits leap seconds, but some systems are exceptions. Because leap seconds are not predictable, the mapping between the seconds count and a future timestamp is not reliable on the atypical systems that include leap seconds in their counts. Here is how the two kinds of systems handle the leap second at the end of the year 2016: # Typical systems ignore leap seconds: date --date='2016-12-31 23:59:59 +0000' +%s 1483228799 date --date='2016-12-31 23:59:60 +0000' +%s date: invalid date '2016-12-31 23:59:60 +0000' date --date='2017-01-01 00:00:00 +0000' +%s 1483228800 # Atypical systems count leap seconds: date --date='2016-12-31 23:59:59 +0000' +%s 1483228825 date --date='2016-12-31 23:59:60 +0000' +%s 1483228826 date --date='2017-01-01 00:00:00 +0000' +%s 1483228827 21.2 ‘arch’: Print machine hardware name ======================================== ‘arch’ prints the machine hardware name, and is equivalent to ‘uname -m’. Synopsis: arch [OPTION] The program accepts the *note Common options:: only. ‘arch’ is not installed by default, so portable scripts should not rely on its existence. An exit status of zero indicates success, and a nonzero value indicates failure. 21.3 ‘nproc’: Print the number of available processors ====================================================== Print the number of processing units available to the current process, which may be less than the number of online processors. If this information is not accessible, then print the number of processors installed. If the ‘OMP_NUM_THREADS’ or ‘OMP_THREAD_LIMIT’ environment variables are set, then they will determine the minimum and maximum returned value respectively. The result is guaranteed to be greater than zero. Synopsis: nproc [OPTION] The program accepts the following options. Also see *note Common options::. ‘--all’ Print the number of installed processors on the system, which may be greater than the number online or available to the current process. The ‘OMP_NUM_THREADS’ or ‘OMP_THREAD_LIMIT’ environment variables are not honored in this case. ‘--ignore=NUMBER’ If possible, exclude this NUMBER of processing units. An exit status of zero indicates success, and a nonzero value indicates failure. 21.4 ‘uname’: Print system information ====================================== ‘uname’ prints information about the machine and operating system it is run on. If no options are given, ‘uname’ acts as if the ‘-s’ option were given. Synopsis: uname [OPTION]... If multiple options or ‘-a’ are given, the selected information is printed in this order: KERNEL-NAME NODENAME KERNEL-RELEASE KERNEL-VERSION MACHINE PROCESSOR HARDWARE-PLATFORM OPERATING-SYSTEM The information may contain internal spaces, so such output cannot be parsed reliably. In the following example, KERNEL-VERSION is ‘#1 SMP Fri Jul 17 17:18:38 UTC 2020’: uname -a ⇒ Linux dumdum.example.org 5.9.16-200.fc33.x86_64 #1 SMP Mon Dec 21 14:08:22 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux The program accepts the following options. Also see *note Common options::. ‘-a’ ‘--all’ Print all of the below information, except omit the processor type and the hardware platform name if they are unknown. ‘-i’ ‘--hardware-platform’ Print the hardware platform name (sometimes called the hardware implementation). Print ‘unknown’ if this information is not available. Note this is non-portable (even across GNU/Linux distributions). ‘-m’ ‘--machine’ Print the machine hardware name (sometimes called the hardware class or hardware type). ‘-n’ ‘--nodename’ Print the network node hostname. ‘-p’ ‘--processor’ Print the processor type (sometimes called the instruction set architecture or ISA). Print ‘unknown’ if this information is not available. Note this is non-portable (even across GNU/Linux distributions). ‘-o’ ‘--operating-system’ Print the name of the operating system. ‘-r’ ‘--kernel-release’ Print the kernel release. ‘-s’ ‘--kernel-name’ Print the kernel name. POSIX 1003.1-2001 (*note Standards conformance::) calls this “the implementation of the operating system”, because the POSIX specification itself has no notion of “kernel”. The kernel name might be the same as the operating system name printed by the ‘-o’ or ‘--operating-system’ option, but it might differ. Some operating systems (e.g., FreeBSD, HP-UX) have the same name as their underlying kernels; others (e.g., GNU/Linux, Solaris) do not. ‘-v’ ‘--kernel-version’ Print the kernel version. An exit status of zero indicates success, and a nonzero value indicates failure. 21.5 ‘hostname’: Print or set system name ========================================= With no arguments, ‘hostname’ prints the name of the current host system. With one argument, it sets the current host name to the specified string. You must have appropriate privileges to set the host name. Synopsis: hostname [NAME] The only options are ‘--help’ and ‘--version’. *Note Common options::. ‘hostname’ is not installed by default, and other packages also supply a ‘hostname’ command, so portable scripts should not rely on its existence or on the exact behavior documented above. An exit status of zero indicates success, and a nonzero value indicates failure. 21.6 ‘hostid’: Print numeric host identifier ============================================ ‘hostid’ prints the numeric identifier of the current host in hexadecimal. This command accepts no arguments. The only options are ‘--help’ and ‘--version’. *Note Common options::. For example, here’s what it prints on one system I use: $ hostid 1bac013d On that system, the 32-bit quantity happens to be closely related to the system’s Internet address, but that isn’t always the case. ‘hostid’ is installed only on systems that have the ‘gethostid’ function, so portable scripts should not rely on its existence. An exit status of zero indicates success, and a nonzero value indicates failure. 21.7 ‘uptime’: Print system uptime and load =========================================== ‘uptime’ prints the current time, the system’s uptime, the number of logged-in users and the current load average. If an argument is specified, it is used as the file to be read to discover how many users are logged in. If no argument is specified, a system default is used (‘uptime --help’ indicates the default setting). The only options are ‘--help’ and ‘--version’. *Note Common options::. For example, here’s what it prints right now on one system I use: $ uptime 14:07 up 3:35, 3 users, load average: 1.39, 1.15, 1.04 The precise method of calculation of load average varies somewhat between systems. Some systems calculate it as the average number of runnable processes over the last 1, 5 and 15 minutes, but some systems also include processes in the uninterruptible sleep state (that is, those processes which are waiting for device I/O). The Linux kernel includes uninterruptible processes. ‘uptime’ is installed only on platforms with infrastructure for obtaining the boot time, and other packages also supply an ‘uptime’ command, so portable scripts should not rely on its existence or on the exact behavior documented above. An exit status of zero indicates success, and a nonzero value indicates failure. 22 SELinux context ****************** This section describes commands for operations with SELinux contexts. 22.1 ‘chcon’: Change SELinux context of file ============================================ ‘chcon’ changes the SELinux security context of the selected files. Synopses: chcon [OPTION]... CONTEXT FILE... chcon [OPTION]... [-u USER] [-r ROLE] [-l RANGE] [-t TYPE] FILE... chcon [OPTION]... --reference=RFILE FILE... Change the SELinux security context of each FILE to CONTEXT. With ‘--reference’, change the security context of each FILE to that of RFILE. The program accepts the following options. Also see *note Common options::. ‘--dereference’ Do not affect symbolic links but what they refer to; this is the default. ‘-h’ ‘--no-dereference’ Affect the symbolic links themselves instead of any referenced file. ‘--reference=RFILE’ Use RFILE’s security context rather than specifying a CONTEXT value. ‘-R’ ‘--recursive’ Operate on files and directories recursively. ‘--preserve-root’ Refuse to operate recursively on the root directory, ‘/’, when used together with the ‘--recursive’ option. *Note Treating / specially::. ‘--no-preserve-root’ Do not treat the root directory, ‘/’, specially when operating recursively; this is the default. *Note Treating / specially::. ‘-H’ If ‘--recursive’ (‘-R’) is specified and a command line argument is a symbolic link to a directory, traverse it. *Note Traversing symlinks::. ‘-L’ In a recursive traversal, traverse every symbolic link to a directory that is encountered. *Note Traversing symlinks::. ‘-P’ Do not traverse any symbolic links. This is the default if none of ‘-H’, ‘-L’, or ‘-P’ is specified. *Note Traversing symlinks::. ‘-v’ ‘--verbose’ Output a diagnostic for every file processed. ‘-u USER’ ‘--user=USER’ Set user USER in the target security context. ‘-r ROLE’ ‘--role=ROLE’ Set role ROLE in the target security context. ‘-t TYPE’ ‘--type=TYPE’ Set type TYPE in the target security context. ‘-l RANGE’ ‘--range=RANGE’ Set range RANGE in the target security context. An exit status of zero indicates success, and a nonzero value indicates failure. 22.2 ‘runcon’: Run a command in specified SELinux context ========================================================= ‘runcon’ runs file in specified SELinux security context. Synopses: runcon CONTEXT COMMAND [ARGS] runcon [ -c ] [-u USER] [-r ROLE] [-t TYPE] [-l RANGE] COMMAND [ARGS] Run COMMAND with completely-specified CONTEXT, or with current or transitioned security context modified by one or more of LEVEL, ROLE, TYPE and USER. If none of ‘-c’, ‘-t’, ‘-u’, ‘-r’, or ‘-l’ is specified, the first argument is used as the complete context. Any additional arguments after COMMAND are interpreted as arguments to the command. With neither CONTEXT nor COMMAND, print the current security context. Note also the ‘setpriv’ command which can be used to set the NO_NEW_PRIVS bit using ‘setpriv --no-new-privs runcon ...’, thus disallowing usage of a security context with more privileges than the process would normally have. ‘runcon’ accepts the following options. Also see *note Common options::. ‘-c’ ‘--compute’ Compute process transition context before modifying. ‘-u USER’ ‘--user=USER’ Set user USER in the target security context. ‘-r ROLE’ ‘--role=ROLE’ Set role ROLE in the target security context. ‘-t TYPE’ ‘--type=TYPE’ Set type TYPE in the target security context. ‘-l RANGE’ ‘--range=RANGE’ Set range RANGE in the target security context. Exit status: 125 if ‘runcon’ itself fails 126 if COMMAND is found but cannot be invoked 127 if COMMAND cannot be found the exit status of COMMAND otherwise 23 Modified command invocation ****************************** This section describes commands that run other commands in some context different than the current one: a modified environment, as a different user, etc. 23.1 ‘chroot’: Run a command with a different root directory ============================================================ ‘chroot’ runs a command with a specified root directory. On many systems, only the super-user can do this.(1). Synopses: chroot OPTION NEWROOT [COMMAND [ARGS]...] chroot OPTION Ordinarily, file names are looked up starting at the root of the directory structure, i.e., ‘/’. ‘chroot’ changes the root to the directory NEWROOT (which must exist), then changes the working directory to ‘/’, and finally runs COMMAND with optional ARGS. If COMMAND is not specified, the default is the value of the ‘SHELL’ environment variable or ‘/bin/sh’ if not set, invoked with the ‘-i’ option. COMMAND must not be a special built-in utility (*note Special built-in utilities::). The program accepts the following options. Also see *note Common options::. Options must precede operands. ‘--groups=GROUPS’ Use this option to override the supplementary GROUPS to be used by the new process. The items in the list (names or numeric IDs) must be separated by commas. Use ‘--groups=''’ to disable the supplementary group look-up implicit in the ‘--userspec’ option. ‘--userspec=USER[:GROUP]’ By default, COMMAND is run with the same credentials as the invoking process. Use this option to run it as a different USER and/or with a different primary GROUP. If a USER is specified then the supplementary groups are set according to the system defined list for that user, unless overridden with the ‘--groups’ option. ‘--skip-chdir’ Use this option to not change the working directory to ‘/’ after changing the root directory to NEWROOT, i.e., inside the chroot. This option is only permitted when NEWROOT is the old ‘/’ directory, and therefore is mostly useful together with the ‘--groups’ and ‘--userspec’ options to retain the previous working directory. The user and group name look-up performed by the ‘--userspec’ and ‘--groups’ options, is done both outside and inside the chroot, with successful look-ups inside the chroot taking precedence. If the specified user or group items are intended to represent a numeric ID, then a name to ID resolving step is avoided by specifying a leading ‘+’. *Note Disambiguating names and IDs::. Here are a few tips to help avoid common problems in using chroot. To start with a simple example, make COMMAND refer to a statically linked binary. If you were to use a dynamically linked executable, then you’d have to arrange to have the shared libraries in the right place under your new root directory. For example, if you create a statically linked ‘ls’ executable, and put it in ‘/tmp/empty’, you can run this command as root: $ chroot /tmp/empty /ls -Rl / Then you’ll see output like this: /: total 1023 -rwxr-xr-x 1 0 0 1041745 Aug 16 11:17 ls If you want to use a dynamically linked executable, say ‘bash’, then first run ‘ldd bash’ to see what shared objects it needs. Then, in addition to copying the actual binary, also copy the listed files to the required positions under your intended new root directory. Finally, if the executable requires any other files (e.g., data, state, device files), copy them into place, too. ‘chroot’ is installed only on systems that have the ‘chroot’ function, so portable scripts should not rely on its existence. Exit status: 125 if ‘chroot’ itself fails 126 if COMMAND is found but cannot be invoked 127 if COMMAND cannot be found the exit status of COMMAND otherwise ---------- Footnotes ---------- (1) However, some systems (e.g., FreeBSD) can be configured to allow certain regular users to use the ‘chroot’ system call, and hence to run this program. Also, on Cygwin, anyone can run the ‘chroot’ command, because the underlying function is non-privileged due to lack of support in MS-Windows. Furthermore, the ‘chroot’ command avoids the ‘chroot’ system call when NEWROOT is identical to the old ‘/’ directory for consistency with systems where this is allowed for non-privileged users. 23.2 ‘env’: Run a command in a modified environment =================================================== ‘env’ runs a command with a modified environment. Synopses: env [OPTION]... [NAME=VALUE]... [COMMAND [ARGS]...] env -[v]S'[OPTION]... [NAME=VALUE]... [COMMAND [ARGS]...]' env ‘env’ is commonly used on first line of scripts (shebang line): #!/usr/bin/env COMMAND #!/usr/bin/env -[v]S[OPTION]... [NAME=VALUE]... COMMAND [ARGS]... Operands of the form ‘VARIABLE=VALUE’ set the environment variable VARIABLE to value VALUE. VALUE may be empty (‘VARIABLE=’). Setting a variable to an empty value is different from unsetting it. These operands are evaluated left-to-right, so if two operands mention the same variable the earlier is ignored. Environment variable names can be empty, and can contain any characters other than ‘=’ and ASCII NUL. However, it is wise to limit yourself to names that consist solely of underscores, digits, and ASCII letters, and that begin with a non-digit, as applications like the shell do not work well with other names. The first operand that does not contain the character ‘=’ specifies the program to invoke; it is searched for according to the ‘PATH’ environment variable. Any remaining arguments are passed as arguments to that program. The program should not be a special built-in utility (*note Special built-in utilities::). Modifications to ‘PATH’ take effect prior to searching for COMMAND. Use caution when reducing ‘PATH’; behavior is not portable when ‘PATH’ is undefined or omits key directories such as ‘/bin’. In the rare case that a utility contains a ‘=’ in the name, the only way to disambiguate it from a variable assignment is to use an intermediate command for COMMAND, and pass the problematic program name via ARGS. For example, if ‘./prog=’ is an executable in the current ‘PATH’: env prog= true # runs 'true', with prog= in environment env ./prog= true # runs 'true', with ./prog= in environment env -- prog= true # runs 'true', with prog= in environment env sh -c '\prog= true' # runs 'prog=' with argument 'true' env sh -c 'exec "$@"' sh prog= true # also runs 'prog=' If no command name is specified following the environment specifications, the resulting environment is printed. This is like specifying the ‘printenv’ program. For some examples, suppose the environment passed to ‘env’ contains ‘LOGNAME=rms’, ‘EDITOR=emacs’, and ‘PATH=.:/gnubin:/hacks’: • Output the current environment. $ env | LC_ALL=C sort EDITOR=emacs LOGNAME=rms PATH=.:/gnubin:/hacks • Run ‘foo’ with a reduced environment, preserving only the original ‘PATH’ to avoid problems in locating ‘foo’. env - PATH="$PATH" foo • Run ‘foo’ with the environment containing ‘LOGNAME=rms’, ‘EDITOR=emacs’, and ‘PATH=.:/gnubin:/hacks’, and guarantees that ‘foo’ was found in the file system rather than as a shell built-in. env foo • Run ‘nemacs’ with the environment containing ‘LOGNAME=foo’, ‘EDITOR=emacs’, ‘PATH=.:/gnubin:/hacks’, and ‘DISPLAY=gnu:0’. env DISPLAY=gnu:0 LOGNAME=foo nemacs • Attempt to run the program ‘/energy/--’ (as that is the only possible path search result); if the command exists, the environment will contain ‘LOGNAME=rms’ and ‘PATH=/energy’, and the arguments will be ‘e=mc2’, ‘bar’, and ‘baz’. env -u EDITOR PATH=/energy -- e=mc2 bar baz 23.2.1 General options ---------------------- The program accepts the following options. Also see *note Common options::. Options must precede operands. ‘-0’ ‘--null’ Output a zero byte (ASCII NUL) at the end of each line, rather than a newline. This option enables other programs to parse the output even when that output would contain data with embedded newlines. ‘-u NAME’ ‘--unset=NAME’ Remove variable NAME from the environment, if it was in the environment. ‘-’ ‘-i’ ‘--ignore-environment’ Start with an empty environment, ignoring the inherited environment. ‘-C DIR’ ‘--chdir=DIR’ Change the working directory to DIR before invoking COMMAND. This differs from the shell built-in ‘cd’ in that it starts COMMAND as a subprocess rather than altering the shell’s own working directory; this allows it to be chained with other commands that run commands in a different context. For example: # Run 'true' with /chroot as its root directory and /srv as its working # directory. chroot /chroot env --chdir=/srv true # Run 'true' with /build as its working directory, FOO=bar in its # environment, and a time limit of five seconds. env --chdir=/build FOO=bar timeout 5 true ‘--default-signal[=SIG]’ Unblock and reset signal SIG to its default signal handler. Without SIG all known signals are unblocked and reset to their defaults. Multiple signals can be comma-separated. An empty SIG argument is a no-op. The following command runs ‘seq’ with SIGINT and SIGPIPE set to their default (which is to terminate the program): env --default-signal=PIPE,INT seq 1000 | head -n1 In the following example, we see how this is not possible to do with traditional shells. Here the first trap command sets SIGPIPE to ignore. The second trap command ostensibly sets it back to its default, but POSIX mandates that the shell must not change inherited state of the signal – so it is a no-op. trap '' PIPE && sh -c 'trap - PIPE ; seq inf | head -n1' Using ‘--default-signal=PIPE’ we can ensure the signal handling is set to its default behavior: trap '' PIPE && sh -c 'env --default-signal=PIPE seq inf | head -n1' ‘--ignore-signal[=SIG]’ Ignore signal SIG when running a program. Without SIG all known signals are set to ignore. Multiple signals can be comma-separated. An empty SIG argument is a no-op. The following command runs ‘seq’ with SIGINT set to be ignored – pressing ‘Ctrl-C’ will not terminate it: env --ignore-signal=INT seq inf > /dev/null ‘SIGCHLD’ is special, in that ‘--ignore-signal=CHLD’ might have no effect (POSIX says it’s unspecified). Most operating systems do not allow ignoring ‘SIGKILL’, ‘SIGSTOP’ (and possibly other signals). Attempting to ignore these signals will fail. Multiple (and contradictory) ‘--default-signal=SIG’ and ‘--ignore-signal=SIG’ options are processed left-to-right, with the latter taking precedence. In the following example, ‘SIGPIPE’ is set to default while ‘SIGINT’ is ignored: env --default-signal=INT,PIPE --ignore-signal=INT ‘--block-signal[=SIG]’ Block signal(s) SIG from being delivered. Without SIG all known signals are set to blocked. Multiple signals can be comma-separated. An empty SIG argument is a no-op. ‘--list-signal-handling’ List blocked or ignored signals to standard error, before executing a command. ‘-v’ ‘--debug’ Show verbose information for each processing step. $ env -v -uTERM A=B uname -s unset: TERM setenv: A=B executing: uname arg[0]= 'uname' arg[1]= '-s' Linux When combined with ‘-S’ it is recommended to list ‘-v’ first, e.g. ‘env -vS'string'’. ‘-S STRING’ ‘--split-string=STRING’ process and split STRING into separate arguments used to pass multiple arguments on shebang lines. ‘env’ supports FreeBSD’s syntax of several escape sequences and environment variable expansions. See below for details and examples. Exit status: 0 if no COMMAND is specified and the environment is output 125 if ‘env’ itself fails 126 if COMMAND is found but cannot be invoked 127 if COMMAND cannot be found the exit status of COMMAND otherwise 23.2.2 ‘-S’/‘--split-string’ usage in scripts --------------------------------------------- The ‘-S’/‘--split-string’ option enables use of multiple arguments on the first line of scripts (the shebang line, ‘#!’). When a script’s interpreter is in a known location, scripts typically contain the absolute file name in their first line: Shell script: #!/bin/sh echo hello Perl script: #!/usr/bin/perl print "hello\n"; Python script: #!/usr/bin/python3 print("hello") When a script’s interpreter is in a non-standard location in the ‘PATH’ environment variable, it is recommended to use ‘env’ on the first line of the script to find the executable and run it: Shell script: #!/usr/bin/env bash echo hello Perl script: #!/usr/bin/env perl print "hello\n"; Python script: #!/usr/bin/env python3 print("hello") Most operating systems (e.g. GNU/Linux, BSDs) treat all text after the first space as a single argument. When using ‘env’ in a script it is thus not possible to specify multiple arguments. In the following example: #!/usr/bin/env perl -T -w print "hello\n"; The operating system treats ‘perl -T -w’ as one argument (the program’s name), and executing the script fails with: /usr/bin/env: 'perl -T -w': No such file or directory The ‘-S’ option instructs ‘env’ to split the single string into multiple arguments. The following example works as expected: $ cat hello.pl #!/usr/bin/env -S perl -T -w print "hello\n"; $ chmod a+x hello.pl $ ./hello.pl hello And is equivalent to running ‘perl -T -w hello.pl’ on the command line prompt. Testing and troubleshooting ........................... To test ‘env -S’ on the command line, use single quotes for the ‘-S’ string to emulate a single parameter. Single quotes are not needed when using ‘env -S’ in a shebang line on the first line of a script (the operating system already treats it as one argument). The following command is equivalent to the ‘hello.pl’ script above: $ env -S'perl -T -w' hello.pl To troubleshoot ‘-S’ usage add the ‘-v’ as the first argument (before ‘-S’). Using ‘-vS’ on a shebang line in a script: $ cat hello-debug.pl #!/usr/bin/env -vS perl -T -w print "hello\n"; $ chmod a+x hello-debug.pl $ ./hello-debug.pl split -S: 'perl -T -w' into: 'perl' & '-T' & '-w' executing: perl arg[0]= 'perl' arg[1]= '-T' arg[2]= '-w' arg[3]= './hello-debug.pl' hello Using ‘-vS’ on the command line prompt (adding single quotes): $ env -vS'perl -T -w' hello-debug.pl split -S: 'perl -T -w' into: 'perl' & '-T' & '-w' executing: perl arg[0]= 'perl' arg[1]= '-T' arg[2]= '-w' arg[3]= 'hello-debug.pl' hello 23.2.3 ‘-S’/‘--split-string’ syntax ----------------------------------- Splitting arguments by whitespace ................................. Running ‘env -Sstring’ splits the STRING into arguments based on unquoted spaces or tab characters. (Newlines, carriage returns, vertical tabs and form feeds are treated like spaces and tabs.) In the following contrived example the ‘awk’ variable ‘OFS’ will be ‘xyz’ as these spaces are inside double quotes. The other space characters are used as argument separators: $ cat one.awk #!/usr/bin/env -S awk -v OFS=" xyz " -f BEGIN {print 1,2,3} $ chmod a+x one.awk $ ./one.awk 1 xyz 2 xyz 3 When using ‘-S’ on the command line prompt, remember to add single quotes around the entire string: $ env -S'awk -v OFS=" xyz " -f' one.awk 1 xyz 2 xyz 3 Escape sequences ................ ‘env’ supports several escape sequences. These sequences are processed when unquoted or inside double quotes (unless otherwise noted). Single quotes disable escape sequences except ‘\'’ and ‘\\’. ‘\c’ Ignore the remaining characters in the string. Cannot be used inside double quotes. ‘\f’ form-feed character (ASCII 0x0C) ‘\n’ new-line character (ASCII 0x0A) ‘\r’ carriage-return character (ASCII 0x0D) ‘\t’ tab character (ASCII 0x09) ‘\v’ vertical tab character (ASCII 0x0B) ‘\#’ A hash ‘#’ character. Used when a ‘#’ character is needed as the first character of an argument (see ’comments’ section below). ‘\$’ A dollar-sign character ‘$’. Unescaped ‘$’ characters are used to expand environment variables (see ’variables’ section below). ‘\_’ Inside double-quotes, replaced with a single space character. Outside quotes, treated as an argument separator. ‘\_’ can be used to avoid space characters in a shebang line (see examples below). ‘\"’ A double-quote character. ‘\'’ A single-quote character. This escape sequence works inside single-quoted strings. ‘\\’ A backslash character. This escape sequence works inside single-quoted strings. The following ‘awk’ script will use tab character as input and output field separator (instead of spaces and tabs): $ cat tabs.awk #!/usr/bin/env -S awk -v FS="\t" -v OFS="\t" -f ... Comments ........ The escape sequence ‘\c’ (used outside single/double quotes) causes ‘env’ to ignore the rest of the string. The ‘#’ character causes ‘env’ to ignore the rest of the string when it appears as the first character of an argument. Use ‘\#’ to reverse this behavior. $ env -S'printf %s\n A B C' A B C $ env -S'printf %s\n A# B C' A# B C $ env -S'printf %s\n A #B C' A $ env -S'printf %s\n A \#B C' A #B C $ env -S'printf %s\n A\cB C' A NOTE: The above examples use single quotes as they are executed on the command-line. Environment variable expansion .............................. The pattern ‘${VARNAME}’ is used to substitute a value from the environment variable. The pattern must include the curly braces (‘{’,‘}’). Without them ‘env’ will reject the string. Special shell variables (such as ‘$@’, ‘$*’, ‘$$’) are not supported. If the environment variable is empty or not set, the pattern will be replaced by an empty string. The value of ‘${VARNAME}’ will be that of the executed ‘env’, before any modifications using ‘-i’/‘--ignore-environment’/‘-u’/‘--unset’ or setting new values using ‘VAR=VALUE’. The following python script prepends ‘/opt/custom/modules’ to the python module search path environment variable (‘PYTHONPATH’): $ cat custom.py #!/usr/bin/env -S PYTHONPATH=/opt/custom/modules/:${PYTHONPATH} python print "hello" ... The expansion of ‘${PYTHONPATH}’ is performed by ‘env’, not by a shell. If the curly braces are omitted, ‘env’ will fail: $ cat custom.py #!/usr/bin/env -S PYTHONPATH=/opt/custom/modules/:$PYTHONPATH python print "hello" ... $ chmod a+x custom.py $ custom.py /usr/bin/env: only ${VARNAME} expansion is supported, error at: $PYTHONPATH python Environment variable expansion happens before clearing the environment (with ‘-i’) or unsetting specific variables (with ‘-u’): $ env -S'-i OLDUSER=${USER} env' OLDUSER=gordon Use ‘-v’ to diagnose the operations step-by-step: $ env -vS'-i OLDUSER=${USER} env' expanding ${USER} into 'gordon' split -S: '-i OLDUSER=${USER} env' into: '-i' & 'OLDUSER=gordon' & 'env' cleaning environ setenv: OLDUSER=gordon executing: env arg[0]= 'env' OLDUSER=gordon 23.3 ‘nice’: Run a command with modified niceness ================================================= ‘nice’ prints a process’s “niceness”, or runs a command with modified niceness. “niceness” affects how favorably the process is scheduled in the system. Synopsis: nice [OPTION]... [COMMAND [ARG]...] If no arguments are given, ‘nice’ prints the current niceness. Otherwise, ‘nice’ runs the given COMMAND with its niceness adjusted. By default, its niceness is incremented by 10. Niceness values range at least from −20 (process has high priority and gets more resources, thus slowing down other processes) through 19 (process has lower priority and runs slowly itself, but has less impact on the speed of other running processes). Some systems may have a wider range of niceness values; conversely, other systems may enforce more restrictive limits. An attempt to set the niceness outside the supported range is treated as an attempt to use the minimum or maximum supported value. A niceness should not be confused with a scheduling priority, which lets applications determine the order in which threads are scheduled to run. Unlike a priority, a niceness is merely advice to the scheduler, which the scheduler is free to ignore. Also, as a point of terminology, POSIX defines the behavior of ‘nice’ in terms of a “nice value”, which is the non-negative difference between a niceness and the minimum niceness. Though ‘nice’ conforms to POSIX, its documentation and diagnostics use the term “niceness” for compatibility with historical practice. COMMAND must not be a special built-in utility (*note Special built-in utilities::). Due to shell aliases and built-in ‘nice’ functions, using an unadorned ‘nice’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env nice ...’) to avoid interference from the shell. Note to change the “niceness” of an existing process, one needs to use the ‘renice’ command. The program accepts the following option. Also see *note Common options::. Options must precede operands. ‘-n ADJUSTMENT’ ‘--adjustment=ADJUSTMENT’ Add ADJUSTMENT instead of 10 to the command’s niceness. If ADJUSTMENT is negative and you lack appropriate privileges, ‘nice’ issues a warning but otherwise acts as if you specified a zero adjustment. For compatibility ‘nice’ also supports an obsolete option syntax ‘-ADJUSTMENT’. New scripts should use ‘-n ADJUSTMENT’ instead. ‘nice’ is installed only on systems that have the POSIX ‘setpriority’ function, so portable scripts should not rely on its existence on non-POSIX platforms. Exit status: 0 if no COMMAND is specified and the niceness is output 125 if ‘nice’ itself fails 126 if COMMAND is found but cannot be invoked 127 if COMMAND cannot be found the exit status of COMMAND otherwise It is sometimes useful to run a non-interactive program with reduced niceness. $ nice factor 4611686018427387903 Since ‘nice’ prints the current niceness, you can invoke it through itself to demonstrate how it works. The default behavior is to increase the niceness by ‘10’: $ nice 0 $ nice nice 10 $ nice -n 10 nice 10 The ADJUSTMENT is relative to the current niceness. In the next example, the first ‘nice’ invocation runs the second one with niceness 10, and it in turn runs the final one with a niceness that is 3 more: $ nice nice -n 3 nice 13 Specifying a niceness larger than the supported range is the same as specifying the maximum supported value: $ nice -n 10000000000 nice 19 Only a privileged user may run a process with lower niceness: $ nice -n -1 nice nice: cannot set niceness: Permission denied 0 $ sudo nice -n -1 nice -1 23.4 ‘nohup’: Run a command immune to hangups ============================================= ‘nohup’ runs the given COMMAND with hangup signals ignored, so that the command can continue running in the background after you log out. Synopsis: nohup COMMAND [ARG]... If standard input is a terminal, redirect it so that terminal sessions do not mistakenly consider the terminal to be used by the command. Make the substitute file descriptor unreadable, so that commands that mistakenly attempt to read from standard input can report an error. This redirection is a GNU extension; programs intended to be portable to non-GNU hosts can use ‘nohup COMMAND [ARG]... 0>/dev/null’ instead. If standard output is a terminal, the command’s standard output is appended to the file ‘nohup.out’; if that cannot be written to, it is appended to the file ‘$HOME/nohup.out’; and if that cannot be written to, the command is not run. Any ‘nohup.out’ or ‘$HOME/nohup.out’ file created by ‘nohup’ is made readable and writable only to the user, regardless of the current umask settings. If standard error is a terminal, it is normally redirected to the same file descriptor as the (possibly-redirected) standard output. However, if standard output is closed, standard error terminal output is instead appended to the file ‘nohup.out’ or ‘$HOME/nohup.out’ as above. To capture the command’s output to a file other than ‘nohup.out’ you can redirect it. For example, to capture the output of ‘make’: nohup make > make.log ‘nohup’ does not automatically put the command it runs in the background; you must do that explicitly, by ending the command line with an ‘&’. Also, ‘nohup’ does not alter the niceness of COMMAND; use ‘nice’ for that, e.g., ‘nohup nice COMMAND’. COMMAND must not be a special built-in utility (*note Special built-in utilities::). The only options are ‘--help’ and ‘--version’. *Note Common options::. Options must precede operands. Exit status: 125 if ‘nohup’ itself fails, and ‘POSIXLY_CORRECT’ is not set 126 if COMMAND is found but cannot be invoked 127 if COMMAND cannot be found the exit status of COMMAND otherwise If ‘POSIXLY_CORRECT’ is set, internal failures give status 127 instead of 125. 23.5 ‘stdbuf’: Run a command with modified I/O stream buffering =============================================================== ‘stdbuf’ allows one to modify the buffering operations of the three standard I/O streams associated with a program. Synopsis: stdbuf OPTION... COMMAND COMMAND must start with the name of a program that 1. uses the ISO C ‘FILE’ streams for input/output (note the programs ‘dd’ and ‘cat’ don’t do that), 2. does not adjust the buffering of its standard streams (note the program ‘tee’ is not in this category). Any additional ARGs are passed as additional arguments to the COMMAND. The program accepts the following options. Also see *note Common options::. ‘-i MODE’ ‘--input=MODE’ Adjust the standard input stream buffering. ‘-o MODE’ ‘--output=MODE’ Adjust the standard output stream buffering. ‘-e MODE’ ‘--error=MODE’ Adjust the standard error stream buffering. The MODE can be specified as follows: ‘L’ Set the stream to line buffered mode. In this mode data is coalesced until a newline is output or input is read from any stream attached to a terminal device. This option is invalid with standard input. ‘0’ Disable buffering of the selected stream. In this mode, data is output immediately and only the amount of data requested is read from input. Note the difference in function for input and output. Disabling buffering for input will not influence the responsiveness or blocking behavior of the stream input functions. For example ‘fread’ will still block until ‘EOF’ or error, even if the underlying ‘read’ returns less data than requested. ‘SIZE’ Specify the size of the buffer to use in fully buffered mode. SIZE may be, or may be an integer optionally followed by, one of the following multiplicative suffixes: ‘KB’ => 1000 (KiloBytes) ‘K’ => 1024 (KibiBytes) ‘MB’ => 1000*1000 (MegaBytes) ‘M’ => 1024*1024 (MebiBytes) ‘GB’ => 1000*1000*1000 (GigaBytes) ‘G’ => 1024*1024*1024 (GibiBytes) and so on for ‘T’, ‘P’, ‘E’, ‘Z’, ‘Y’, ‘R’, and ‘Q’. Binary prefixes can be used, too: ‘KiB’=‘K’, ‘MiB’=‘M’, and so on. ‘stdbuf’ is installed only on platforms that use the Executable and Linkable Format (ELF) and support the ‘constructor’ attribute, so portable scripts should not rely on its existence. Exit status: 125 if ‘stdbuf’ itself fails 126 if COMMAND is found but cannot be invoked 127 if COMMAND cannot be found the exit status of COMMAND otherwise 23.6 ‘timeout’: Run a command with a time limit =============================================== ‘timeout’ runs the given COMMAND and kills it if it is still running after the specified time interval. Synopsis: timeout [OPTION] DURATION COMMAND [ARG]... COMMAND must not be a special built-in utility (*note Special built-in utilities::). The program accepts the following options. Also see *note Common options::. Options must precede operands. ‘--preserve-status’ Return the exit status of the managed COMMAND on timeout, rather than a specific exit status indicating a timeout. This is useful if the managed COMMAND supports running for an indeterminate amount of time. ‘--foreground’ Don’t create a separate background program group, so that the managed COMMAND can use the foreground TTY normally. This is needed to support two situations when timing out commands, when not invoking ‘timeout’ from an interactive shell. 1. COMMAND is interactive and needs to read from the terminal for example 2. the user wants to support sending signals directly to COMMAND from the terminal (like Ctrl-C for example) Note in this mode of operation, any children of COMMAND will not be timed out. Also SIGCONT will not be sent to COMMAND, as it’s generally not needed with foreground processes, and can cause intermittent signal delivery issues with programs that are monitors themselves (like GDB for example). ‘-k DURATION’ ‘--kill-after=DURATION’ Ensure the monitored COMMAND is killed by also sending a ‘KILL’ signal. The specified DURATION starts from the point in time when ‘timeout’ sends the initial signal to COMMAND, i.e., not from the beginning when the COMMAND is started. This option has no effect if either the main DURATION of the ‘timeout’ command, or the DURATION specified to this option, is 0. This option may be useful if the selected signal did not kill the COMMAND, either because the signal was blocked or ignored, or if the COMMAND takes too long (e.g. for cleanup work) to terminate itself within a certain amount of time. ‘-s SIGNAL’ ‘--signal=SIGNAL’ Send this SIGNAL to COMMAND on timeout, rather than the default ‘TERM’ signal. SIGNAL may be a name like ‘HUP’ or a number. *Note Signal specifications::. ‘-v’ ‘--verbose’ Diagnose to standard error, any signal sent upon timeout. DURATION is a floating point number in either the current or the C locale (*note Floating point::) followed by an optional unit: ‘s’ for seconds (the default) ‘m’ for minutes ‘h’ for hours ‘d’ for days A duration of 0 disables the associated timeout. Note that the actual timeout duration is dependent on system conditions, which should be especially considered when specifying sub-second timeouts. Exit status: 124 if COMMAND times out, and ‘--preserve-status’ is not specified 125 if ‘timeout’ itself fails 126 if COMMAND is found but cannot be invoked 127 if COMMAND cannot be found 137 if COMMAND or ‘timeout’ is sent the KILL(9) signal (128+9) the exit status of COMMAND otherwise In the case of the ‘KILL(9)’ signal, ‘timeout’ returns with exit status 137, regardless of whether that signal is sent to COMMAND or to ‘timeout’ itself, i.e., these cases cannot be distinguished. In the latter case, the COMMAND process may still be alive after ‘timeout’ has forcefully been terminated. Examples: # Send the default TERM signal after 20s to a short-living 'sleep 1'. # As that terminates long before the given duration, 'timeout' returns # with the same exit status as the command, 0 in this case. timeout 20 sleep 1 # Send the INT signal after 5s to the 'sleep' command. Returns after # 5 seconds with exit status 124 to indicate the sending of the signal. timeout -s INT 5 sleep 20 # Likewise, but the command ignoring the INT signal due to being started # via 'env --ignore-signal'. Thus, 'sleep' terminates regularly after # the full 20 seconds, still 'timeout' returns with exit status 124. timeout -s INT 5s env --ignore-signal=INT sleep 20 # Likewise, but sending the KILL signal 3 seconds after the initial # INT signal. Hence, 'sleep' is forcefully terminated after about # 8 seconds (5+3), and 'timeout' returns with an exit status of 137. timeout -s INT -k 3s 5s env --ignore-signal=INT sleep 20 24 Process control ****************** 24.1 ‘kill’: Send a signal to processes ======================================= The ‘kill’ command sends a signal to processes, causing them to terminate or otherwise act upon receiving the signal in some way. Alternatively, it lists information about signals. Synopses: kill [-s SIGNAL | --signal SIGNAL | -SIGNAL] PID... kill [-l | --list | -t | --table] [SIGNAL]... Due to shell aliases and built-in ‘kill’ functions, using an unadorned ‘kill’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env kill ...’) to avoid interference from the shell. The first form of the ‘kill’ command sends a signal to all PID arguments. The default signal to send if none is specified is ‘TERM’. The special signal number ‘0’ does not denote a valid signal, but can be used to test whether the PID arguments specify processes to which a signal could be sent. If PID is positive, the signal is sent to the process with the process ID PID. If PID is zero, the signal is sent to all processes in the process group of the current process. If PID is −1, the signal is sent to all processes for which the user has permission to send a signal. If PID is less than −1, the signal is sent to all processes in the process group that equals the absolute value of PID. If PID is not positive, a system-dependent set of system processes is excluded from the list of processes to which the signal is sent. If a negative PID argument is desired as the first one, it should be preceded by ‘--’. However, as a common extension to POSIX, ‘--’ is not required with ‘kill -SIGNAL -PID’. The following commands are equivalent: kill -15 -1 kill -TERM -1 kill -s TERM -- -1 kill -- -1 The first form of the ‘kill’ command succeeds if every PID argument specifies at least one process that the signal was sent to. The second form of the ‘kill’ command lists signal information. Either the ‘-l’ or ‘--list’ option, or the ‘-t’ or ‘--table’ option must be specified. Without any SIGNAL argument, all supported signals are listed. The output of ‘-l’ or ‘--list’ is a list of the signal names, one per line; if SIGNAL is already a name, the signal number is printed instead. The output of ‘-t’ or ‘--table’ is a table of signal numbers, names, and descriptions. This form of the ‘kill’ command succeeds if all SIGNAL arguments are valid and if there is no output error. The ‘kill’ command also supports the ‘--help’ and ‘--version’ options. *Note Common options::. A SIGNAL may be a signal name like ‘HUP’, or a signal number like ‘1’, or an exit status of a process terminated by the signal. A signal name can be given in canonical form or prefixed by ‘SIG’. The case of the letters is ignored, except for the ‘-SIGNAL’ option which must use upper case to avoid ambiguity with lower case option letters. *Note Signal specifications::, for a list of supported signal names and numbers. 25 Delaying *********** 25.1 ‘sleep’: Delay for a specified time ======================================== ‘sleep’ pauses for an amount of time specified by the sum of the values of the command line arguments. Synopsis: sleep NUMBER[smhd]... Each argument is a non-negative number followed by an optional unit; the default is seconds. The units are: ‘s’ seconds ‘m’ minutes ‘h’ hours ‘d’ days Although portable POSIX scripts must give ‘sleep’ a single non-negative integer argument without a suffix, GNU ‘sleep’ also accepts two or more arguments, unit suffixes, and floating-point numbers in either the current or the C locale. *Note Floating point::. For instance, the following could be used to ‘sleep’ for 1 second, 234 milli-, 567 micro- and 890 nanoseconds: sleep 1234e-3 567.89e-6 Also one could sleep indefinitely like: sleep inf The only options are ‘--help’ and ‘--version’. *Note Common options::. Due to shell aliases and built-in ‘sleep’ functions, using an unadorned ‘sleep’ interactively or in a script may get you different functionality than that described here. Invoke it via ‘env’ (i.e., ‘env sleep ...’) to avoid interference from the shell. An exit status of zero indicates success, and a nonzero value indicates failure. 26 Numeric operations ********************* These programs do numerically-related operations. 26.1 ‘factor’: Print prime factors ================================== ‘factor’ prints prime factors. Synopsis: factor [OPTION]... [NUMBER]... If no NUMBER is specified on the command line, ‘factor’ reads numbers from standard input, delimited by newlines, tabs, or spaces. The program accepts the following options. Also see *note Common options::. ‘-h’ ‘--exponents’ print factors in the form p^e, rather than repeating the prime ‘p’, ‘e’ times. If the exponent ‘e’ is 1, then it is omitted. $ factor --exponents 3000 3000: 2^3 3 5^3 If the number to be factored is small (less than 2^{127} on typical machines), ‘factor’ uses a faster algorithm. For example, on a circa-2017 Intel Xeon Silver 4116, factoring the product of the eighth and ninth Mersenne primes (approximately 2^{92}) takes about 4 ms of CPU time: $ M8=$(echo 2^31-1 | bc) $ M9=$(echo 2^61-1 | bc) $ n=$(echo "$M8 * $M9" | bc) $ bash -c "time factor $n" 4951760154835678088235319297: 2147483647 2305843009213693951 real 0m0.004s user 0m0.004s sys 0m0.000s For larger numbers, ‘factor’ uses a slower algorithm. On the same platform, factoring the eighth Fermat number 2^{256} + 1 takes about 14 seconds, and the slower algorithm would have taken about 750 ms to factor 2^{127} - 3 instead of the 50 ms needed by the faster algorithm. Factoring large numbers is, in general, hard. The Pollard-Brent rho algorithm used by ‘factor’ is particularly effective for numbers with relatively small factors. If you wish to factor large numbers which do not have small factors (for example, numbers which are the product of two large primes), other methods are far better. An exit status of zero indicates success, and a nonzero value indicates failure. 26.2 ‘numfmt’: Reformat numbers =============================== ‘numfmt’ reads numbers in various representations and reformats them as requested. The most common usage is converting numbers to/from _human_ representation (e.g. ‘4G’ ↦ ‘4,000,000,000’). numfmt [OPTION]... [NUMBER] ‘numfmt’ converts each NUMBER on the command-line according to the specified options (see below). If no NUMBERs are given, it reads numbers from standard input. ‘numfmt’ can optionally extract numbers from specific columns, maintaining proper line padding and alignment. An exit status of zero indicates success, and a nonzero value indicates failure. See ‘--invalid’ for additional information regarding exit status. 26.2.1 General options ---------------------- The program accepts the following options. Also see *note Common options::. ‘--debug’ Print (to standard error) warning messages about possible erroneous usage. ‘-d D’ ‘--delimiter=D’ Use the character D as input field separator (default: whitespace). _Note_: Using non-default delimiter turns off automatic padding. ‘--field=FIELDS’ Convert the number in input field FIELDS (default: 1). FIELDS supports ‘cut’ style field ranges: N N'th field, counted from 1 N- from N'th field, to end of line N-M from N'th to M'th field (inclusive) -M from first to M'th field (inclusive) - all fields ‘--format=FORMAT’ Use printf-style floating FORMAT string. The FORMAT string must contain one ‘%f’ directive, optionally with ‘'’, ‘-’, ‘0’, width or precision modifiers. The ‘'’ modifier will enable ‘--grouping’, the ‘-’ modifier will enable left-aligned ‘--padding’ and the width modifier will enable right-aligned ‘--padding’. The ‘0’ width modifier (without the ‘-’ modifier) will generate leading zeros on the number, up to the specified width. A precision specification like ‘%.1f’ will override the precision determined from the input data or set due to ‘--to’ option auto scaling. ‘--from=UNIT’ Auto-scales input numbers according to UNIT. See UNITS below. The default is no scaling, meaning suffixes (e.g. ‘M’, ‘G’) will trigger an error. ‘--from-unit=N’ Specify the input unit size (instead of the default 1). Use this option when the input numbers represent other units (e.g. if the input number ‘10’ represents 10 units of 512 bytes, use ‘--from-unit=512’). Suffixes are handled as with ‘--from=auto’. ‘--grouping’ Group digits in output numbers according to the current locale’s grouping rules (e.g _Thousands Separator_ character, commonly ‘.’ (dot) or ‘,’ comma). This option has no effect in ‘POSIX/C’ locale. ‘--header[=N]’ Print the first N (default: 1) lines without any conversion. ‘--invalid=MODE’ The default action on input errors is to exit immediately with status code 2. ‘--invalid=‘abort’’ explicitly specifies this default mode. With a MODE of ‘fail’, print a warning for _each_ conversion error, and exit with status 2. With a MODE of ‘warn’, exit with status 0, even in the presence of conversion errors, and with a MODE of ‘ignore’ do not even print diagnostics. ‘--padding=N’ Pad the output numbers to N characters, by adding spaces. If N is a positive number, numbers will be right-aligned. If N is a negative number, numbers will be left-aligned. By default, numbers are automatically aligned based on the input line’s width (only with the default delimiter). ‘--round=METHOD’ When converting number representations, round the number according to METHOD, which can be ‘up’, ‘down’, ‘from-zero’ (the default), ‘towards-zero’, ‘nearest’. ‘--suffix=SUFFIX’ Add ‘SUFFIX’ to the output numbers, and accept optional ‘SUFFIX’ in input numbers. ‘--to=UNIT’ Auto-scales output numbers according to UNIT. See _Units_ below. The default is no scaling, meaning all the digits of the number are printed. ‘--to-unit=N’ Specify the output unit size (instead of the default 1). Use this option when the output numbers represent other units (e.g. to represent ‘4,000,000’ bytes in blocks of 1KB, use ‘--to=si --to-unit=1000’). Suffixes are handled as with ‘--from=auto’. ‘-z’ ‘--zero-terminated’ Delimit items with a zero byte rather than a newline (ASCII LF). I.e., treat input as items separated by ASCII NUL and terminate output items with ASCII NUL. This option can be useful in conjunction with ‘perl -0’ or ‘find -print0’ and ‘xargs -0’ which do the same in order to reliably handle arbitrary file names (even those containing blanks or other special characters). Note with ‘-z’ the newline character is treated as a field separator. 26.2.2 Possible UNITs: ---------------------- The following are the possible UNIT options with ‘--from=UNITS’ and ‘--to=UNITS’: NONE No scaling is performed. For input numbers, no suffixes are accepted, and any trailing characters following the number will trigger an error. For output numbers, all digits of the numbers will be printed. SI Auto-scale numbers according to the _International System of Units (SI)_ standard. For input numbers, accept one of the following suffixes. For output numbers, values larger than 1000 will be rounded, and printed with one of the following suffixes: ‘K’ => 1000^1 = 10^3 (Kilo) ‘M’ => 1000^2 = 10^6 (Mega) ‘G’ => 1000^3 = 10^9 (Giga) ‘T’ => 1000^4 = 10^{12} (Tera) ‘P’ => 1000^5 = 10^{15} (Peta) ‘E’ => 1000^6 = 10^{18} (Exa) ‘Z’ => 1000^7 = 10^{21} (Zetta) ‘Y’ => 1000^8 = 10^{24} (Yotta) ‘R’ => 1000^9 = 10^{27} (Ronna) ‘Q’ => 1000^{10} = 10^{30} (Quetta) IEC Auto-scale numbers according to the _International Electrotechnical Commission (IEC)_ standard. For input numbers, accept one of the following suffixes. For output numbers, values larger than 1024 will be rounded, and printed with one of the following suffixes: ‘K’ => 1024^1 = 2^{10} (Kibi) ‘M’ => 1024^2 = 2^{20} (Mebi) ‘G’ => 1024^3 = 2^{30} (Gibi) ‘T’ => 1024^4 = 2^{40} (Tebi) ‘P’ => 1024^5 = 2^{50} (Pebi) ‘E’ => 1024^6 = 2^{60} (Exbi) ‘Z’ => 1024^7 = 2^{70} (Zebi) ‘Y’ => 1024^8 = 2^{80} (Yobi) ‘R’ => 1024^9 = 2^{90} (Robi) ‘Q’ => 1024^{10} = 2^{100} (Quebi) The ‘iec’ option uses a single letter suffix (e.g. ‘G’), which is not fully standard, as the _iec_ standard recommends a two-letter symbol (e.g ‘Gi’) – but in practice, this method is common. Compare with the ‘iec-i’ option. IEC-I Auto-scale numbers according to the _International Electrotechnical Commission (IEC)_ standard. For input numbers, accept one of the following suffixes. For output numbers, values larger than 1024 will be rounded, and printed with one of the following suffixes: ‘Ki’ => 1024^1 = 2^{10} (Kibi) ‘Mi’ => 1024^2 = 2^{20} (Mebi) ‘Gi’ => 1024^3 = 2^{30} (Gibi) ‘Ti’ => 1024^4 = 2^{40} (Tebi) ‘Pi’ => 1024^5 = 2^{50} (Pebi) ‘Ei’ => 1024^6 = 2^{60} (Exbi) ‘Zi’ => 1024^7 = 2^{70} (Zebi) ‘Yi’ => 1024^8 = 2^{80} (Yobi) ‘Ri’ => 1024^9 = 2^{90} (Robi) ‘Qi’ => 1024^{10} = 2^{100} (Quebi) The ‘iec-i’ option uses a two-letter suffix symbol (e.g. ‘Gi’), as the _iec_ standard recommends, but this is not always common in practice. Compare with the ‘iec’ option. AUTO ‘auto’ can only be used with ‘--from’. With this method, numbers with single-letter suffixes like ‘K’ suffixes are interpreted as _SI_ values, and numbers with two-letter suffixes like ‘Ki’ are interpreted as _IEC_ values. 26.2.3 Examples of using ‘numfmt’ --------------------------------- Converting a single number from/to _human_ representation: $ numfmt --to=si 500000 500K $ numfmt --to=iec 500000 489K $ numfmt --to=iec-i 500000 489Ki $ numfmt --from=si 1M 1000000 $ numfmt --from=iec 1M 1048576 # with '--from=auto', M=Mega, Mi=Mebi $ numfmt --from=auto 1M 1000000 $ numfmt --from=auto 1Mi 1048576 Converting from ‘SI’ to ‘IEC’ scales (e.g. when a drive’s capacity is advertised as ‘1TB’, while checking the drive’s capacity gives lower values): $ numfmt --from=si --to=iec 1T 932G With both input and output scales specified, the largest defined prefixes are supported: $ numfmt --from=si --to=iec-i 2000R 1.6Qi Converting a single field from an input file / piped input (these contrived examples are for demonstration purposes only, as both ‘ls’ and ‘df’ support the ‘--human-readable’ option to output sizes in human-readable format): # Third field (file size) will be shown in SI representation $ ls -log | numfmt --field 3 --header --to=si | head -n4 -rw-r--r-- 1 94K Aug 23 2011 ABOUT-NLS -rw-r--r-- 1 3.7K Jan 7 16:15 AUTHORS -rw-r--r-- 1 36K Jun 1 2011 COPYING -rw-r--r-- 1 0 Jan 7 15:15 ChangeLog # Second field (size) will be shown in IEC representation $ df --block-size=1 | numfmt --field 2 --header --to=iec | head -n4 File system 1B-blocks Used Available Use% Mounted on rootfs 132G 104741408 26554036 80% / tmpfs 794M 7580 804960 1% /run/shm /dev/sdb1 694G 651424756 46074696 94% /home Output can be tweaked using ‘--padding’ or ‘--format’: # Pad to 10 characters, right-aligned $ du -s * | numfmt --to=si --padding=10 2.5K config.log 108 config.status 1.7K configure 20 configure.ac # Pad to 10 characters, left-aligned $ du -s * | numfmt --to=si --padding=-10 2.5K config.log 108 config.status 1.7K configure 20 configure.ac # Pad to 10 characters, left-aligned, using 'format' $ du -s * | numfmt --to=si --format="%10f" 2.5K config.log 108 config.status 1.7K configure 20 configure.ac # Pad to 10 characters, left-aligned, using 'format' $ du -s * | numfmt --to=si --padding="%-10f" 2.5K config.log 108 config.status 1.7K configure 20 configure.ac With locales that support grouping digits, using ‘--grouping’ or ‘--format’ enables grouping. In ‘POSIX’ locale, grouping is silently ignored: $ LC_ALL=C numfmt --from=iec --grouping 2G 2147483648 $ LC_ALL=en_US.utf8 numfmt --from=iec --grouping 2G 2,147,483,648 $ LC_ALL=ta_IN numfmt --from=iec --grouping 2G 2,14,74,83,648 $ LC_ALL=C numfmt --from=iec --format="==%'15f==" 2G == 2147483648== $ LC_ALL=en_US.utf8 numfmt --from=iec --format="==%'15f==" 2G == 2,147,483,648== $ LC_ALL=en_US.utf8 numfmt --from=iec --format="==%'-15f==" 2G ==2,147,483,648 == $ LC_ALL=ta_IN numfmt --from=iec --format="==%'15f==" 2G == 2,14,74,83,648== 26.3 ‘seq’: Print numeric sequences =================================== ‘seq’ prints a sequence of numbers to standard output. Synopses: seq [OPTION]... LAST seq [OPTION]... FIRST LAST seq [OPTION]... FIRST INCREMENT LAST ‘seq’ prints the numbers from FIRST to LAST by INCREMENT. By default, each number is printed on a separate line. When INCREMENT is not specified, it defaults to ‘1’, even when FIRST is larger than LAST. FIRST also defaults to ‘1’. So ‘seq 1’ prints ‘1’, but ‘seq 0’ and ‘seq 10 5’ produce no output. The sequence of numbers ends when the sum of the current number and INCREMENT would become greater than LAST, so ‘seq 1 10 10’ only produces ‘1’. INCREMENT must not be ‘0’; use the tool ‘yes’ to get repeated output of a constant number. FIRST, INCREMENT and LAST must not be ‘NaN’, but ‘inf’ is supported. Floating-point numbers may be specified in either the current or the C locale. *Note Floating point::. The program accepts the following options. Also see *note Common options::. Options must precede operands. ‘-f FORMAT’ ‘--format=FORMAT’ Print all numbers using FORMAT. FORMAT must contain exactly one of the ‘printf’-style floating point conversion specifications ‘%a’, ‘%e’, ‘%f’, ‘%g’, ‘%A’, ‘%E’, ‘%F’, ‘%G’. The ‘%’ may be followed by zero or more flags taken from the set ‘-+#0 '’, then an optional width containing one or more digits, then an optional precision consisting of a ‘.’ followed by zero or more digits. FORMAT may also contain any number of ‘%%’ conversion specifications. All conversion specifications have the same meaning as with ‘printf’. The default format is derived from FIRST, STEP, and LAST. If these all use a fixed point decimal representation, the default format is ‘%.Pf’, where P is the minimum precision that can represent the output numbers exactly. Otherwise, the default format is ‘%g’. ‘-s STRING’ ‘--separator=STRING’ Separate numbers with STRING; default is a newline. The output always terminates with a newline. ‘-w’ ‘--equal-width’ Print all numbers with the same width, by padding with leading zeros. FIRST, STEP, and LAST should all use a fixed point decimal representation. (To have other kinds of padding, use ‘--format’). You can get finer-grained control over output with ‘-f’: $ seq -f '(%9.2E)' -9e5 1.1e6 1.3e6 (-9.00E+05) ( 2.00E+05) ( 1.30E+06) If you want hexadecimal integer output, you can use ‘printf’ to perform the conversion: $ printf '%x\n' $(seq 1048575 1024 1050623) fffff 1003ff 1007ff For very long lists of numbers, use xargs to avoid system limitations on the length of an argument list: $ seq 1000000 | xargs printf '%x\n' | tail -n 3 f423e f423f f4240 To generate octal output, use the printf ‘%o’ format instead of ‘%x’. On most systems, seq can produce whole-number output for values up to at least 2^{53}. Larger integers are approximated. The details differ depending on your floating-point implementation. *Note Floating point::. A common case is that ‘seq’ works with integers through 2^{64}, and larger integers may not be numerically correct: $ seq 50000000000000000000 2 50000000000000000004 50000000000000000000 50000000000000000000 50000000000000000004 However, note that when limited to non-negative whole numbers, an increment of less than 200, and no format-specifying option, seq can print arbitrarily large numbers. Therefore ‘seq inf’ can be used to generate an infinite sequence of numbers. Be careful when using ‘seq’ with outlandish values: otherwise you may see surprising results, as ‘seq’ uses floating point internally. For example, on the x86 platform, where the internal representation uses a 64-bit fraction, the command: seq 1 0.0000000000000000001 1.0000000000000000009 outputs 1.0000000000000000007 twice and skips 1.0000000000000000008. An exit status of zero indicates success, and a nonzero value indicates failure. 27 File permissions ******************* Each file has a set of “file mode bits” that control the kinds of access that users have to that file. They can be represented either in symbolic form or as an octal number. 27.1 Structure of File Mode Bits ================================ The file mode bits have two parts: the “file permission bits”, which control ordinary access to the file, and “special mode bits”, which affect only some files. There are three kinds of permissions that a user can have for a file: 1. permission to read the file. For directories, this means permission to list the contents of the directory. 2. permission to write to (change) the file. For directories, this means permission to create and remove files in the directory. 3. permission to execute the file (run it as a program). For directories, this means permission to access files in the directory. There are three categories of users who may have different permissions to perform any of the above operations on a file: 1. the file’s owner; 2. other users who are in the file’s group; 3. everyone else. Files are given an owner and group when they are created. Usually the owner is the current user and the group is the group of the directory the file is in, but this varies with the operating system, the file system the file is created on, and the way the file is created. You can change the owner and group of a file by using the ‘chown’ and ‘chgrp’ commands. In addition to the three sets of three permissions listed above, the file mode bits have three special components, which affect only executable files (programs) and, on most systems, directories: The “set-user-ID bit” (“setuid bit”). On execution, set the process’s effective user ID to that of the file. For directories on a few systems, give files created in the directory the same owner as the directory, no matter who creates them, and set the set-user-ID bit of newly-created subdirectories. The “set-group-ID bit” (“setgid bit”). On execution, set the process’s effective group ID to that of the file. For directories on most systems, give files created in the directory the same group as the directory, no matter what group the user who creates them is in, and set the set-group-ID bit of newly-created subdirectories. The “restricted deletion flag” or “sticky bit”. Prevent unprivileged users from removing or renaming a file in a directory unless they own the file or the directory; this is commonly found on world-writable directories like ‘/tmp’. For regular files on some older systems, save the program’s text image on the swap device so it will load more quickly when run, so that the image is “sticky”. In addition to the file mode bits listed above, there may be file attributes specific to the file system, e.g., access control lists (ACLs), whether a file is compressed, whether a file can be modified (immutability), and whether a file can be dumped. These are usually set using programs specific to the file system. For example: ext2 On GNU and GNU/Linux the file attributes specific to the ext2 file system are set using ‘chattr’. FFS On FreeBSD the file flags specific to the FFS file system are set using ‘chflags’. Even if a file’s mode bits allow an operation on that file, that operation may still fail, because: • the file-system-specific attributes or flags do not permit it; or • the file system is mounted as read-only. For example, if the immutable attribute is set on a file, it cannot be modified, regardless of the fact that you may have just run ‘chmod a+w FILE’. 27.2 Symbolic Modes =================== “Symbolic modes” represent changes to files’ mode bits as operations on single-character symbols. They allow you to modify either all or selected parts of files’ mode bits, optionally based on their previous values, and perhaps on the current ‘umask’ as well (*note Umask and Protection::). The format of symbolic modes is: [ugoa...][-+=]PERMS...[,...] where PERMS is either zero or more letters from the set ‘rwxXst’, or a single letter from the set ‘ugo’. The following sections describe the operators and other details of symbolic modes. 27.2.1 Setting Permissions -------------------------- The basic symbolic operations on a file’s permissions are adding, removing, and setting the permission that certain users have to read, write, and execute or search the file. These operations have the following format: USERS OPERATION PERMISSIONS The spaces between the three parts above are shown for readability only; symbolic modes cannot contain spaces. The USERS part tells which users’ access to the file is changed. It consists of one or more of the following letters (or it can be empty; *note Umask and Protection::, for a description of what happens then). When more than one of these letters is given, the order that they are in does not matter. ‘u’ the user who owns the file; ‘g’ other users who are in the file’s group; ‘o’ all other users; ‘a’ all users; the same as ‘ugo’. The OPERATION part tells how to change the affected users’ access to the file, and is one of the following symbols: ‘+’ to add the PERMISSIONS to whatever permissions the USERS already have for the file; ‘-’ to remove the PERMISSIONS from whatever permissions the USERS already have for the file; ‘=’ to make the PERMISSIONS the only permissions that the USERS have for the file. The PERMISSIONS part tells what kind of access to the file should be changed; it is normally zero or more of the following letters. As with the USERS part, the order does not matter when more than one letter is given. Omitting the PERMISSIONS part is useful only with the ‘=’ operation, where it gives the specified USERS no access at all to the file. ‘r’ the permission the USERS have to read the file; ‘w’ the permission the USERS have to write to the file; ‘x’ the permission the USERS have to execute the file, or search it if it is a directory. For example, to give everyone permission to read and write a regular file, but not to execute it, use: a=rw To remove write permission for all users other than the file’s owner, use: go-w The above command does not affect the access that the owner of the file has to it, nor does it affect whether other users can read or execute the file. To give everyone except a file’s owner no permission to do anything with that file, use the mode below. Other users could still remove the file, if they have write permission on the directory it is in. go= Another way to specify the same thing is: og-rwx 27.2.2 Copying Existing Permissions ----------------------------------- You can base a file’s permissions on its existing permissions. To do this, instead of using a series of ‘r’, ‘w’, or ‘x’ letters after the operator, you use the letter ‘u’, ‘g’, or ‘o’. For example, the mode o+g adds the permissions for users who are in a file’s group to the permissions that other users have for the file. Thus, if the file started out as mode 664 (‘rw-rw-r--’), the above mode would change it to mode 666 (‘rw-rw-rw-’). If the file had started out as mode 741 (‘rwxr----x’), the above mode would change it to mode 745 (‘rwxr--r-x’). The ‘-’ and ‘=’ operations work analogously. 27.2.3 Changing Special Mode Bits --------------------------------- In addition to changing a file’s read, write, and execute/search permissions, you can change its special mode bits. *Note Mode Structure::, for a summary of these special mode bits. To change the file mode bits to set the user ID on execution, use ‘u’ in the USERS part of the symbolic mode and ‘s’ in the PERMISSIONS part. To change the file mode bits to set the group ID on execution, use ‘g’ in the USERS part of the symbolic mode and ‘s’ in the PERMISSIONS part. To set both user and group ID on execution, omit the USERS part of the symbolic mode (or use ‘a’) and use ‘s’ in the PERMISSIONS part. To change the file mode bits to set the restricted deletion flag or sticky bit, omit the USERS part of the symbolic mode (or use ‘a’) and use ‘t’ in the PERMISSIONS part. For example, to set the set-user-ID mode bit of a program, you can use the mode: u+s To remove both set-user-ID and set-group-ID mode bits from it, you can use the mode: a-s To set the restricted deletion flag or sticky bit, you can use the mode: +t The combination ‘o+s’ has no effect. On GNU systems the combinations ‘u+t’ and ‘g+t’ have no effect, and ‘o+t’ acts like plain ‘+t’. The ‘=’ operator is not very useful with special mode bits. For example, the mode: o=t does set the restricted deletion flag or sticky bit, but it also removes all read, write, and execute/search permissions that users not in the file’s group might have had for it. *Note Directory Setuid and Setgid::, for additional rules concerning set-user-ID and set-group-ID bits and directories. 27.2.4 Conditional Executability -------------------------------- There is one more special type of symbolic permission: if you use ‘X’ instead of ‘x’, execute/search permission is affected only if the file is a directory or already had execute permission. For example, this mode: a+X gives all users permission to search directories, or to execute files if anyone could execute them before. 27.2.5 Making Multiple Changes ------------------------------ The format of symbolic modes is actually more complex than described above (*note Setting Permissions::). It provides two ways to make multiple changes to files’ mode bits. The first way is to specify multiple OPERATION and PERMISSIONS parts after a USERS part in the symbolic mode. For example, the mode: og+rX-w gives users other than the owner of the file read permission and, if it is a directory or if someone already had execute permission to it, gives them execute/search permission; and it also denies them write permission to the file. It does not affect the permission that the owner of the file has for it. The above mode is equivalent to the two modes: og+rX og-w The second way to make multiple changes is to specify more than one simple symbolic mode, separated by commas. For example, the mode: a+r,go-w gives everyone permission to read the file and removes write permission on it for all users except its owner. Another example: u=rwx,g=rx,o= sets all of the permission bits for the file explicitly. (It gives users who are not in the file’s group no permission at all for it.) The two methods can be combined. The mode: a+r,g+x-w gives all users permission to read the file, and gives users who are in the file’s group permission to execute/search it as well, but not permission to write to it. The above mode could be written in several different ways; another is: u+r,g+rx,o+r,g-w 27.2.6 The Umask and Protection ------------------------------- If the USERS part of a symbolic mode is omitted, it defaults to ‘a’ (affect all users), except that any permissions that are _set_ in the system variable ‘umask’ are _not affected_. The value of ‘umask’ can be set using the ‘umask’ command. Its default value varies from system to system. Omitting the USERS part of a symbolic mode is generally not useful with operations other than ‘+’. It is useful with ‘+’ because it allows you to use ‘umask’ as an easily customizable protection against giving away more permission to files than you intended to. As an example, if ‘umask’ has the value 2, which removes write permission for users who are not in the file’s group, then the mode: +w adds permission to write to the file to its owner and to other users who are in the file’s group, but _not_ to other users. In contrast, the mode: a+w ignores ‘umask’, and _does_ give write permission for the file to all users. 27.3 Numeric Modes ================== As an alternative to giving a symbolic mode, you can give an octal (base 8) number that represents the mode. The permissions granted to the user, to other users in the file’s group, and to other users not in the file’s group each require three bits: one bit for read, one for write, and one for execute/search permission. These three bits are represented as one octal digit; for example, if all three are present, the resulting 111 (in binary) is represented as the digit 7 (in octal). The three special mode bits also require one bit each, and they are as a group represented as another octal digit. Here is how the bits are arranged, starting with the highest valued bit: Value in Corresponding Mode Mode Bit Special mode bits: 4000 Set user ID 2000 Set group ID 1000 Restricted deletion flag or sticky bit The file's owner: 400 Read 200 Write 100 Execute/search Other users in the file's group: 40 Read 20 Write 10 Execute/search Other users not in the file's group: 4 Read 2 Write 1 Execute/search For example, numeric mode ‘4751’ corresponds to symbolic mode ‘u=srwx,g=rx,o=x’, and numeric mode ‘664’ corresponds to symbolic mode ‘ug=rw,o=r’. Numeric mode ‘0’ corresponds to symbolic mode ‘a=’. A numeric mode is usually shorter than the corresponding symbolic mode, but it is limited in that normally it cannot take into account the previous file mode bits; it can only set them absolutely. The set-user-ID and set-group-ID bits of directories are an exception to this general limitation. *Note Directory Setuid and Setgid::. Also, operator numeric modes can take previous file mode bits into account. *Note Operator Numeric Modes::. Numeric modes are always interpreted in octal; you do not have to add a leading ‘0’, as you do in C. Mode ‘0055’ is the same as mode ‘55’. However, modes of five digits or more, such as ‘00055’, are sometimes special (*note Directory Setuid and Setgid::). 27.4 Operator Numeric Modes =========================== An operator numeric mode is a numeric mode that is prefixed by a ‘-’, ‘+’, or ‘=’ operator, which has the same interpretation as in symbolic modes. For example, ‘+440’ enables read permission for the file’s owner and group, ‘-1’ disables execute permission for other users, and ‘=600’ clears all permissions except for enabling read-write permissions for the file’s owner. Operator numeric modes can be combined with symbolic modes by separating them with a comma; for example, ‘=0,u+r’ clears all permissions except for enabling read permission for the file’s owner. The commands ‘chmod =755 DIR’ and ‘chmod 755 DIR’ differ in that the former clears the directory DIR’s setuid and setgid bits, whereas the latter preserves them. *Note Directory Setuid and Setgid::. Operator numeric modes are a GNU extension. 27.5 Directories and the Set-User-ID and Set-Group-ID Bits ========================================================== On most systems, if a directory’s set-group-ID bit is set, newly created subfiles inherit the same group as the directory, and newly created subdirectories inherit the set-group-ID bit of the parent directory. On a few systems, a directory’s set-user-ID bit has a similar effect on the ownership of new subfiles and the set-user-ID bits of new subdirectories. These mechanisms let users share files more easily, by lessening the need to use ‘chmod’ or ‘chown’ to share new files. These convenience mechanisms rely on the set-user-ID and set-group-ID bits of directories. If commands like ‘chmod’ and ‘mkdir’ routinely cleared these bits on directories, the mechanisms would be less convenient and it would be harder to share files. Therefore, a command like ‘chmod’ does not affect the set-user-ID or set-group-ID bits of a directory unless the user specifically mentions them in a symbolic mode, or uses an operator numeric mode such as ‘=755’, or sets them in a numeric mode, or clears them in a numeric mode that has five or more octal digits. For example, on systems that support set-group-ID inheritance: # These commands leave the set-user-ID and # set-group-ID bits of the subdirectories alone, # so that they retain their default values. mkdir A B C chmod 755 A chmod 0755 B chmod u=rwx,go=rx C mkdir -m 755 D mkdir -m 0755 E mkdir -m u=rwx,go=rx F If you want to try to set these bits, you must mention them explicitly in the symbolic or numeric modes, e.g.: # These commands try to set the set-user-ID # and set-group-ID bits of the subdirectories. mkdir G chmod 6755 G chmod +6000 G chmod u=rwx,go=rx,a+s G mkdir -m 6755 H mkdir -m +6000 I mkdir -m u=rwx,go=rx,a+s J If you want to try to clear these bits, you must mention them explicitly in a symbolic mode, or use an operator numeric mode, or specify a numeric mode with five or more octal digits, e.g.: # These commands try to clear the set-user-ID # and set-group-ID bits of the directory D. chmod a-s D chmod -6000 D chmod =755 D chmod 00755 D This behavior is a GNU extension. Portable scripts should not rely on requests to set or clear these bits on directories, as POSIX allows implementations to ignore these requests. The GNU behavior with numeric modes of four or fewer digits is intended for scripts portable to systems that preserve these bits; the behavior with numeric modes of five or more digits is for scripts portable to systems that do not preserve the bits. 28 File timestamps ****************** Standard POSIX files have three timestamps: the access timestamp (atime) of the last read, the modification timestamp (mtime) of the last write, and the status change timestamp (ctime) of the last change to the file’s meta-information. Some file systems support a fourth time: the birth timestamp (birthtime) of when the file was created; by definition, birthtime never changes. One common example of a ctime change is when the permissions of a file change. Changing the permissions doesn’t access the file, so atime doesn’t change, nor does it modify the file, so the mtime doesn’t change. Yet, something about the file itself has changed, and this must be noted somewhere. This is the job of the ctime field. This is necessary, so that, for example, a backup program can make a fresh copy of the file, including the new permissions value. Another operation that modifies a file’s ctime without affecting the others is renaming. Naively, a file’s atime, mtime, and ctime are set to the current time whenever you read, write, or change the attributes of the file respectively, and searching a directory counts as reading it. A file’s atime and mtime can also be set directly, via the ‘touch’ command (*note touch invocation::). In practice, though, timestamps are not updated quite that way. For efficiency reasons, many systems are lazy about updating atimes: when a program accesses a file, they may delay updating the file’s atime, or may not update the file’s atime if the file has been accessed recently, or may not update the atime at all. Similar laziness, though typically not quite so extreme, applies to mtimes and ctimes. Some systems emulate timestamps instead of supporting them directly, and these emulations may disagree with the naive interpretation. For example, a system may fake an atime or ctime by using the mtime. The determination of what time is “current” depends on the platform. Platforms with network file systems often use different clocks for the operating system and for file systems; because updates typically uses file systems’ clocks by default, clock skew can cause the resulting file timestamps to appear to be in a program’s “future” or “past”. When the system updates a file timestamp to a desired time T (which is either the current time, or a time specified via the ‘touch’ command), there are several reasons the file’s timestamp may be set to a value that differs from T. First, T may have a higher resolution than supported. Second, a file system may use different resolutions for different types of times. Third, file timestamps may use a different resolution than operating system timestamps. Fourth, the operating system primitives used to update timestamps may employ yet a different resolution. For example, in theory a file system might use 10-microsecond resolution for access timestamp and 100-nanosecond resolution for modification timestamp, and the operating system might use nanosecond resolution for the current time and microsecond resolution for the primitive that ‘touch’ uses to set a file’s timestamp to an arbitrary value. 29 Date input formats ********************* First, a quote: Our units of temporal measurement, from seconds on up to months, are so complicated, asymmetrical and disjunctive so as to make coherent mental reckoning in time all but impossible. Indeed, had some tyrannical god contrived to enslave our minds to time, to make it all but impossible for us to escape subjection to sodden routines and unpleasant surprises, he could hardly have done better than handing down our present system. It is like a set of trapezoidal building blocks, with no vertical or horizontal surfaces, like a language in which the simplest thought demands ornate constructions, useless particles and lengthy circumlocutions. Unlike the more successful patterns of language and science, which enable us to face experience boldly or at least level-headedly, our system of temporal calculation silently and persistently encourages our terror of time. ... It is as though architects had to measure length in feet, width in meters and height in ells; as though basic instruction manuals demanded a knowledge of five different languages. It is no wonder then that we often look into our own immediate past or future, last Tuesday or a week from Sunday, with feelings of helpless confusion. ... —Robert Grudin, ‘Time and the Art of Living’. This section describes the textual date representations that GNU programs accept. These are the strings you, as a user, can supply as arguments to the various programs. The C interface (via the ‘parse_datetime’ function) is not described here. 29.1 General date syntax ======================== A “date” is a string, possibly empty, containing many items separated by whitespace. The whitespace may be omitted when no ambiguity arises. The empty string means the beginning of today (i.e., midnight). Order of the items is immaterial. A date string may contain many flavors of items: • calendar date items • time of day items • time zone items • combined date and time of day items • day of the week items • relative items • pure numbers. We describe each of these item types in turn, below. A few ordinal numbers may be written out in words in some contexts. This is most useful for specifying day of the week items or relative items (see below). Among the most commonly used ordinal numbers, the word ‘last’ stands for -1, ‘this’ stands for 0, and ‘first’ and ‘next’ both stand for 1. Because the word ‘second’ stands for the unit of time there is no way to write the ordinal number 2, but for convenience ‘third’ stands for 3, ‘fourth’ for 4, ‘fifth’ for 5, ‘sixth’ for 6, ‘seventh’ for 7, ‘eighth’ for 8, ‘ninth’ for 9, ‘tenth’ for 10, ‘eleventh’ for 11 and ‘twelfth’ for 12. When a month is written this way, it is still considered to be written numerically, instead of being “spelled in full”; this changes the allowed strings. In the current implementation, only English is supported for words and abbreviations like ‘AM’, ‘DST’, ‘EST’, ‘first’, ‘January’, ‘Sunday’, ‘tomorrow’, and ‘year’. The output of the ‘date’ command is not always acceptable as a date string, not only because of the language problem, but also because there is no standard meaning for time zone items like ‘IST’. When using ‘date’ to generate a date string intended to be parsed later, specify a date format that is independent of language and that does not use time zone items other than ‘UTC’ and ‘Z’. Here are some ways to do this: $ LC_ALL=C TZ=UTC0 date Tue Nov 15 02:02:42 UTC 2022 $ TZ=UTC0 date +'%Y-%m-%d %H:%M:%SZ' 2022-11-15 02:02:42Z $ date --rfc-3339=ns # --rfc-3339 is a GNU extension. 2022-11-14 21:02:42.000000000-05:00 $ date --rfc-email # a GNU extension Mon, 14 Nov 2022 21:02:42 -0500 $ date +'%Y-%m-%d %H:%M:%S %z' # %z is a GNU extension. 2022-11-14 21:02:42 -0500 $ date +'@%s.%N' # %s and %N are GNU extensions. @1668477762.692722128 Alphabetic case is completely ignored in dates. Comments may be introduced between round parentheses, as long as included parentheses are properly nested. Hyphens not followed by a digit are currently ignored. Leading zeros on numbers are ignored. Invalid dates like ‘2022-02-29’ or times like ‘24:00’ are rejected. In the typical case of a host that does not support leap seconds, a time like ‘23:59:60’ is rejected even if it corresponds to a valid leap second. 29.2 Calendar date items ======================== A “calendar date item” specifies a day of the year. It is specified differently, depending on whether the month is specified numerically or literally. All these strings specify the same calendar date: 2022-11-14 # ISO 8601. 22-11-14 # Assume 19xx for 69 through 99, # 20xx for 00 through 68 (not recommended). 11/14/2022 # Common U.S. writing. 14 November 2022 14 Nov 2022 # Three-letter abbreviations always allowed. November 14, 2022 14-nov-2022 14nov2022 The year can also be omitted. In this case, the last specified year is used, or the current year if none. For example: 11/14 nov 14 Here are the rules. For numeric months, the ISO 8601 format ‘YEAR-MONTH-DAY’ is allowed, where YEAR is any positive number, MONTH is a number between 01 and 12, and DAY is a number between 01 and 31. A leading zero must be present if a number is less than ten. If YEAR is 68 or smaller, then 2000 is added to it; otherwise, if YEAR is less than 100, then 1900 is added to it. The construct ‘MONTH/DAY/YEAR’, popular in the United States, is accepted. Also ‘MONTH/DAY’, omitting the year. Literal months may be spelled out in full: ‘January’, ‘February’, ‘March’, ‘April’, ‘May’, ‘June’, ‘July’, ‘August’, ‘September’, ‘October’, ‘November’ or ‘December’. Literal months may be abbreviated to their first three letters, possibly followed by an abbreviating dot. It is also permitted to write ‘Sept’ instead of ‘September’. When months are written literally, the calendar date may be given as any of the following: DAY MONTH YEAR DAY MONTH MONTH DAY YEAR DAY-MONTH-YEAR Or, omitting the year: MONTH DAY 29.3 Time of day items ====================== A “time of day item” in date strings specifies the time on a given day. Here are some examples, all of which represent the same time: 20:02:00.000000 20:02 8:02pm 20:02-0500 # In EST (U.S. Eastern Standard Time). More generally, the time of day may be given as ‘HOUR:MINUTE:SECOND’, where HOUR is a number between 0 and 23, MINUTE is a number between 0 and 59, and SECOND is a number between 0 and 59 possibly followed by ‘.’ or ‘,’ and a fraction containing one or more digits. Alternatively, ‘:SECOND’ can be omitted, in which case it is taken to be zero. On the rare hosts that support leap seconds, SECOND may be 60. If the time is followed by ‘am’ or ‘pm’ (or ‘a.m.’ or ‘p.m.’), HOUR is restricted to run from 1 to 12, and ‘:MINUTE’ may be omitted (taken to be zero). ‘am’ indicates the first half of the day, ‘pm’ indicates the second half of the day. In this notation, 12 is the predecessor of 1: midnight is ‘12am’ while noon is ‘12pm’. (This is the zero-oriented interpretation of ‘12am’ and ‘12pm’, as opposed to the old tradition derived from Latin which uses ‘12m’ for noon and ‘12pm’ for midnight.) The time may alternatively be followed by a time zone correction, expressed as ‘SHHMM’, where S is ‘+’ or ‘-’, HH is a number of zone hours and MM is a number of zone minutes. The zone minutes term, MM, may be omitted, in which case the one- or two-digit correction is interpreted as a number of hours. You can also separate HH from MM with a colon. When a time zone correction is given this way, it forces interpretation of the time relative to Coordinated Universal Time (UTC), overriding any previous specification for the time zone or the local time zone. For example, ‘+0530’ and ‘+05:30’ both stand for the time zone 5.5 hours ahead of UTC (e.g., India). This is the best way to specify a time zone correction by fractional parts of an hour. The maximum zone correction is 24 hours. Either ‘am’/‘pm’ or a time zone correction may be specified, but not both. 29.4 Time zone items ==================== A “time zone item” specifies an international time zone, indicated by a small set of letters, e.g., ‘UTC’ or ‘Z’ for Coordinated Universal Time. Any included periods are ignored. By following a non-daylight-saving time zone by the string ‘DST’ in a separate word (that is, separated by some white space), the corresponding daylight saving time zone may be specified. Alternatively, a non-daylight-saving time zone can be followed by a time zone correction, to add the two values. This is normally done only for ‘UTC’; for example, ‘UTC+05:30’ is equivalent to ‘+05:30’. Time zone items other than ‘UTC’ and ‘Z’ are obsolescent and are not recommended, because they are ambiguous; for example, ‘EST’ has a different meaning in Australia than in the United States, and ‘A’ has different meaning as a military time zone than as an obsolete RFC 822 time zone. Instead, it’s better to use unambiguous numeric time zone corrections like ‘-0500’, as described in the previous section. If neither a time zone item nor a time zone correction is supplied, timestamps are interpreted using the rules of the default time zone (*note Specifying time zone rules::). 29.5 Combined date and time of day items ======================================== The ISO 8601 date and time of day extended format consists of an ISO 8601 date, a ‘T’ character separator, and an ISO 8601 time of day. This format is also recognized if the ‘T’ is replaced by a space. In this format, the time of day should use 24-hour notation. Fractional seconds are allowed, with either comma or period preceding the fraction. ISO 8601 fractional minutes and hours are not supported. Typically, hosts support nanosecond timestamp resolution; excess precision is silently discarded. Here are some examples: 2022-09-24T20:02:00.052-05:00 2022-12-31T23:59:59,999999999+11:00 1970-01-01 00:00Z 29.6 Day of week items ====================== The explicit mention of a day of the week will forward the date (only if necessary) to reach that day of the week in the future. Days of the week may be spelled out in full: ‘Sunday’, ‘Monday’, ‘Tuesday’, ‘Wednesday’, ‘Thursday’, ‘Friday’ or ‘Saturday’. Days may be abbreviated to their first three letters, optionally followed by a period. The special abbreviations ‘Tues’ for ‘Tuesday’, ‘Wednes’ for ‘Wednesday’ and ‘Thur’ or ‘Thurs’ for ‘Thursday’ are also allowed. A number may precede a day of the week item to move forward supplementary weeks. It is best used in expression like ‘third monday’. In this context, ‘last DAY’ or ‘next DAY’ is also acceptable; they move one week before or after the day that DAY by itself would represent. A comma following a day of the week item is ignored. 29.7 Relative items in date strings =================================== “Relative items” adjust a date (or the current date if none) forward or backward. The effects of relative items accumulate. Here are some examples: 1 year 1 year ago 3 years 2 days The unit of time displacement may be selected by the string ‘year’ or ‘month’ for moving by whole years or months. These are fuzzy units, as years and months are not all of equal duration. More precise units are ‘fortnight’ which is worth 14 days, ‘week’ worth 7 days, ‘day’ worth 24 hours, ‘hour’ worth 60 minutes, ‘minute’ or ‘min’ worth 60 seconds, and ‘second’ or ‘sec’ worth one second. An ‘s’ suffix on these units is accepted and ignored. The unit of time may be preceded by a multiplier, given as an optionally signed number. Unsigned numbers are taken as positively signed. No number at all implies 1 for a multiplier. Following a relative item by the string ‘ago’ is equivalent to preceding the unit by a multiplier with value -1. The string ‘tomorrow’ is worth one day in the future (equivalent to ‘day’), the string ‘yesterday’ is worth one day in the past (equivalent to ‘day ago’). The strings ‘now’ or ‘today’ are relative items corresponding to zero-valued time displacement, these strings come from the fact a zero-valued time displacement represents the current time when not otherwise changed by previous items. They may be used to stress other items, like in ‘12:00 today’. The string ‘this’ also has the meaning of a zero-valued time displacement, but is preferred in date strings like ‘this thursday’. When a relative item causes the resulting date to cross a boundary where the clocks were adjusted, typically for daylight saving time, the resulting date and time are adjusted accordingly. The fuzz in units can cause problems with relative items. For example, ‘2022-12-31 -1 month’ might evaluate to 2022-12-01, because 2022-11-31 is an invalid date. To determine the previous month more reliably, you can ask for the month before the 15th of the current month. For example: $ date -R Thu, 31 Dec 2022 13:02:39 -0400 $ date --date='-1 month' +'Last month was %B?' Last month was December? $ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!' Last month was November! Also, take care when manipulating dates around clock changes such as daylight saving leaps. In a few cases these have added or subtracted as much as 24 hours from the clock, so it is often wise to adopt universal time by setting the ‘TZ’ environment variable to ‘UTC0’ before embarking on calendrical calculations. 29.8 Pure numbers in date strings ================================= The precise interpretation of a pure decimal number depends on the context in the date string. If the decimal number is of the form YYYYMMDD and no other calendar date item (*note Calendar date items::) appears before it in the date string, then YYYY is read as the year, MM as the month number and DD as the day of the month, for the specified calendar date. If the decimal number is of the form HHMM and no other time of day item appears before it in the date string, then HH is read as the hour of the day and MM as the minute of the hour, for the specified time of day. MM can also be omitted. If both a calendar date and a time of day appear to the left of a number in the date string, but no relative item, then the number overrides the year. 29.9 Seconds since the Epoch ============================ If you precede a number with ‘@’, it represents an internal timestamp as a count of seconds. The number can contain an internal decimal point (either ‘.’ or ‘,’); any excess precision not supported by the internal representation is truncated toward minus infinity. Such a number cannot be combined with any other date item, as it specifies a complete timestamp. Internally, computer times are represented as a count of seconds since an Epoch—a well-defined point of time. On GNU and POSIX systems, the Epoch is 1970-01-01 00:00:00 UTC, so ‘@0’ represents this time, ‘@1’ represents 1970-01-01 00:00:01 UTC, and so forth. GNU and most other POSIX-compliant systems support such times as an extension to POSIX, using negative counts, so that ‘@-1’ represents 1969-12-31 23:59:59 UTC. Most modern systems count seconds with 64-bit two’s-complement integers of seconds with nanosecond subcounts, which is a range that includes the known lifetime of the universe with nanosecond resolution. Some obsolescent systems count seconds with 32-bit two’s-complement integers and can represent times from 1901-12-13 20:45:52 through 2038-01-19 03:14:07 UTC. A few systems sport other time ranges. On most hosts, these counts ignore the presence of leap seconds. For example, on most hosts ‘@1483228799’ represents 2016-12-31 23:59:59 UTC, ‘@1483228800’ represents 2017-01-01 00:00:00 UTC, and there is no way to represent the intervening leap second 2016-12-31 23:59:60 UTC. 29.10 Specifying time zone rules ================================ Normally, dates are interpreted using the rules of the current time zone, which in turn are specified by the ‘TZ’ environment variable, or by a system default if ‘TZ’ is not set. To specify a different set of default time zone rules that apply just to one date, start the date with a string of the form ‘TZ="RULE"’. The two quote characters (‘"’) must be present in the date, and any quotes or backslashes within RULE must be escaped by a backslash. For example, with the GNU ‘date’ command you can answer the question “What time is it in New York when a Paris clock shows 6:30am on October 31, 2022?” by using a date beginning with ‘TZ="Europe/Paris"’ as shown in the following shell transcript: $ export TZ="America/New_York" $ date --date='TZ="Europe/Paris" 2022-10-31 06:30' Mon Oct 31 01:30:00 EDT 2022 In this example, the ‘--date’ operand begins with its own ‘TZ’ setting, so the rest of that operand is processed according to ‘Europe/Paris’ rules, treating the string ‘2022-11-14 06:30’ as if it were in Paris. However, since the output of the ‘date’ command is processed according to the overall time zone rules, it uses New York time. (Paris was normally six hours ahead of New York in 2022, but this example refers to a brief Halloween period when the gap was five hours.) A ‘TZ’ value is a rule that typically names a location in the ‘tz’ database (https://www.iana.org/time-zones). A recent catalog of location names appears in the TWiki Date and Time Gateway (https://twiki.org/cgi-bin/xtra/tzdatepick.html). A few non-GNU hosts require a colon before a location name in a ‘TZ’ setting, e.g., ‘TZ=":America/New_York"’. The ‘tz’ database includes a wide variety of locations ranging from ‘Africa/Abidjan’ to ‘Pacific/Tongatapu’, but if you are at sea and have your own private time zone, or if you are using a non-GNU host that does not support the ‘tz’ database, you may need to use a POSIX rule instead. The previously-mentioned POSIX rule ‘UTC0’ says that the time zone abbreviation is ‘UTC’, the zone is zero hours away from Greenwich, and there is no daylight saving time. POSIX rules can also specify nonzero Greenwich offsets. For example, the following shell transcript answers the question “What time is it five and a half hours east of Greenwich when a clock seven hours west of Greenwich shows 9:50pm on July 12, 2022?” $ TZ="<+0530>-5:30" date --date='TZ="<-07>+7" 2022-07-12 21:50' Wed Jul 13 10:20:00 +0530 2022 This example uses the somewhat-confusing POSIX convention for rules. ‘TZ="<-07>+7"’ says that the time zone abbreviation is ‘-07’ and the time zone is 7 hours west of Greenwich, and ‘TZ="<+0530>-5:30"’ says that the time zone abbreviation is ‘+0530’ and the time zone is 5 hours 30 minutes east of Greenwich. (One should never use a setting like ‘TZ="UTC-5"’, since this would incorrectly imply that local time is five hours east of Greenwich and the time zone is called “UTC”.) Although trickier POSIX ‘TZ’ settings like ‘TZ="<-05>+5<-04>,M3.2.0/2,M11.1.0/2"’ can specify some daylight saving regimes, location-based settings like ‘TZ="America/New_York"’ are typically simpler and more accurate historically. *Note Specifying the Time Zone with ‘TZ’: (libc)TZ Variable. 29.11 Authors of ‘parse_datetime’ ================================= ‘parse_datetime’ started life as ‘getdate’, as originally implemented by Steven M. Bellovin () while at the University of North Carolina at Chapel Hill. The code was later tweaked by a couple of people on Usenet, then completely overhauled by Rich $alz () and Jim Berets () in August, 1990. Various revisions for the GNU system were made by David MacKenzie, Jim Meyering, Paul Eggert and others, including renaming it to ‘get_date’ to avoid a conflict with the alternative Posix function ‘getdate’, and a later rename to ‘parse_datetime’. The Posix function ‘getdate’ can parse more locale-specific dates using ‘strptime’, but relies on an environment variable and external file, and lacks the thread-safety of ‘parse_datetime’. This chapter was originally produced by François Pinard () from the ‘parse_datetime.y’ source code, and then edited by K. Berry (). 30 Version sort ordering ************************ 30.1 Version sort overview ========================== “Version sort” puts items such as file names and lines of text in an order that feels natural to people, when the text contains a mixture of letters and digits. Lexicographic sorting usually does not produce the order that one expects because comparisons are made on a character-by-character basis. Compare the sorting of the following items: Lexicographic sort: Version Sort: a1 a1 a120 a2 a13 a13 a2 a120 Version sort functionality in GNU Coreutils is available in the ‘ls -v’, ‘ls --sort=version’, ‘sort -V’, and ‘sort --version-sort’ commands. 30.1.1 Using version sort in GNU Coreutils ------------------------------------------ Two GNU Coreutils programs use version sort: ‘ls’ and ‘sort’. To list files in version sort order, use ‘ls’ with the ‘-v’ or ‘--sort=version’ option: default sort: version sort: $ ls -1 $ ls -1 -v a1 a1 a100 a1.4 a1.13 a1.13 a1.4 a1.40 a1.40 a2 a2 a100 To sort text files in version sort order, use ‘sort’ with the ‘-V’ or ‘--version-sort’ option: $ cat input b3 b11 b1 b20 lexicographic order: version sort order: $ sort input $ sort -V input b1 b1 b11 b3 b20 b11 b3 b20 To sort a specific field in a file, use ‘-k/--key’ with ‘V’ type sorting, which is often combined with ‘b’ to ignore leading blanks in the field: $ cat input2 100 b3 apples 2000 b11 oranges 3000 b1 potatoes 4000 b20 bananas $ sort -k 2bV,2 input2 3000 b1 potatoes 100 b3 apples 2000 b11 oranges 4000 b20 bananas 30.1.2 Version sort and natural sort ------------------------------------ In GNU Coreutils, the name “version sort” was chosen because it is based on Debian GNU/Linux’s algorithm of sorting packages’ versions. Its goal is to answer questions like “Which package is newer, ‘firefox-60.7.2’ or ‘firefox-60.12.3’?” In Coreutils this algorithm was slightly modified to work on more general input such as textual strings and file names (see *note Differences from Debian version sort::). In other contexts, such as other programs and other programming languages, a similar sorting functionality is called natural sort (https://en.wikipedia.org/wiki/Natural_sort_order). 30.1.3 Variations in version sort order --------------------------------------- Currently there is no standard for version sort. That is: there is no one correct way or universally agreed-upon way to order items. Each program and each programming language can decide its own ordering algorithm and call it “version sort”, “natural sort”, or other names. See *note Other version/natural sort implementations:: for many examples of differing sorting possibilities, each with its own rules and variations. If you find a bug in the Coreutils implementation of version-sort, please report it. *Note Reporting version sort bugs::. 30.2 Version sort implementation ================================ GNU Coreutils version sort is based on the “upstream version” part of Debian’s versioning scheme (https://www.debian.org/doc/debian-policy/ch-controlfields.html#version). This section describes the GNU Coreutils sort ordering rules. The next section (*note Differences from Debian version sort::) describes some differences between GNU Coreutils and Debian version sort. 30.2.1 Version-sort ordering rules ---------------------------------- The version sort ordering rules are: 1. The strings are compared from left to right. 2. First the initial part of each string consisting entirely of non-digit bytes is determined. A. These two parts (either of which may be empty) are compared lexically. If a difference is found it is returned. B. The lexical comparison is a lexicographic comparison of byte strings, except that: a. ASCII letters sort before other bytes. b. A tilde sorts before anything, even an empty string. 3. Then the initial part of the remainder of each string that contains all the leading digits is determined. The numerical values represented by these two parts are compared, and any difference found is returned as the result of the comparison. A. For these purposes an empty string (which can only occur at the end of one or both version strings being compared) counts as zero. B. Because the numerical value is used, non-identical strings can compare equal. For example, ‘123’ compares equal to ‘00123’, and the empty string compares equal to ‘0’. 4. These two steps (comparing and removing initial non-digit strings and initial digit strings) are repeated until a difference is found or both strings are exhausted. Consider the version-sort comparison of two file names: ‘foo07.7z’ and ‘foo7a.7z’. The two strings will be broken down to the following parts, and the parts compared respectively from each string: foo vs foo (rule 2, non-digits) 07 vs 7 (rule 3, digits) . vs a. (rule 2) 7 vs 7 (rule 3) z vs z (rule 2) Comparison flow based on above algorithm: 1. The first parts (‘foo’) are identical. 2. The second parts (‘07’ and ‘7’) are compared numerically, and compare equal. 3. The third parts (‘.’ vs ‘a.’) are compared lexically by ASCII value (rule 2.B). 4. The first byte of the first string (‘.’) is compared to the first byte of the second string (‘a’). 5. Rule 2.B.a says letters sorts before non-letters. Hence, ‘a’ comes before ‘.’. 6. The returned result is that ‘foo7a.7z’ comes before ‘foo07.7z’. Result when using sort: $ cat input3 foo07.7z foo7a.7z $ sort -V input3 foo7a.7z foo07.7z See *note Differences from Debian version sort:: for additional rules that extend the Debian algorithm in Coreutils. 30.2.2 Version sort is not the same as numeric sort --------------------------------------------------- Consider the following text file: $ cat input4 8.10 8.5 8.1 8.01 8.010 8.100 8.49 Numerical Sort: Version Sort: $ sort -n input4 $ sort -V input4 8.01 8.01 8.010 8.1 8.1 8.5 8.10 8.010 8.100 8.10 8.49 8.49 8.5 8.100 Numeric sort (‘sort -n’) treats the entire string as a single numeric value, and compares it to other values. For example, ‘8.1’, ‘8.10’ and ‘8.100’ are numerically equivalent, and are ordered together. Similarly, ‘8.49’ is numerically less than ‘8.5’, and appears before first. Version sort (‘sort -V’) first breaks down the string into digit and non-digit parts, and only then compares each part (see annotated example in *note Version-sort ordering rules::). Comparing the string ‘8.1’ to ‘8.01’, first the ‘8’s are compared (and are identical), then the dots (‘.’) are compared and are identical, and lastly the remaining digits are compared numerically (‘1’ and ‘01’) – which are numerically equal. Hence, ‘8.01’ and ‘8.1’ are grouped together. Similarly, comparing ‘8.5’ to ‘8.49’ – the ‘8’ and ‘.’ parts are identical, then the numeric values ‘5’ and ‘49’ are compared. The resulting ‘5’ appears before ‘49’. This sorting order (where ‘8.5’ comes before ‘8.49’) is common when assigning versions to computer programs (while perhaps not intuitive or “natural” for people). 30.2.3 Version sort punctuation ------------------------------- Punctuation is sorted by ASCII order (rule 2.B). $ touch 1.0.5_src.tar.gz 1.0_src.tar.gz $ ls -v -1 1.0.5_src.tar.gz 1.0_src.tar.gz Why is ‘1.0.5_src.tar.gz’ listed before ‘1.0_src.tar.gz’? Based on the version-sort ordering rules, the strings are broken down into the following parts: 1 vs 1 (rule 3, all digits) . vs . (rule 2, all non-digits) 0 vs 0 (rule 3) . vs _src.tar.gz (rule 2) 5 vs empty string (no more bytes in the file name) _src.tar.gz vs empty string The fourth parts (‘.’ and ‘_src.tar.gz’) are compared lexically by ASCII order. The ‘.’ (ASCII value 46) is less than ‘_’ (ASCII value 95) – and should be listed before it. Hence, ‘1.0.5_src.tar.gz’ is listed first. If a different byte appears instead of the underscore (for example, percent sign ‘%’ ASCII value 37, which is less than dot’s ASCII value of 46), that file will be listed first: $ touch 1.0.5_src.tar.gz 1.0%zzzzz.gz 1.0%zzzzz.gz 1.0.5_src.tar.gz The same reasoning applies to the following example, as ‘.’ with ASCII value 46 is less than ‘/’ with ASCII value 47: $ cat input5 3.0/ 3.0.5 $ sort -V input5 3.0.5 3.0/ 30.2.4 Punctuation vs letters ----------------------------- Rule 2.B.a says letters sort before non-letters (after breaking down a string to digit and non-digit parts). $ cat input6 a% az $ sort -V input6 az a% The input strings consist entirely of non-digits, and based on the above algorithm have only one part, all non-digits (‘a%’ vs ‘az’). Each part is then compared lexically, byte-by-byte; ‘a’ compares identically in both strings. Rule 2.B.a says a letter like ‘z’ sorts before a non-letter like ‘%’ – hence ‘az’ appears first (despite ‘z’ having ASCII value of 122, much larger than ‘%’ with ASCII value 37). 30.2.5 The tilde ‘~’ -------------------- Rule 2.B.b says the tilde ‘~’ (ASCII 126) sorts before other bytes, and before an empty string. $ cat input7 1 1% 1.2 1~ ~ $ sort -V input7 ~ 1~ 1 1% 1.2 The sorting algorithm starts by breaking down the string into non-digit (rule 2) and digit parts (rule 3). In the above input file, only the last line in the input file starts with a non-digit (‘~’). This is the first part. All other lines in the input file start with a digit – their first non-digit part is empty. Based on rule 2.B.b, tilde ‘~’ sorts before other bytes and before the empty string – hence it comes before all other strings, and is listed first in the sorted output. The remaining lines (‘1’, ‘1%’, ‘1.2’, ‘1~’) follow similar logic: The digit part is extracted (1 for all strings) and compares equal. The following extracted parts for the remaining input lines are: empty part, ‘%’, ‘.’, ‘~’. Tilde sorts before all others, hence the line ‘1~’ appears next. The remaining lines (‘1’, ‘1%’, ‘1.2’) are sorted based on previously explained rules. 30.2.6 Version sort ignores locale ---------------------------------- In version sort, Unicode characters are compared byte-by-byte according to their binary representation, ignoring their Unicode value or the current locale. Most commonly, Unicode characters are encoded as UTF-8 bytes; for example, GREEK SMALL LETTER ALPHA (U+03B1, ‘α’) is encoded as the UTF-8 sequence ‘0xCE 0xB1’). The encoding is compared byte-by-byte, e.g., first ‘0xCE’ (decimal value 206) then ‘0xB1’ (decimal value 177). $ touch aa az "a%" "aα" $ ls -1 -v aa az a% aα Ignoring the first letter (‘a’) which is identical in all strings, the compared values are: ‘a’ and ‘z’ are letters, and sort before all other non-digits. Then, percent sign ‘%’ (ASCII value 37) is compared to the first byte of the UTF-8 sequence of ‘α’, which is 0xCE or 206). The value 37 is smaller, hence ‘a%’ is listed before ‘aα’. 30.3 Differences from Debian version sort ========================================= GNU Coreutils version sort differs slightly from the official Debian algorithm, in order to accommodate more general usage and file name listing. 30.3.1 Hyphen-minus ‘-’ and colon ‘:’ ------------------------------------- In Debian’s version string syntax the version consists of three parts: [epoch:]upstream_version[-debian_revision] The ‘epoch’ and ‘debian_revision’ parts are optional. Example of such version strings: 60.7.2esr-1~deb9u1 52.9.0esr-1~deb9u1 1:2.3.4-1+b2 327-2 1:1.0.13-3 2:1.19.2-1+deb9u5 If the ‘debian_revision part’ is not present, hyphens ‘-’ are not allowed. If epoch is not present, colons ‘:’ are not allowed. If these parts are present, hyphen and/or colons can appear only once in valid Debian version strings. In GNU Coreutils, such restrictions are not reasonable (a file name can have many hyphens, a line of text can have many colons). As a result, in GNU Coreutils hyphens and colons are treated exactly like all other punctuation, i.e., they are sorted after letters. *Note Version sort punctuation::. In Debian, these characters are treated differently than in Coreutils: a version string with hyphen will sort before similar strings without hyphens. Compare: $ touch 1ab-cd 1abb $ ls -v -1 1abb 1ab-cd $ if dpkg --compare-versions 1abb lt 1ab-cd > then echo sorted > else echo out of order > fi out of order For further details, see *note Comparing two strings using Debian's algorithm:: and GNU Bug 35939 (https://bugs.gnu.org/35939). 30.3.2 Special priority in GNU Coreutils version sort ----------------------------------------------------- In GNU Coreutils version sort, the following items have special priority and sort before all other strings (listed in order): 1. The empty string 2. The string ‘.’ (a single dot, ASCII 46) 3. The string ‘..’ (two dots) 4. Strings starting with dot (‘.’) sort before strings starting with any other byte. Example: $ printf '%s\n' a "" b "." c ".." ".d20" ".d3" | sort -V . .. .d3 .d20 a b c These priorities make perfect sense for ‘ls -v’: The special files dot ‘.’ and dot-dot ‘..’ will be listed first, followed by any hidden files (files starting with a dot), followed by non-hidden files. For ‘sort -V’ these priorities might seem arbitrary. However, because the sorting code is shared between the ‘ls’ and ‘sort’ program, the ordering rules are the same. 30.3.3 Special handling of file extensions ------------------------------------------ GNU Coreutils version sort implements specialized handling of strings that look like file names with extensions. This enables slightly more natural ordering of file names. The following additional rules apply when comparing two strings where both begin with non-‘.’. They also apply when comparing two strings where both begin with ‘.’ but neither is ‘.’ or ‘..’. 1. A suffix (i.e., a file extension) is defined as: a dot, followed by an ASCII letter or tilde, followed by zero or more ASCII letters, digits, or tildes; all repeated zero or more times, and ending at string end. This is equivalent to matching the extended regular expression ‘(\.[A-Za-z~][A-Za-z0-9~]*)*$’ in the C locale. The longest such match is used, except that a suffix is not allowed to match an entire nonempty string. 2. The suffixes are temporarily removed, and the strings are compared without them, using version sort (see *note Version-sort ordering rules::) without special priority (see *note Special priority in GNU Coreutils version sort::). 3. If the suffix-less strings do not compare equal, this comparison result is used and the suffixes are effectively ignored. 4. If the suffix-less strings compare equal, the suffixes are restored and the entire strings are compared using version sort. Examples for rule 1: • ‘hello-8.txt’: the suffix is ‘.txt’ • ‘hello-8.2.txt’: the suffix is ‘.txt’ (‘.2’ is not included because the dot is not followed by a letter) • ‘hello-8.0.12.tar.gz’: the suffix is ‘.tar.gz’ (‘.0.12’ is not included) • ‘hello-8.2’: no suffix (suffix is an empty string) • ‘hello.foobar65’: the suffix is ‘.foobar65’ • ‘gcc-c++-10.8.12-0.7rc2.fc9.tar.bz2’: the suffix is ‘.fc9.tar.bz2’ (‘.7rc2’ is not included as it begins with a digit) • ‘.autom4te.cfg’: the suffix is the entire string. Examples for rule 2: • Comparing ‘hello-8.txt’ to ‘hello-8.2.12.txt’, the ‘.txt’ suffix is temporarily removed from both strings. • Comparing ‘foo-10.3.tar.gz’ to ‘foo-10.tar.xz’, the suffixes ‘.tar.gz’ and ‘.tar.xz’ are temporarily removed from the strings. Example for rule 3: • Comparing ‘hello.foobar65’ to ‘hello.foobar4’, the suffixes (‘.foobar65’ and ‘.foobar4’) are temporarily removed. The remaining strings are identical (‘hello’). The suffixes are then restored, and the entire strings are compared (‘hello.foobar4’ comes first). Examples for rule 4: • When comparing the strings ‘hello-8.2.txt’ and ‘hello-8.10.txt’, the suffixes (‘.txt’) are temporarily removed. The remaining strings (‘hello-8.2’ and ‘hello-8.10’) are compared as previously described (‘hello-8.2’ comes first). (In this case the suffix removal algorithm does not have a noticeable effect on the resulting order.) How does the suffix-removal algorithm effect ordering results? Consider the comparison of hello-8.txt and hello-8.2.txt. Without the suffix-removal algorithm, the strings will be broken down to the following parts: hello- vs hello- (rule 2, all non-digits) 8 vs 8 (rule 3, all digits) .txt vs . (rule 2) empty vs 2 empty vs .txt The comparison of the third parts (‘.’ vs ‘.txt’) will determine that the shorter string comes first – resulting in ‘hello-8.2.txt’ appearing first. Indeed this is the order in which Debian’s ‘dpkg’ compares the strings. A more natural result is that ‘hello-8.txt’ should come before ‘hello-8.2.txt’, and this is where the suffix-removal comes into play: The suffixes (‘.txt’) are removed, and the remaining strings are broken down into the following parts: hello- vs hello- (rule 2, all non-digits) 8 vs 8 (rule 3, all digits) empty vs . (rule 2) empty vs 2 As empty strings sort before non-empty strings, the result is ‘hello-8’ being first. A real-world example would be listing files such as: ‘gcc_10.fc9.tar.gz’ and ‘gcc_10.8.12.7rc2.fc9.tar.bz2’: Debian’s algorithm would list ‘gcc_10.8.12.7rc2.fc9.tar.bz2’ first, while ‘ls -v’ will list ‘gcc_10.fc9.tar.gz’ first. These priorities make sense for ‘ls -v’: Versioned files will be listed in a more natural order. For ‘sort -V’ these priorities might seem arbitrary. However, because the sorting code is shared between the ‘ls’ and ‘sort’ program, the ordering rules are the same. 30.3.4 Comparing two strings using Debian’s algorithm ----------------------------------------------------- The Debian program ‘dpkg’ (available on all Debian and Ubuntu installations) can compare two strings using the ‘--compare-versions’ option. To use it, create a helper shell function (simply copy & paste the following snippet to your shell command-prompt): compver() { if dpkg --compare-versions "$1" lt "$2" then printf '%s\n' "$1" "$2" else printf '%s\n' "$2" "$1" fi } Then compare two strings by calling ‘compver’: $ compver 8.49 8.5 8.5 8.49 Note that ‘dpkg’ will warn if the strings have invalid syntax: $ compver "foo07.7z" "foo7a.7z" dpkg: warning: version 'foo07.7z' has bad syntax: version number does not start with digit dpkg: warning: version 'foo7a.7z' has bad syntax: version number does not start with digit foo7a.7z foo07.7z $ compver "3.0/" "3.0.5" dpkg: warning: version '3.0/' has bad syntax: invalid character in version number 3.0.5 3.0/ To illustrate the different handling of hyphens between Debian and Coreutils algorithms (see *note Hyphen-minus and colon::): $ compver abb ab-cd 2>/dev/null $ printf 'abb\nab-cd\n' | sort -V ab-cd abb abb ab-cd To illustrate the different handling of file extension: (see *note Special handling of file extensions::): $ compver hello-8.txt hello-8.2.txt 2>/dev/null hello-8.2.txt hello-8.txt $ printf '%s\n' hello-8.txt hello-8.2.txt | sort -V hello-8.txt hello-8.2.txt 30.4 Advanced Topics ==================== 30.4.1 Reporting version sort bugs ---------------------------------- If you suspect a bug in GNU Coreutils version sort (i.e., in the output of ‘ls -v’ or ‘sort -V’), please first check the following: 1. Is the result consistent with Debian’s own ordering (using ‘dpkg’, see *note Comparing two strings using Debian's algorithm::)? If it is, then this is not a bug – please do not report it. 2. If the result differs from Debian’s, is it explained by one of the sections in *note Differences from Debian version sort::? If it is, then this is not a bug – please do not report it. 3. If you have a question about specific ordering which is not explained here, please write to , and provide a concise example that will help us diagnose the issue. 4. If you still suspect a bug which is not explained by the above, please write to with a concrete example of the suspected incorrect output, with details on why you think it is incorrect. 30.4.2 Other version/natural sort implementations ------------------------------------------------- As previously mentioned, there are multiple variations on version/natural sort, each with its own rules. Some examples are: • Natural Sorting variants in Rosetta Code (https://rosettacode.org/wiki/Natural_sorting). • Python’s natsort package (https://pypi.org/project/natsort/) (includes detailed description of their sorting rules: natsort – how it works (https://natsort.readthedocs.io/en/master/howitworks.html)). • Ruby’s version_sorter (https://github.com/github/version_sorter). • Perl has multiple packages for natural and version sorts (each likely with its own rules and nuances): Sort::Naturally (https://metacpan.org/pod/Sort::Naturally), Sort::Versions (https://metacpan.org/pod/Sort::Versions), CPAN::Version (https://metacpan.org/pod/CPAN::Version). • PHP has a built-in function natsort (https://www.php.net/manual/en/function.natsort.php). • NodeJS’s natural-sort package (https://www.npmjs.com/package/natural-sort). • In zsh, the glob modifier (http://zsh.sourceforge.net/Doc/Release/Expansion.html#Glob-Qualifiers) ‘*(n)’ will expand to files in natural sort order. • When writing C programs, the GNU libc library (‘glibc’) provides the strverscmp(3) (https://man7.org/linux/man-pages/man3/strverscmp.3.html) function to compare two strings, and versionsort(3) (https://man7.org/linux/man-pages/man3/versionsort.3.html) function to compare two directory entries (despite the names, they are not identical to GNU Coreutils version sort ordering). • Using Debian’s sorting algorithm in: • python: Stack Overflow Example #4957741 (https://stackoverflow.com/a/4957741). • NodeJS: deb-version-compare (https://www.npmjs.com/package/deb-version-compare). 30.4.3 Related source code -------------------------- • Debian’s code which splits a version string into ‘epoch/upstream_version/debian_revision’ parts: parsehelp.c:parseversion() (https://git.dpkg.org/cgit/dpkg/dpkg.git/tree/lib/dpkg/parsehelp.c#n191). • Debian’s code which performs the ‘upstream_version’ comparison: version.c (https://git.dpkg.org/cgit/dpkg/dpkg.git/tree/lib/dpkg/version.c#n140). • Gnulib code (used by GNU Coreutils) which performs the version comparison: filevercmp.c (https://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/filevercmp.c). 31 Opening the Software Toolbox ******************************* An earlier version of this chapter appeared in the ‘What’s GNU?’ column of the June 1994 ‘Linux Journal’ (https://www.linuxjournal.com/article.php?sid=2762). It was written by Arnold Robbins. Toolbox Introduction ==================== This month’s column is only peripherally related to the GNU Project, in that it describes a number of the GNU tools on your GNU/Linux system and how they might be used. What it’s really about is the “Software Tools” philosophy of program development and usage. The software tools philosophy was an important and integral concept in the initial design and development of Unix (of which GNU/Linux and GNU are essentially clones). Unfortunately, in the modern day press of Internetworking and flashy GUIs, it seems to have fallen by the wayside. This is a shame, since it provides a powerful mental model for solving many kinds of problems. Many people carry a Swiss Army knife around in their pants pockets (or purse). A Swiss Army knife is a handy tool to have: it has several knife blades, a screwdriver, tweezers, toothpick, nail file, corkscrew, and perhaps a number of other things on it. For the everyday, small miscellaneous jobs where you need a simple, general purpose tool, it’s just the thing. On the other hand, an experienced carpenter doesn’t build a house using a Swiss Army knife. Instead, he has a toolbox chock full of specialized tools – a saw, a hammer, a screwdriver, a plane, and so on. And he knows exactly when and where to use each tool; you won’t catch him hammering nails with the handle of his screwdriver. The Unix developers at Bell Labs were all professional programmers and trained computer scientists. They had found that while a one-size-fits-all program might appeal to a user because there’s only one program to use, in practice such programs are a. difficult to write, b. difficult to maintain and debug, and c. difficult to extend to meet new situations. Instead, they felt that programs should be specialized tools. In short, each program “should do one thing well.” No more and no less. Such programs are simpler to design, write, and get right – they only do one thing. Furthermore, they found that with the right machinery for hooking programs together, that the whole was greater than the sum of the parts. By combining several special purpose programs, you could accomplish a specific task that none of the programs was designed for, and accomplish it much more quickly and easily than if you had to write a special purpose program. We will see some (classic) examples of this further on in the column. (An important additional point was that, if necessary, take a detour and build any software tools you may need first, if you don’t already have something appropriate in the toolbox.) I/O Redirection =============== Hopefully, you are familiar with the basics of I/O redirection in the shell, in particular the concepts of “standard input,” “standard output,” and “standard error”. Briefly, “standard input” is a data source, where data comes from. A program should not need to either know or care if the data source is a regular file, a keyboard, a magnetic tape, or even a punched card reader. Similarly, “standard output” is a data sink, where data goes to. The program should neither know nor care where this might be. Programs that only read their standard input, do something to the data, and then send it on, are called “filters”, by analogy to filters in a water pipeline. With the Unix shell, it’s very easy to set up data pipelines: program_to_create_data | filter1 | ... | filterN > final.pretty.data We start out by creating the raw data; each filter applies some successive transformation to the data, until by the time it comes out of the pipeline, it is in the desired form. This is fine and good for standard input and standard output. Where does the standard error come in to play? Well, think about ‘filter1’ in the pipeline above. What happens if it encounters an error in the data it sees? If it writes an error message to standard output, it will just disappear down the pipeline into ‘filter2’’s input, and the user will probably never see it. So programs need a place where they can send error messages so that the user will notice them. This is standard error, and it is usually connected to your console or window, even if you have redirected standard output of your program away from your screen. For filter programs to work together, the format of the data has to be agreed upon. The most straightforward and easiest format to use is simply lines of text. Unix data files are generally just streams of bytes, with lines delimited by the ASCII LF (Line Feed) character, conventionally called a “newline” in the Unix literature. (This is ‘'\n'’ if you’re a C programmer.) This is the format used by all the traditional filtering programs. (Many earlier operating systems had elaborate facilities and special purpose programs for managing binary data. Unix has always shied away from such things, under the philosophy that it’s easiest to simply be able to view and edit your data with a text editor.) OK, enough introduction. Let’s take a look at some of the tools, and then we’ll see how to hook them together in interesting ways. In the following discussion, we will only present those command line options that interest us. As you should always do, double check your system documentation for the full story. The ‘who’ Command ================= The first program is the ‘who’ command. By itself, it generates a list of the users who are currently logged in. Although I’m writing this on a single-user system, we’ll pretend that several people are logged in: $ who ⊣ arnold console Jan 22 19:57 ⊣ miriam ttyp0 Jan 23 14:19(:0.0) ⊣ bill ttyp1 Jan 21 09:32(:0.0) ⊣ arnold ttyp2 Jan 23 20:48(:0.0) Here, the ‘$’ is the usual shell prompt, at which I typed ‘who’. There are three people logged in, and I am logged in twice. On traditional Unix systems, user names are never more than eight characters long. This little bit of trivia will be useful later. The output of ‘who’ is nice, but the data is not all that exciting. The ‘cut’ Command ================= The next program we’ll look at is the ‘cut’ command. This program cuts out columns or fields of input data. For example, we can tell it to print just the login name and full name from the ‘/etc/passwd’ file. The ‘/etc/passwd’ file has seven fields, separated by colons: arnold:xyzzy:2076:10:Arnold D. Robbins:/home/arnold:/bin/bash To get the first and fifth fields, we would use ‘cut’ like this: $ cut -d: -f1,5 /etc/passwd ⊣ root:Operator ... ⊣ arnold:Arnold D. Robbins ⊣ miriam:Miriam A. Robbins ... With the ‘-c’ option, ‘cut’ will cut out specific characters (i.e., columns) in the input lines. This is useful for input data that has fixed width fields, and does not have a field separator. For example, list the Monday dates for the current month: $ cal | cut -c 3-5 ⊣Mo ⊣ ⊣ 6 ⊣ 13 ⊣ 20 ⊣ 27 The ‘sort’ Command ================== Next we’ll look at the ‘sort’ command. This is one of the most powerful commands on a Unix-style system; one that you will often find yourself using when setting up fancy data plumbing. The ‘sort’ command reads and sorts each file named on the command line. It then merges the sorted data and writes it to standard output. It will read standard input if no files are given on the command line (thus making it into a filter). The sort is based on the character collating sequence or based on user-supplied ordering criteria. The ‘uniq’ Command ================== Finally (at least for now), we’ll look at the ‘uniq’ program. When sorting data, you will often end up with duplicate lines, lines that are identical. Usually, all you need is one instance of each line. This is where ‘uniq’ comes in. The ‘uniq’ program reads its standard input. It prints only one copy of each repeated line. It does have several options. Later on, we’ll use the ‘-c’ option, which prints each unique line, preceded by a count of the number of times that line occurred in the input. Putting the Tools Together ========================== Now, let’s suppose this is a large ISP server system with dozens of users logged in. The management wants the system administrator to write a program that will generate a sorted list of logged in users. Furthermore, even if a user is logged in multiple times, his or her name should only show up in the output once. The administrator could sit down with the system documentation and write a C program that did this. It would take perhaps a couple of hundred lines of code and about two hours to write it, test it, and debug it. However, knowing the software toolbox, the administrator can instead start out by generating just a list of logged on users: $ who | cut -c1-8 ⊣ arnold ⊣ miriam ⊣ bill ⊣ arnold Next, sort the list: $ who | cut -c1-8 | sort ⊣ arnold ⊣ arnold ⊣ bill ⊣ miriam Finally, run the sorted list through ‘uniq’, to weed out duplicates: $ who | cut -c1-8 | sort | uniq ⊣ arnold ⊣ bill ⊣ miriam The ‘sort’ command actually has a ‘-u’ option that does what ‘uniq’ does. However, ‘uniq’ has other uses for which one cannot substitute ‘sort -u’. The administrator puts this pipeline into a shell script, and makes it available for all the users on the system (‘#’ is the system administrator, or ‘root’, prompt): # cat > /usr/local/bin/listusers who | cut -c1-8 | sort | uniq ^D # chmod +x /usr/local/bin/listusers There are four major points to note here. First, with just four programs, on one command line, the administrator was able to save about two hours worth of work. Furthermore, the shell pipeline is just about as efficient as the C program would be, and it is much more efficient in terms of programmer time. People time is much more expensive than computer time, and in our modern “there’s never enough time to do everything” society, saving two hours of programmer time is no mean feat. Second, it is also important to emphasize that with the _combination_ of the tools, it is possible to do a special purpose job never imagined by the authors of the individual programs. Third, it is also valuable to build up your pipeline in stages, as we did here. This allows you to view the data at each stage in the pipeline, which helps you acquire the confidence that you are indeed using these tools correctly. Finally, by bundling the pipeline in a shell script, other users can use your command, without having to remember the fancy plumbing you set up for them. In terms of how you run them, shell scripts and compiled programs are indistinguishable. After the previous warm-up exercise, we’ll look at two additional, more complicated pipelines. For them, we need to introduce two more tools. The first is the ‘tr’ command, which stands for “transliterate.” The ‘tr’ command works on a character-by-character basis, changing characters. Normally it is used for things like mapping upper case to lower case: $ echo ThIs ExAmPlE HaS MIXED case! | tr '[:upper:]' '[:lower:]' ⊣ this example has mixed case! There are several options of interest: ‘-c’ work on the complement of the listed characters, i.e., operations apply to characters not in the given set ‘-d’ delete characters in the first set from the output ‘-s’ squeeze repeated characters in the output into just one character. We will be using all three options in a moment. The other command we’ll look at is ‘comm’. The ‘comm’ command takes two sorted input files as input data, and prints out the files’ lines in three columns. The output columns are the data lines unique to the first file, the data lines unique to the second file, and the data lines that are common to both. The ‘-1’, ‘-2’, and ‘-3’ command line options _omit_ the respective columns. (This is non-intuitive and takes a little getting used to.) For example: $ cat f1 ⊣ 11111 ⊣ 22222 ⊣ 33333 ⊣ 44444 $ cat f2 ⊣ 00000 ⊣ 22222 ⊣ 33333 ⊣ 55555 $ comm f1 f2 ⊣ 00000 ⊣ 11111 ⊣ 22222 ⊣ 33333 ⊣ 44444 ⊣ 55555 The file name ‘-’ tells ‘comm’ to read standard input instead of a regular file. Now we’re ready to build a fancy pipeline. The first application is a word frequency counter. This helps an author determine if he or she is over-using certain words. The first step is to change the case of all the letters in our input file to one case. “The” and “the” are the same word when doing counting. $ tr '[:upper:]' '[:lower:]' < whats.gnu | ... The next step is to get rid of punctuation. Quoted words and unquoted words should be treated identically; it’s easiest to just get the punctuation out of the way. $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | ... The second ‘tr’ command operates on the complement of the listed characters, which are all the letters, the digits, the underscore, and the blank. The ‘\n’ represents the newline character; it has to be left alone. (The ASCII tab character should also be included for good measure in a production script.) At this point, we have data consisting of words separated by blank space. The words only contain alphanumeric characters (and the underscore). The next step is break the data apart so that we have one word per line. This makes the counting operation much easier, as we will see shortly. $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | > tr -s ' ' '\n' | ... This command turns blanks into newlines. The ‘-s’ option squeezes multiple newline characters in the output into just one, removing blank lines. (The ‘>’ is the shell’s “secondary prompt.” This is what the shell prints when it notices you haven’t finished typing in all of a command.) We now have data consisting of one word per line, no punctuation, all one case. We’re ready to count each word: $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | > tr -s ' ' '\n' | sort | uniq -c | ... At this point, the data might look something like this: 60 a 2 able 6 about 1 above 2 accomplish 1 acquire 1 actually 2 additional The output is sorted by word, not by count! What we want is the most frequently used words first. Fortunately, this is easy to accomplish, with the help of two more ‘sort’ options: ‘-n’ do a numeric sort, not a textual one ‘-r’ reverse the order of the sort The final pipeline looks like this: $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | > tr -s ' ' '\n' | sort | uniq -c | sort -n -r ⊣ 156 the ⊣ 60 a ⊣ 58 to ⊣ 51 of ⊣ 51 and ... Whew! That’s a lot to digest. Yet, the same principles apply. With six commands, on two lines (really one long one split for convenience), we’ve created a program that does something interesting and useful, in much less time than we could have written a C program to do the same thing. A minor modification to the above pipeline can give us a simple spelling checker! To determine if you’ve spelled a word correctly, all you have to do is look it up in a dictionary. If it is not there, then chances are that your spelling is incorrect. So, we need a dictionary. The conventional location for a dictionary is ‘/usr/share/dict/words’. Now, how to compare our file with the dictionary? As before, we generate a sorted list of words, one per line: $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | > tr -s ' ' '\n' | sort -u | ... Now, all we need is a list of words that are _not_ in the dictionary. Here is where the ‘comm’ command comes in. Unfortunately ‘comm’ operates on sorted input and ‘/usr/share/dict/words’ is not sorted the way that ‘sort’ and ‘comm’ normally use, so we first create a properly-sorted copy of the dictionary and then run a pipeline that uses the copy. $ sort /usr/share/dict/words > sorted-words $ tr '[:upper:]' '[:lower:]' < whats.gnu | tr -cd '[:alnum:]_ \n' | > tr -s ' ' '\n' | sort -u | > comm -23 - sorted-words The ‘-2’ and ‘-3’ options eliminate lines that are only in the dictionary (the second file), and lines that are in both files. Lines only in the first file (standard input, our stream of words), are words that are not in the dictionary. These are likely candidates for spelling errors. This pipeline was the first cut at a production spelling checker on Unix. There are some other tools that deserve brief mention. ‘grep’ search files for text that matches a regular expression ‘wc’ count lines, words, characters ‘tee’ a T-fitting for data pipes, copies data to files and to standard output ‘sed’ the stream editor, an advanced tool ‘awk’ a data manipulation language, another advanced tool The software tools philosophy also espoused the following bit of advice: “Let someone else do the hard part.” This means, take something that gives you most of what you need, and then massage it the rest of the way until it’s in the form that you want. To summarize: 1. Each program should do one thing well. No more, no less. 2. Combining programs with appropriate plumbing leads to results where the whole is greater than the sum of the parts. It also leads to novel uses of programs that the authors might never have imagined. 3. Programs should never print extraneous header or trailer data, since these could get sent on down a pipeline. (A point we didn’t mention earlier.) 4. Let someone else do the hard part. 5. Know your toolbox! Use each program appropriately. If you don’t have an appropriate tool, build one. All the programs discussed are available as described in GNU core utilities (https://www.gnu.org/software/coreutils/coreutils.html). None of what I have presented in this column is new. The Software Tools philosophy was first introduced in the book ‘Software Tools’, by Brian Kernighan and P.J. Plauger (Addison-Wesley, ISBN 0-201-03669-X). This book showed how to write and use software tools. It was written in 1976, using a preprocessor for FORTRAN named ‘ratfor’ (RATional FORtran). At the time, C was not as ubiquitous as it is now; FORTRAN was. The last chapter presented a ‘ratfor’ to FORTRAN processor, written in ‘ratfor’. ‘ratfor’ looks an awful lot like C; if you know C, you won’t have any problem following the code. In 1981, the book was updated and made available as ‘Software Tools in Pascal’ (Addison-Wesley, ISBN 0-201-10342-7). Both books are still in print and are well worth reading if you’re a programmer. They certainly made a major change in how I view programming. The programs in both books are available from Brian Kernighan’s home page (https://www.cs.princeton.edu/~bwk/). For a number of years, there was an active Software Tools Users Group, whose members had ported the original ‘ratfor’ programs to essentially every computer system with a FORTRAN compiler. The popularity of the group waned in the middle 1980s as Unix began to spread beyond universities. With the current proliferation of GNU code and other clones of Unix programs, these programs now receive little attention; modern C versions are much more efficient and do more than these programs do. Nevertheless, as exposition of good programming style, and evangelism for a still-valuable philosophy, these books are unparalleled, and I recommend them highly. Acknowledgment: I would like to express my gratitude to Brian Kernighan of Bell Labs, the original Software Toolsmith, for reviewing this column. Appendix A GNU Free Documentation License ***************************************** Version 1.3, 3 November 2008 Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document “free” in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference. 1. APPLICABILITY AND DEFINITIONS This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law. A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language. A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document’s overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them. The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none. The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words. A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”. Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only. The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning of the body of the text. The “publisher” means any person or entity that distributes copies of the Document to the public. A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition. The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License. 2. VERBATIM COPYING You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3. You may also lend copies, under the same conditions stated above, and you may publicly display copies. 3. COPYING IN QUANTITY If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects. If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages. If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public. It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document. 4. MODIFICATIONS You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version: A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission. B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement. C. State on the Title page the name of the publisher of the Modified Version, as the publisher. D. Preserve all the copyright notices of the Document. E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices. F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below. G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice. H. Include an unaltered copy of this License. I. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence. J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission. K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein. L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles. M. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version. N. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section. O. Preserve any Warranty Disclaimers. If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles. You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard. You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one. The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version. 5. COMBINING DOCUMENTS You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers. The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work. In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.” 6. COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects. You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document. 7. AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document. If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate. 8. TRANSLATION Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail. If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title. 9. TERMINATION You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License. However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation. Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice. Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it. 10. FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See . Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document. 11. RELICENSING “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site. “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization. “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document. An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008. The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing. ADDENDUM: How to use this License for your documents ==================================================== To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page: Copyright (C) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this: with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation. If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software. Index ***** * Menu: * !: Connectives for test. (line 10394) * !=: String tests. (line 10346) * %: Numeric expressions. (line 10523) * %b: printf invocation. (line 10030) * %q: printf invocation. (line 10037) * &: Relations for expr. (line 10540) * *: Numeric expressions. (line 10523) * +: String expressions. (line 10500) * + <1>: Numeric expressions. (line 10519) * +PAGE_RANGE: pr invocation. (line 1713) * -: Numeric expressions. (line 10519) * - <1>: env invocation. (line 13127) * - and Unix rm: rm invocation. (line 7734) * -, removing files beginning with: rm invocation. (line 7722) * --: Common options. (line 388) * --across: pr invocation. (line 1736) * --additional-suffix: split invocation. (line 2407) * --address-radix: od invocation. (line 1266) * --adjustment: nice invocation. (line 13551) * --algorithm: cksum general options. (line 2846) * --all: unexpand invocation. (line 5536) * --all <1>: Which files are listed. (line 5603) * --all <2>: df invocation. (line 9086) * --all <3>: du invocation. (line 9317) * --all <4>: stty invocation. (line 11236) * --all <5>: who invocation. (line 11949) * --all <6>: nproc invocation. (line 12627) * --all <7>: uname invocation. (line 12665) * --all-repeated: uniq invocation. (line 3856) * --almost-all: Which files are listed. (line 5607) * --apparent-size: du invocation. (line 9320) * --append: tee invocation. (line 10611) * --archive: cp invocation. (line 6513) * --attributes-only: cp invocation. (line 6522) * --author: What information is listed. (line 5690) * --auto-reference: Output formatting in ptx. (line 4270) * --backup: Backup options. (line 409) * --backup <1>: cp invocation. (line 6529) * --backup <2>: install invocation. (line 7362) * --backup <3>: mv invocation. (line 7528) * --backup <4>: ln invocation. (line 8081) * --base16: basenc invocation. (line 1542) * --base2lsbf: basenc invocation. (line 1548) * --base2msbf: basenc invocation. (line 1552) * --base32: basenc invocation. (line 1528) * --base32hex: basenc invocation. (line 1535) * --base64: basenc invocation. (line 1516) * --base64 <1>: cksum general options. (line 2864) * --base64url: basenc invocation. (line 1522) * --batch-size: sort invocation. (line 3390) * --before: tac invocation. (line 1089) * --binary: cksum common options. (line 2906) * --block-size: Block size. (line 578) * --block-size <1>: df invocation. (line 9098) * --block-size <2>: du invocation. (line 9340) * --block-size=SIZE: Block size. (line 459) * --body-numbering: nl invocation. (line 1155) * --boot: who invocation. (line 11953) * --bourne-shell: dircolors invocation. (line 6425) * --break-file: Input processing in ptx. (line 4117) * --buffer-size: sort invocation. (line 3444) * --bytes: fold invocation. (line 1948) * --bytes <1>: head invocation. (line 1994) * --bytes <2>: tail invocation. (line 2080) * --bytes <3>: split invocation. (line 2309) * --bytes <4>: wc invocation. (line 2692) * --bytes <5>: cut invocation. (line 4596) * --bytes <6>: du invocation. (line 9345) * --c-shell: dircolors invocation. (line 6431) * --cached=MODE: stat invocation. (line 9609) * --canonicalize: readlink invocation. (line 8407) * --canonicalize-existing: readlink invocation. (line 8414) * --canonicalize-existing <1>: realpath invocation. (line 11063) * --canonicalize-missing: readlink invocation. (line 8421) * --canonicalize-missing <1>: realpath invocation. (line 11071) * --changes: chown invocation. (line 8604) * --changes <1>: chgrp invocation. (line 8740) * --changes <2>: chmod invocation. (line 8869) * --characters: cut invocation. (line 4604) * --chars: wc invocation. (line 2696) * --chdir: env invocation. (line 13132) * --check: sort invocation. (line 3161) * --check <1>: sort invocation. (line 3169) * --check-chars: uniq invocation. (line 3920) * --classify: General output formatting. (line 6076) * --color: General output formatting. (line 6052) * --columns: pr invocation. (line 1723) * --compare: install invocation. (line 7367) * --complement: cut invocation. (line 4656) * --complement <1>: tr invocation. (line 5152) * --compute: runcon invocation. (line 12904) * --context: What information is listed. (line 5939) * --context <1>: cp invocation. (line 6867) * --context <2>: install invocation. (line 7464) * --context <3>: mv invocation. (line 7616) * --context <4>: mkdir invocation. (line 8274) * --context <5>: mkfifo invocation. (line 8309) * --context <6>: mknod invocation. (line 8369) * --context <7>: id invocation. (line 11812) * --count: uniq invocation. (line 3842) * --count <1>: who invocation. (line 11983) * --count-links: du invocation. (line 9411) * --crown-margin: fmt invocation. (line 1633) * --csh: dircolors invocation. (line 6431) * --data: sync invocation. (line 9832) * --date: touch invocation. (line 8973) * --date <1>: Options for date. (line 12341) * --dead: who invocation. (line 11957) * --debug: cksum general options. (line 2875) * --debug <1>: cp invocation. (line 6564) * --debug <2>: install invocation. (line 7393) * --debug <3>: mv invocation. (line 7532) * --debug <4>: Options for date. (line 12355) * --debug <5>: env invocation. (line 13202) * --debug <6>: numfmt invocation. (line 14044) * --decode: base64 invocation. (line 1484) * --delete: tr invocation. (line 5159) * --delimiter: cut invocation. (line 4636) * --delimiter <1>: numfmt invocation. (line 14049) * --delimiters: paste invocation. (line 4732) * --dereference: Which files are listed. (line 5674) * --dereference <1>: cp invocation. (line 6603) * --dereference <2>: chown invocation. (line 8637) * --dereference <3>: chgrp invocation. (line 8750) * --dereference <4>: du invocation. (line 9405) * --dereference <5>: stat invocation. (line 9598) * --dereference <6>: chcon invocation. (line 12814) * --dereference-args: du invocation. (line 9355) * --dereference-command-line: Which files are listed. (line 5626) * --dereference-command-line-symlink-to-dir: Which files are listed. (line 5631) * --dictionary-order: sort invocation. (line 3209) * --digits: csplit invocation. (line 2568) * --dir: rm invocation. (line 7655) * --directory: Which files are listed. (line 5618) * --directory <1>: install invocation. (line 7388) * --directory <2>: ln invocation. (line 8087) * --directory <3>: mktemp invocation. (line 10996) * --dired: What information is listed. (line 5696) * --double-space: pr invocation. (line 1748) * --dry-run: mktemp invocation. (line 11008) * --echo: shuf invocation. (line 3696) * --elide-empty-files: split invocation. (line 2412) * --elide-empty-files <1>: csplit invocation. (line 2582) * --endian: od invocation. (line 1281) * --equal-width: seq invocation. (line 14375) * --error: stdbuf invocation. (line 13695) * --escape: Formatting the file names. (line 6306) * --exact: shred invocation. (line 7901) * --exclude-from=FILE: du invocation. (line 9536) * --exclude-type: df invocation. (line 9268) * --exclude=PATTERN: du invocation. (line 9541) * --expand-tabs: pr invocation. (line 1772) * --exponents: factor invocation. (line 13982) * --field: numfmt invocation. (line 14053) * --field-separator: sort invocation. (line 3460) * --fields: cut invocation. (line 4613) * --file: stty invocation. (line 11241) * --file <1>: Options for date. (line 12360) * --file-system: stat invocation. (line 9604) * --file-system <1>: sync invocation. (line 9837) * --file-type: General output formatting. (line 6092) * --files0-from=FILE: wc invocation. (line 2730) * --files0-from=FILE <1>: sort invocation. (line 3346) * --files0-from=FILE <2>: du invocation. (line 9367) * --filter: split invocation. (line 2331) * --first-line-number: pr invocation. (line 1848) * --flag-truncation: Output formatting in ptx. (line 4296) * --follow: tail invocation. (line 2098) * --footer-numbering: nl invocation. (line 1182) * --force: cp invocation. (line 6569) * --force <1>: mv invocation. (line 7537) * --force <2>: rm invocation. (line 7659) * --force <3>: shred invocation. (line 7860) * --force <4>: ln invocation. (line 8093) * --foreground: timeout invocation. (line 13759) * --form-feed: pr invocation. (line 1780) * --format: od invocation. (line 1320) * --format <1>: What information is listed. (line 5813) * --format <2>: General output formatting. (line 6033) * --format <3>: General output formatting. (line 6045) * --format <4>: General output formatting. (line 6138) * --format <5>: General output formatting. (line 6149) * --format <6>: numfmt invocation. (line 14063) * --format <7>: seq invocation. (line 14354) * --format=FORMAT: stat invocation. (line 9626) * --format=roff: Output formatting in ptx. (line 4325) * --format=tex: Output formatting in ptx. (line 4343) * --from: chown invocation. (line 8614) * --from <1>: numfmt invocation. (line 14074) * --from-unit: numfmt invocation. (line 14079) * --full-time: What information is listed. (line 5783) * --gap-size: Output formatting in ptx. (line 4251) * --general-numeric-sort: sort invocation. (line 3227) * --goal: fmt invocation. (line 1663) * --group: uniq invocation. (line 3887) * --group <1>: install invocation. (line 7398) * --group <2>: id invocation. (line 11790) * --group-directories-first: Which files are listed. (line 5640) * --grouping: numfmt invocation. (line 14085) * --groups: id invocation. (line 11794) * --groups <1>: chroot invocation. (line 12957) * --hardware-platform: uname invocation. (line 12670) * --head-count: shuf invocation. (line 3708) * --header: pr invocation. (line 1785) * --header <1>: General options in join. (line 4808) * --header <2>: numfmt invocation. (line 14091) * --header-numbering: nl invocation. (line 1186) * --header=N: numfmt invocation. (line 14091) * --heading: who invocation. (line 11961) * --help: Common options. (line 381) * --hex-suffixes: split invocation. (line 2403) * --hide-control-chars: Formatting the file names. (line 6318) * --hide=PATTERN: Which files are listed. (line 5648) * --human-numeric-sort: sort invocation. (line 3254) * --human-readable: Block size. (line 578) * --human-readable <1>: What information is listed. (line 5798) * --human-readable <2>: df invocation. (line 9103) * --human-readable <3>: du invocation. (line 9384) * --hyperlink: General output formatting. (line 6097) * --ignore: nproc invocation. (line 12633) * --ignore-backups: Which files are listed. (line 5613) * --ignore-case: sort invocation. (line 3216) * --ignore-case <1>: uniq invocation. (line 3846) * --ignore-case <2>: Charset selection in ptx. (line 4110) * --ignore-case <3>: General options in join. (line 4818) * --ignore-environment: env invocation. (line 13127) * --ignore-fail-on-non-empty: rmdir invocation. (line 8467) * --ignore-file: Input processing in ptx. (line 4135) * --ignore-garbage: base64 invocation. (line 1490) * --ignore-interrupts: tee invocation. (line 10616) * --ignore-leading-blanks: sort invocation. (line 3201) * --ignore-missing: cksum common options. (line 2955) * --ignore-nonprinting: sort invocation. (line 3271) * --ignore=PATTERN: Which files are listed. (line 5660) * --indent: pr invocation. (line 1854) * --indicator-style: General output formatting. (line 6076) * --indicator-style <1>: General output formatting. (line 6092) * --indicator-style <2>: General output formatting. (line 6106) * --indicator-style <3>: General output formatting. (line 6144) * --initial: expand invocation. (line 5482) * --inode: What information is listed. (line 5805) * --inodes: df invocation. (line 9113) * --inodes <1>: du invocation. (line 9390) * --input: stdbuf invocation. (line 13687) * --input-range: shuf invocation. (line 3700) * --interactive: cp invocation. (line 6593) * --interactive <1>: mv invocation. (line 7543) * --interactive <2>: rm invocation. (line 7675) * --interactive <3>: ln invocation. (line 8097) * --invalid: numfmt invocation. (line 14094) * --io-blocks: truncate invocation. (line 9873) * --iso-8601[=TIMESPEC]: Options for date. (line 12368) * --iterations=NUMBER: shred invocation. (line 7864) * --join-blank-lines: nl invocation. (line 1195) * --join-lines: pr invocation. (line 1798) * --keep-files: csplit invocation. (line 2573) * --kernel-name: uname invocation. (line 12701) * --kernel-release: uname invocation. (line 12697) * --kernel-version: uname invocation. (line 12712) * --key: sort invocation. (line 3359) * --kibibytes: General output formatting. (line 6125) * --kill-after: timeout invocation. (line 13776) * --length: pr invocation. (line 1807) * --length <1>: cksum general options. (line 2880) * --length <2>: b2sum invocation. (line 3068) * --line-bytes: split invocation. (line 2324) * --line-increment: nl invocation. (line 1190) * --lines: head invocation. (line 2010) * --lines <1>: tail invocation. (line 2167) * --lines <2>: split invocation. (line 2301) * --lines <3>: wc invocation. (line 2706) * --link: cp invocation. (line 6599) * --literal: Formatting the file names. (line 6312) * --local: df invocation. (line 9125) * --logical: ln invocation. (line 8102) * --logical <1>: realpath invocation. (line 11076) * --logical <2>: pwd invocation. (line 11190) * --login: who invocation. (line 11965) * --lookup: who invocation. (line 11970) * --machine: uname invocation. (line 12677) * --macro-name: Output formatting in ptx. (line 4319) * --max-depth=DEPTH: du invocation. (line 9362) * --max-line-length: wc invocation. (line 2712) * --max-unchanged-stats: tail invocation. (line 2155) * --merge: pr invocation. (line 1814) * --merge <1>: sort invocation. (line 3175) * --mesg: who invocation. (line 12008) * --message: who invocation. (line 12008) * --mode: install invocation. (line 7404) * --mode <1>: mkdir invocation. (line 8234) * --mode <2>: mkfifo invocation. (line 8302) * --mode <3>: mknod invocation. (line 8363) * --month-sort: sort invocation. (line 3278) * --multiple: basename invocation. (line 10787) * --name: id invocation. (line 11798) * --no-clobber: cp invocation. (line 6610) * --no-clobber <1>: mv invocation. (line 7550) * --no-copy: mv invocation. (line 7557) * --no-create: touch invocation. (line 8969) * --no-create <1>: truncate invocation. (line 9869) * --no-dereference: cp invocation. (line 6616) * --no-dereference <1>: ln invocation. (line 8108) * --no-dereference <2>: chown invocation. (line 8649) * --no-dereference <3>: chgrp invocation. (line 8762) * --no-dereference <4>: touch invocation. (line 8987) * --no-dereference <5>: du invocation. (line 9421) * --no-dereference <6>: chcon invocation. (line 12819) * --no-file-warnings: pr invocation. (line 1861) * --no-group: What information is listed. (line 5792) * --no-newline: readlink invocation. (line 8426) * --no-preserve-root: rm invocation. (line 7709) * --no-preserve-root <1>: chown invocation. (line 8662) * --no-preserve-root <2>: chgrp invocation. (line 8775) * --no-preserve-root <3>: chmod invocation. (line 8884) * --no-preserve-root <4>: chcon invocation. (line 12836) * --no-renumber: nl invocation. (line 1214) * --no-symlinks: realpath invocation. (line 11105) * --no-sync: df invocation. (line 9129) * --no-target-directory: Target directory. (line 749) * --no-target-directory <1>: cp invocation. (line 6817) * --no-target-directory <2>: install invocation. (line 7455) * --no-target-directory <3>: mv invocation. (line 7611) * --no-target-directory <4>: ln invocation. (line 8176) * --nodename: uname invocation. (line 12682) * --null: du invocation. (line 9311) * --null <1>: printenv invocation. (line 11724) * --null <2>: env invocation. (line 13115) * --number: cat invocation. (line 1029) * --number <1>: split invocation. (line 2347) * --number-format: nl invocation. (line 1203) * --number-lines: pr invocation. (line 1827) * --number-nonblank: cat invocation. (line 1017) * --number-separator: nl invocation. (line 1218) * --number-width: nl invocation. (line 1228) * --numeric-sort: sort invocation. (line 3288) * --numeric-suffixes: split invocation. (line 2389) * --numeric-uid-gid: What information is listed. (line 5907) * --omit-header: pr invocation. (line 1884) * --omit-pagination: pr invocation. (line 1894) * --one-file-system: cp invocation. (line 6861) * --one-file-system <1>: rm invocation. (line 7686) * --one-file-system <2>: du invocation. (line 9547) * --only-delimited: cut invocation. (line 4644) * --only-file: Input processing in ptx. (line 4144) * --operating-system: uname invocation. (line 12693) * --output: sort invocation. (line 3416) * --output <1>: shuf invocation. (line 3713) * --output <2>: df invocation. (line 9136) * --output <3>: stdbuf invocation. (line 13691) * --output-delimiter: cut invocation. (line 4649) * --output-duplicates: od invocation. (line 1380) * --output-error: tee invocation. (line 10620) * --output-tabs: pr invocation. (line 1791) * --owner: install invocation. (line 7416) * --padding: numfmt invocation. (line 14102) * --pages=PAGE_RANGE: pr invocation. (line 1713) * --page_width: pr invocation. (line 1913) * --parallel: sort invocation. (line 3488) * --parents: cp invocation. (line 6699) * --parents <1>: mkdir invocation. (line 8251) * --parents <2>: rmdir invocation. (line 8471) * --physical: ln invocation. (line 8127) * --physical <1>: realpath invocation. (line 11081) * --physical <2>: pwd invocation. (line 11197) * --pid: tail invocation. (line 2174) * --portability: df invocation. (line 9193) * --portability <1>: pathchk invocation. (line 10906) * --prefix: csplit invocation. (line 2550) * --preserve: cp invocation. (line 6623) * --preserve-context: install invocation. (line 7421) * --preserve-root: rm invocation. (line 7702) * --preserve-root <1>: chown invocation. (line 8657) * --preserve-root <2>: chgrp invocation. (line 8770) * --preserve-root <3>: chmod invocation. (line 8879) * --preserve-root <4>: chcon invocation. (line 12831) * --preserve-status: timeout invocation. (line 13753) * --preserve-timestamps: install invocation. (line 7428) * --print-database: dircolors invocation. (line 6436) * --print-ls-colors: dircolors invocation. (line 6441) * --print-type: df invocation. (line 9245) * --printf=FORMAT: stat invocation. (line 9635) * --process: who invocation. (line 11979) * --processor: uname invocation. (line 12686) * --quiet: head invocation. (line 2017) * --quiet <1>: tail invocation. (line 2196) * --quiet <2>: csplit invocation. (line 2593) * --quiet <3>: cksum common options. (line 2961) * --quiet <4>: readlink invocation. (line 8433) * --quiet <5>: chown invocation. (line 8610) * --quiet <6>: chgrp invocation. (line 8746) * --quiet <7>: chmod invocation. (line 8875) * --quiet <8>: mktemp invocation. (line 11003) * --quiet <9>: realpath invocation. (line 11087) * --quiet <10>: tty invocation. (line 11749) * --quote-name: Formatting the file names. (line 6325) * --quoting-style: Formatting the file names. (line 6306) * --quoting-style <1>: Formatting the file names. (line 6312) * --quoting-style <2>: Formatting the file names. (line 6325) * --quoting-style <3>: Formatting the file names. (line 6329) * --random-sort: sort invocation. (line 3317) * --random-source: sort invocation. (line 3432) * --random-source <1>: shuf invocation. (line 3719) * --random-source <2>: shred invocation. (line 7870) * --range: chcon invocation. (line 12870) * --range <1>: runcon invocation. (line 12920) * --raw: cksum general options. (line 2886) * --read-bytes: od invocation. (line 1306) * --real: id invocation. (line 11803) * --recursive: Which files are listed. (line 5681) * --recursive <1>: cp invocation. (line 6712) * --recursive <2>: rm invocation. (line 7716) * --recursive <3>: chown invocation. (line 8681) * --recursive <4>: chgrp invocation. (line 8793) * --recursive <5>: chmod invocation. (line 8899) * --recursive <6>: chcon invocation. (line 12828) * --reference: chown invocation. (line 8666) * --reference <1>: chgrp invocation. (line 8779) * --reference <2>: chmod invocation. (line 8892) * --reference <3>: touch invocation. (line 9006) * --reference <4>: truncate invocation. (line 9877) * --reference <5>: Options for date. (line 12397) * --reference <6>: chcon invocation. (line 12823) * --references: Input processing in ptx. (line 4157) * --reflink[=WHEN]: cp invocation. (line 6725) * --regex: tac invocation. (line 1094) * --relative: ln invocation. (line 8136) * --relative-base: realpath invocation. (line 11095) * --relative-base <1>: Realpath usage examples. (line 11124) * --relative-to: realpath invocation. (line 11090) * --relative-to <1>: Realpath usage examples. (line 11124) * --remove: shred invocation. (line 7881) * --remove-destination: cp invocation. (line 6752) * --remove=unlink: shred invocation. (line 7881) * --remove=wipe: shred invocation. (line 7881) * --remove=wipesync: shred invocation. (line 7881) * --repeat: shuf invocation. (line 3724) * --repeated: uniq invocation. (line 3850) * --resolution: Options for date. (line 12401) * --retry: tail invocation. (line 2199) * --reverse: sort invocation. (line 3311) * --reverse <1>: Sorting the output. (line 5965) * --rfc-2822: Options for date. (line 12417) * --rfc-3339=TIMESPEC: Options for date. (line 12426) * --rfc-822: Options for date. (line 12417) * --rfc-email: Options for date. (line 12411) * --right-side-refs: Output formatting in ptx. (line 4281) * --role: chcon invocation. (line 12862) * --role <1>: runcon invocation. (line 12912) * --round: numfmt invocation. (line 14109) * --round=down: numfmt invocation. (line 14109) * --round=from-zero: numfmt invocation. (line 14109) * --round=nearest: numfmt invocation. (line 14109) * --round=towards-zero: numfmt invocation. (line 14109) * --round=up: numfmt invocation. (line 14109) * --runlevel: who invocation. (line 11988) * --save: stty invocation. (line 11251) * --section-delimiter: nl invocation. (line 1173) * --sentence-regexp: Input processing in ptx. (line 4174) * --sep-string: pr invocation. (line 1875) * --separate-dirs: du invocation. (line 9426) * --separator: tac invocation. (line 1098) * --separator <1>: pr invocation. (line 1866) * --separator <2>: split invocation. (line 2420) * --separator <3>: seq invocation. (line 14370) * --serial: paste invocation. (line 4723) * --set: Options for date. (line 12455) * --sh: dircolors invocation. (line 6425) * --show-all: cat invocation. (line 1013) * --show-control-chars: pr invocation. (line 1742) * --show-control-chars <1>: Formatting the file names. (line 6373) * --show-ends: cat invocation. (line 1024) * --show-nonprinting: cat invocation. (line 1049) * --show-nonprinting <1>: pr invocation. (line 1899) * --show-tabs: cat invocation. (line 1042) * --si: Block size. (line 578) * --si <1>: What information is listed. (line 5931) * --si <2>: df invocation. (line 9212) * --si <3>: du invocation. (line 9433) * --signal: timeout invocation. (line 13793) * --silent: head invocation. (line 2017) * --silent <1>: tail invocation. (line 2196) * --silent <2>: csplit invocation. (line 2593) * --silent <3>: readlink invocation. (line 8433) * --silent <4>: chown invocation. (line 8610) * --silent <5>: chgrp invocation. (line 8746) * --silent <6>: chmod invocation. (line 8875) * --silent <7>: tty invocation. (line 11749) * --size: What information is listed. (line 5916) * --size <1>: truncate invocation. (line 9881) * --size=BYTES: shred invocation. (line 7875) * --skip-bytes: od invocation. (line 1289) * --skip-chars: uniq invocation. (line 3828) * --skip-chdir: chroot invocation. (line 12970) * --skip-fields: uniq invocation. (line 3817) * --sleep-interval: tail invocation. (line 2216) * --sort: sort invocation. (line 3227) * --sort <1>: sort invocation. (line 3254) * --sort <2>: sort invocation. (line 3278) * --sort <3>: sort invocation. (line 3288) * --sort <4>: sort invocation. (line 3317) * --sort <5>: Sorting the output. (line 5972) * --sort <6>: Sorting the output. (line 5976) * --sort <7>: Sorting the output. (line 6004) * --sort <8>: Sorting the output. (line 6011) * --sort <9>: Sorting the output. (line 6017) * --sort <10>: Sorting the output. (line 6023) * --spaces: fold invocation. (line 1954) * --sparse=WHEN: cp invocation. (line 6756) * --split-only: fmt invocation. (line 1646) * --split-string: env invocation. (line 13217) * --squeeze-blank: cat invocation. (line 1034) * --squeeze-repeats: tr invocation. (line 5163) * --stable: sort invocation. (line 3437) * --starting-line-number: nl invocation. (line 1223) * --status: cksum common options. (line 2969) * --strict: cksum common options. (line 3005) * --strings: od invocation. (line 1311) * --strip: install invocation. (line 7438) * --strip <1>: realpath invocation. (line 11105) * --strip-program: install invocation. (line 7441) * --strip-trailing-slashes: cp invocation. (line 6795) * --strip-trailing-slashes <1>: mv invocation. (line 7597) * --suffix: Backup options. (line 445) * --suffix <1>: cp invocation. (line 6808) * --suffix <2>: install invocation. (line 7445) * --suffix <3>: mv invocation. (line 7602) * --suffix <4>: ln invocation. (line 8167) * --suffix <5>: basename invocation. (line 10793) * --suffix <6>: mktemp invocation. (line 11024) * --suffix <7>: numfmt invocation. (line 14114) * --suffix-format: csplit invocation. (line 2554) * --suffix-length: split invocation. (line 2381) * --summarize: du invocation. (line 9441) * --suppress-matched: csplit invocation. (line 2576) * --symbolic: ln invocation. (line 8161) * --symbolic-link: cp invocation. (line 6800) * --sync: df invocation. (line 9219) * --sysv: sum invocation. (line 2776) * --tabs: expand invocation. (line 5458) * --tabs <1>: unexpand invocation. (line 5509) * --tabsize: General output formatting. (line 6153) * --tag: cksum common options. (line 2978) * --tagged-paragraph: fmt invocation. (line 1639) * --target-directory: Target directory. (line 765) * --target-directory <1>: cp invocation. (line 6813) * --target-directory <2>: install invocation. (line 7450) * --target-directory <3>: mv invocation. (line 7607) * --target-directory <4>: ln invocation. (line 8172) * --temporary-directory: sort invocation. (line 3480) * --terse: stat invocation. (line 9646) * --text: cksum common options. (line 2990) * --threshold: du invocation. (line 9445) * --time: Sorting the output. (line 5953) * --time <1>: Sorting the output. (line 5984) * --time <2>: Sorting the output. (line 5990) * --time <3>: Sorting the output. (line 5997) * --time <4>: touch invocation. (line 8965) * --time <5>: touch invocation. (line 9002) * --time <6>: du invocation. (line 9481) * --time <7>: du invocation. (line 9488) * --time <8>: du invocation. (line 9494) * --time <9>: who invocation. (line 11996) * --time-style: Formatting file timestamps. (line 6209) * --time-style <1>: du invocation. (line 9498) * --tmpdir: mktemp invocation. (line 11016) * --to: numfmt invocation. (line 14118) * --to-unit: numfmt invocation. (line 14123) * --total: df invocation. (line 9225) * --total <1>: du invocation. (line 9349) * --total=WHEN: wc invocation. (line 2717) * --traditional: od invocation. (line 1430) * --truncate-set1: tr invocation. (line 5168) * --type: df invocation. (line 9239) * --type <1>: chcon invocation. (line 12866) * --type <2>: runcon invocation. (line 12916) * --unbuffered: split invocation. (line 2427) * --uniform-spacing: fmt invocation. (line 1652) * --unique: sort invocation. (line 3496) * --unique <1>: uniq invocation. (line 3914) * --universal: Options for date. (line 12461) * --unset: env invocation. (line 13121) * --untagged: cksum general options. (line 2893) * --update: mv invocation. (line 7563) * --update[=WHICH]: cp invocation. (line 6822) * --user: id invocation. (line 11808) * --user <1>: chcon invocation. (line 12858) * --user <2>: runcon invocation. (line 12908) * --userspec: chroot invocation. (line 12963) * --utc: Options for date. (line 12461) * --verbose: head invocation. (line 2021) * --verbose <1>: tail invocation. (line 2227) * --verbose <2>: split invocation. (line 2431) * --verbose <3>: cp invocation. (line 6857) * --verbose <4>: install invocation. (line 7460) * --verbose <5>: mv invocation. (line 7594) * --verbose <6>: rm invocation. (line 7720) * --verbose <7>: shred invocation. (line 7896) * --verbose <8>: ln invocation. (line 8181) * --verbose <9>: mkdir invocation. (line 8269) * --verbose <10>: readlink invocation. (line 8437) * --verbose <11>: rmdir invocation. (line 8480) * --verbose <12>: chown invocation. (line 8673) * --verbose <13>: chgrp invocation. (line 8785) * --verbose <14>: chmod invocation. (line 8889) * --verbose <15>: chcon invocation. (line 12854) * --verbose <16>: timeout invocation. (line 13799) * --version: Common options. (line 385) * --version-sort: sort invocation. (line 3305) * --warn: cksum common options. (line 3000) * --width: od invocation. (line 1387) * --width <1>: fmt invocation. (line 1658) * --width <2>: pr invocation. (line 1903) * --width <3>: fold invocation. (line 1960) * --width <4>: Output formatting in ptx. (line 4257) * --width <5>: General output formatting. (line 6173) * --word-regexp: Input processing in ptx. (line 4214) * --words: wc invocation. (line 2701) * --wrap: base64 invocation. (line 1476) * --writable: who invocation. (line 12008) * --z85: basenc invocation. (line 1556) * --zero: cksum common options. (line 3010) * --zero <1>: General output formatting. (line 6181) * --zero <2>: shred invocation. (line 7912) * --zero <3>: readlink invocation. (line 8441) * --zero <4>: basename invocation. (line 10797) * --zero <5>: dirname invocation. (line 10846) * --zero <6>: realpath invocation. (line 11112) * --zero <7>: id invocation. (line 11819) * --zero-terminated: head invocation. (line 2025) * --zero-terminated <1>: tail invocation. (line 2231) * --zero-terminated <2>: sort invocation. (line 3511) * --zero-terminated <3>: shuf invocation. (line 3732) * --zero-terminated <4>: uniq invocation. (line 3926) * --zero-terminated <5>: comm invocation. (line 4023) * --zero-terminated <6>: cut invocation. (line 4664) * --zero-terminated <7>: paste invocation. (line 4743) * --zero-terminated <8>: General options in join. (line 4876) * --zero-terminated <9>: numfmt invocation. (line 14130) * -0: du invocation. (line 9311) * -0 <1>: printenv invocation. (line 11724) * -0 <2>: env invocation. (line 13115) * -1: comm invocation. (line 3957) * -1 <1>: General options in join. (line 4823) * -1 <2>: General output formatting. (line 6040) * -2: comm invocation. (line 3957) * -2 <1>: General options in join. (line 4826) * -3: comm invocation. (line 3957) * -A: cat invocation. (line 1013) * -A <1>: od invocation. (line 1266) * -a: od invocation. (line 1399) * -a <1>: pr invocation. (line 1736) * -a <2>: split invocation. (line 2381) * -a <3>: cksum general options. (line 2846) * -A <2>: Output formatting in ptx. (line 4270) * -a <4>: General options in join. (line 4793) * -a <5>: unexpand invocation. (line 5536) * -a <6>: Which files are listed. (line 5603) * -A <3>: Which files are listed. (line 5607) * -a <7>: cp invocation. (line 6513) * -a <8>: touch invocation. (line 8965) * -a <9>: df invocation. (line 9086) * -a <10>: du invocation. (line 9317) * -a <11>: Connectives for test. (line 10400) * -a <12>: tee invocation. (line 10611) * -a <13>: basename invocation. (line 10787) * -a <14>: stty invocation. (line 11236) * -a <15>: who invocation. (line 11949) * -a <16>: uname invocation. (line 12665) * -b: Backup options. (line 409) * -b <1>: cat invocation. (line 1017) * -b <2>: tac invocation. (line 1089) * -b <3>: nl invocation. (line 1155) * -b <4>: od invocation. (line 1402) * -b <5>: fold invocation. (line 1948) * -b <6>: split invocation. (line 2309) * -b <7>: csplit invocation. (line 2554) * -b <8>: cksum common options. (line 2906) * -b <9>: sort invocation. (line 3201) * -b <10>: Input processing in ptx. (line 4117) * -b <11>: cut invocation. (line 4596) * -B: Which files are listed. (line 5613) * -b <12>: Formatting the file names. (line 6306) * -b <13>: dircolors invocation. (line 6425) * -b <14>: cp invocation. (line 6529) * -b <15>: install invocation. (line 7362) * -b <16>: mv invocation. (line 7528) * -b <17>: ln invocation. (line 8081) * -B <1>: df invocation. (line 9098) * -B <2>: du invocation. (line 9340) * -b <18>: du invocation. (line 9345) * -b <19>: File type tests. (line 10237) * -b <20>: who invocation. (line 11953) * -c: od invocation. (line 1405) * -c <1>: fmt invocation. (line 1633) * -c <2>: pr invocation. (line 1742) * -c <3>: head invocation. (line 1994) * -c <4>: tail invocation. (line 2080) * -C: split invocation. (line 2324) * -c <5>: wc invocation. (line 2692) * -c <6>: sort invocation. (line 3161) * -c <7>: sort invocation. (line 3169) * -c <8>: shuf invocation. (line 3696) * -c <9>: uniq invocation. (line 3842) * -c <10>: cut invocation. (line 4604) * -c <11>: tr invocation. (line 5152) * -C <1>: tr invocation. (line 5152) * -c <12>: Sorting the output. (line 5953) * -C <2>: General output formatting. (line 6045) * -c <13>: dircolors invocation. (line 6431) * -C <3>: install invocation. (line 7367) * -c <14>: install invocation. (line 7378) * -c <15>: chown invocation. (line 8604) * -c <16>: chgrp invocation. (line 8740) * -c <17>: chmod invocation. (line 8869) * -c <18>: touch invocation. (line 8969) * -c <19>: du invocation. (line 9349) * -c <20>: stat invocation. (line 9626) * -c <21>: truncate invocation. (line 9869) * -c <22>: File type tests. (line 10240) * -c <23>: runcon invocation. (line 12904) * -C <4>: env invocation. (line 13132) * -COLUMN: pr invocation. (line 1723) * -d: nl invocation. (line 1173) * -d <1>: od invocation. (line 1409) * -d <2>: base64 invocation. (line 1484) * -d <3>: pr invocation. (line 1748) * -d <4>: split invocation. (line 2389) * -d <5>: sort invocation. (line 3209) * -d <6>: uniq invocation. (line 3850) * -D: uniq invocation. (line 3856) * -d <7>: cut invocation. (line 4636) * -d <8>: paste invocation. (line 4732) * -d <9>: tr invocation. (line 5159) * -d <10>: Which files are listed. (line 5618) * -D <1>: What information is listed. (line 5696) * -d <11>: cp invocation. (line 6559) * -D <2>: install invocation. (line 7381) * -d <12>: install invocation. (line 7388) * -d <13>: rm invocation. (line 7655) * -d <14>: ln invocation. (line 8087) * -d <15>: touch invocation. (line 8973) * -D <3>: du invocation. (line 9355) * -d <16>: File type tests. (line 10243) * -d <17>: mktemp invocation. (line 10996) * -d <18>: who invocation. (line 11957) * -d <19>: Options for date. (line 12341) * -d <20>: numfmt invocation. (line 14049) * -d DEPTH: du invocation. (line 9362) * -e: cat invocation. (line 1020) * -E: cat invocation. (line 1024) * -e <1>: pr invocation. (line 1772) * -e <2>: split invocation. (line 2412) * -e <3>: General options in join. (line 4804) * -e <4>: readlink invocation. (line 8414) * -e <5>: echo invocation. (line 9940) * -E <1>: echo invocation. (line 9976) * -e <6>: File characteristic tests. (line 10299) * -e <7>: realpath invocation. (line 11063) * -e <8>: stdbuf invocation. (line 13695) * -ef: File characteristic tests. (line 10313) * -eq: Numeric tests. (line 10361) * -f: nl invocation. (line 1182) * -f <1>: od invocation. (line 1412) * -F: pr invocation. (line 1780) * -f <2>: pr invocation. (line 1780) * -f <3>: tail invocation. (line 2098) * -F <1>: tail invocation. (line 2150) * -f <4>: csplit invocation. (line 2550) * -f <5>: sort invocation. (line 3216) * -f <6>: uniq invocation. (line 3817) * -f <7>: Charset selection in ptx. (line 4110) * -F <2>: Output formatting in ptx. (line 4296) * -f <8>: cut invocation. (line 4613) * -f <9>: Sorting the output. (line 5958) * -F <3>: General output formatting. (line 6076) * -f <10>: cp invocation. (line 6569) * -f <11>: mv invocation. (line 7537) * -f <12>: rm invocation. (line 7659) * -f <13>: shred invocation. (line 7860) * -F <4>: ln invocation. (line 8087) * -f <14>: ln invocation. (line 8093) * -f <15>: readlink invocation. (line 8407) * -f <16>: chown invocation. (line 8610) * -f <17>: chgrp invocation. (line 8746) * -f <18>: chmod invocation. (line 8875) * -f <19>: touch invocation. (line 8983) * -f <20>: stat invocation. (line 9604) * -f <21>: File type tests. (line 10246) * -F <5>: stty invocation. (line 11241) * -f <22>: Options for date. (line 12360) * -f <23>: seq invocation. (line 14354) * -g: fmt invocation. (line 1663) * -g <1>: sort invocation. (line 3227) * -g <2>: Output formatting in ptx. (line 4251) * -g <3>: What information is listed. (line 5788) * -G: What information is listed. (line 5792) * -g <4>: install invocation. (line 7398) * -g <5>: Access permission tests. (line 10269) * -G <1>: Access permission tests. (line 10291) * -g <6>: stty invocation. (line 11251) * -g <7>: id invocation. (line 11790) * -G <2>: id invocation. (line 11794) * -ge: Numeric tests. (line 10361) * -gt: Numeric tests. (line 10361) * -h: Block size. (line 578) * -H: Traversing symlinks. (line 843) * -h <1>: nl invocation. (line 1186) * -h <2>: pr invocation. (line 1785) * -h <3>: sort invocation. (line 3254) * -H <1>: Which files are listed. (line 5626) * -h <4>: What information is listed. (line 5798) * -H <2>: cp invocation. (line 6586) * -h <5>: chown invocation. (line 8649) * -H <3>: chown invocation. (line 8684) * -h <6>: chgrp invocation. (line 8762) * -H <4>: chgrp invocation. (line 8797) * -h <7>: touch invocation. (line 8987) * -h <8>: df invocation. (line 9103) * -H <5>: df invocation. (line 9109) * -H <6>: du invocation. (line 9380) * -h <9>: du invocation. (line 9384) * -h <10>: File type tests. (line 10250) * -H <7>: who invocation. (line 11961) * -h <11>: chcon invocation. (line 12819) * -H <8>: chcon invocation. (line 12840) * -h <12>: factor invocation. (line 13982) * -i: nl invocation. (line 1190) * -i <1>: od invocation. (line 1415) * -i <2>: base64 invocation. (line 1490) * -i <3>: pr invocation. (line 1791) * -i <4>: sort invocation. (line 3271) * -i <5>: shuf invocation. (line 3700) * -i <6>: uniq invocation. (line 3846) * -i <7>: Input processing in ptx. (line 4135) * -i <8>: General options in join. (line 4818) * -i <9>: expand invocation. (line 5482) * -I: Which files are listed. (line 5660) * -i <10>: What information is listed. (line 5805) * -i <11>: cp invocation. (line 6593) * -i <12>: mv invocation. (line 7543) * -i <13>: rm invocation. (line 7663) * -I <1>: rm invocation. (line 7669) * -i <14>: ln invocation. (line 8097) * -i <15>: df invocation. (line 9113) * -i <16>: tee invocation. (line 10616) * -i <17>: uname invocation. (line 12670) * -i <18>: env invocation. (line 13127) * -i <19>: stdbuf invocation. (line 13687) * -I[TIMESPEC]: Options for date. (line 12368) * -j: od invocation. (line 1289) * -J: pr invocation. (line 1798) * -k: Block size. (line 578) * -k <1>: csplit invocation. (line 2573) * -k <2>: sort invocation. (line 3359) * -k <3>: General output formatting. (line 6125) * -k <4>: df invocation. (line 9119) * -k <5>: du invocation. (line 9399) * -k <6>: Access permission tests. (line 10272) * -k <7>: timeout invocation. (line 13776) * -L: Traversing symlinks. (line 847) * -l: nl invocation. (line 1195) * -l <1>: od invocation. (line 1418) * -l <2>: pr invocation. (line 1807) * -l <3>: split invocation. (line 2301) * -l <4>: wc invocation. (line 2706) * -L <1>: wc invocation. (line 2712) * -l <5>: cksum general options. (line 2880) * -l <6>: b2sum invocation. (line 3068) * -L <2>: Which files are listed. (line 5674) * -l <7>: What information is listed. (line 5813) * -l <8>: cp invocation. (line 6599) * -L <3>: cp invocation. (line 6603) * -L <4>: ln invocation. (line 8102) * -L <5>: chown invocation. (line 8689) * -L <6>: chgrp invocation. (line 8802) * -l <9>: df invocation. (line 9125) * -L <7>: du invocation. (line 9405) * -l <10>: du invocation. (line 9411) * -L <8>: stat invocation. (line 9598) * -L <9>: File type tests. (line 10250) * -L <10>: realpath invocation. (line 11076) * -L <11>: pwd invocation. (line 11190) * -l <11>: who invocation. (line 11965) * -L <12>: chcon invocation. (line 12845) * -l <12>: chcon invocation. (line 12870) * -l <13>: runcon invocation. (line 12920) * -le: Numeric tests. (line 10361) * -lt: Numeric tests. (line 10361) * -m: pr invocation. (line 1814) * -m <1>: wc invocation. (line 2696) * -m <2>: sort invocation. (line 3175) * -M: sort invocation. (line 3278) * -M <1>: Output formatting in ptx. (line 4319) * -m <3>: General output formatting. (line 6138) * -m <4>: install invocation. (line 7404) * -m <5>: mkdir invocation. (line 8234) * -m <6>: mkfifo invocation. (line 8302) * -m <7>: mknod invocation. (line 8363) * -m <8>: readlink invocation. (line 8421) * -m <9>: touch invocation. (line 9002) * -m <10>: du invocation. (line 9415) * -m <11>: realpath invocation. (line 11071) * -m <12>: who invocation. (line 11975) * -m <13>: uname invocation. (line 12677) * -n: cat invocation. (line 1029) * -n <1>: nl invocation. (line 1203) * -N: od invocation. (line 1306) * -n <2>: pr invocation. (line 1827) * -N <1>: pr invocation. (line 1848) * -n <3>: head invocation. (line 2010) * -n <4>: tail invocation. (line 2167) * -n <5>: split invocation. (line 2347) * -n <6>: csplit invocation. (line 2568) * -n <7>: sort invocation. (line 3288) * -n <8>: shuf invocation. (line 3708) * -n <9>: cut invocation. (line 4640) * -n <10>: What information is listed. (line 5907) * -N <2>: Formatting the file names. (line 6312) * -n <11>: cp invocation. (line 6610) * -n <12>: mv invocation. (line 7550) * -n <13>: ln invocation. (line 8108) * -n <14>: readlink invocation. (line 8426) * -n <15>: echo invocation. (line 9937) * -N <3>: File characteristic tests. (line 10317) * -n <16>: String tests. (line 10336) * -n <17>: id invocation. (line 11798) * -n <18>: uname invocation. (line 12682) * -n <19>: nice invocation. (line 13551) * -n NUMBER: shred invocation. (line 7864) * -ne: Numeric tests. (line 10361) * -nt: File characteristic tests. (line 10305) * -o: od invocation. (line 1421) * -o <1>: pr invocation. (line 1854) * -o <2>: sort invocation. (line 3416) * -o <3>: shuf invocation. (line 3713) * -o <4>: Input processing in ptx. (line 4144) * -O: Output formatting in ptx. (line 4325) * -o <5>: What information is listed. (line 5911) * -o <6>: install invocation. (line 7416) * -o <7>: truncate invocation. (line 9873) * -O <1>: Access permission tests. (line 10288) * -o <8>: Connectives for test. (line 10404) * -o <9>: uname invocation. (line 12693) * -o <10>: stdbuf invocation. (line 13691) * -ot: File characteristic tests. (line 10309) * -P: Traversing symlinks. (line 851) * -p: nl invocation. (line 1214) * -p <1>: General output formatting. (line 6144) * -p <2>: dircolors invocation. (line 6436) * -P <1>: cp invocation. (line 6616) * -p <3>: cp invocation. (line 6623) * -p <4>: install invocation. (line 7428) * -P <2>: ln invocation. (line 8127) * -p <5>: mkdir invocation. (line 8251) * -p <6>: rmdir invocation. (line 8471) * -P <3>: chown invocation. (line 8702) * -P <4>: chgrp invocation. (line 8815) * -P <5>: df invocation. (line 9193) * -P <6>: du invocation. (line 9421) * -p <7>: File type tests. (line 10255) * -p <8>: tee invocation. (line 10620) * -p <9>: pathchk invocation. (line 10889) * -P <7>: pathchk invocation. (line 10902) * -p <10>: mktemp invocation. (line 11016) * -P <8>: realpath invocation. (line 11081) * -P <9>: pwd invocation. (line 11197) * -p <11>: who invocation. (line 11979) * -p <12>: uname invocation. (line 12686) * -P <10>: chcon invocation. (line 12849) * -q: head invocation. (line 2017) * -q <1>: tail invocation. (line 2196) * -q <2>: csplit invocation. (line 2593) * -q <3>: Formatting the file names. (line 6318) * -Q: Formatting the file names. (line 6325) * -q <4>: readlink invocation. (line 8433) * -q <5>: mktemp invocation. (line 11003) * -q <6>: realpath invocation. (line 11087) * -q <7>: who invocation. (line 11983) * -r: tac invocation. (line 1094) * -r <1>: pr invocation. (line 1861) * -r <2>: sum invocation. (line 2770) * -r <3>: sort invocation. (line 3311) * -R: sort invocation. (line 3317) * -r <4>: shuf invocation. (line 3724) * -r <5>: Input processing in ptx. (line 4157) * -R <1>: Output formatting in ptx. (line 4281) * -R <2>: Which files are listed. (line 5681) * -r <6>: Sorting the output. (line 5965) * -R <3>: cp invocation. (line 6712) * -r <7>: cp invocation. (line 6712) * -r <8>: rm invocation. (line 7716) * -R <4>: rm invocation. (line 7716) * -r <9>: ln invocation. (line 8136) * -R <5>: chown invocation. (line 8681) * -R <6>: chgrp invocation. (line 8793) * -R <7>: chmod invocation. (line 8899) * -r <10>: touch invocation. (line 9006) * -r <11>: truncate invocation. (line 9877) * -r <12>: Access permission tests. (line 10275) * -r <13>: id invocation. (line 11803) * -r <14>: who invocation. (line 11988) * -r <15>: Options for date. (line 12397) * -R <8>: Options for date. (line 12411) * -r <16>: uname invocation. (line 12697) * -R <9>: chcon invocation. (line 12828) * -r <17>: chcon invocation. (line 12862) * -r <18>: runcon invocation. (line 12912) * -S: Backup options. (line 445) * -s: cat invocation. (line 1034) * -s <1>: tac invocation. (line 1098) * -s <2>: nl invocation. (line 1218) * -S <1>: od invocation. (line 1311) * -s <3>: od invocation. (line 1424) * -s <4>: fmt invocation. (line 1646) * -s <5>: pr invocation. (line 1866) * -S <2>: pr invocation. (line 1875) * -s <6>: fold invocation. (line 1954) * -s <7>: tail invocation. (line 2216) * -s <8>: csplit invocation. (line 2593) * -s <9>: sum invocation. (line 2776) * -s <10>: sort invocation. (line 3437) * -S <3>: sort invocation. (line 3444) * -s <11>: uniq invocation. (line 3828) * -S <4>: Input processing in ptx. (line 4174) * -s <12>: cut invocation. (line 4644) * -s <13>: paste invocation. (line 4723) * -s <14>: tr invocation. (line 5163) * -s <15>: What information is listed. (line 5916) * -S <5>: Sorting the output. (line 5972) * -s <16>: cp invocation. (line 6800) * -S <6>: cp invocation. (line 6808) * -s <17>: install invocation. (line 7438) * -S <7>: install invocation. (line 7445) * -S <8>: mv invocation. (line 7602) * -s <18>: ln invocation. (line 8161) * -S <9>: ln invocation. (line 8167) * -s <19>: readlink invocation. (line 8433) * -S <10>: du invocation. (line 9426) * -s <20>: du invocation. (line 9441) * -s <21>: truncate invocation. (line 9881) * -S <11>: File type tests. (line 10258) * -s <22>: File characteristic tests. (line 10302) * -s <23>: basename invocation. (line 10793) * -s <24>: realpath invocation. (line 11105) * -s <25>: tty invocation. (line 11749) * -s <26>: who invocation. (line 11992) * -s <27>: Options for date. (line 12455) * -s <28>: uname invocation. (line 12701) * -S <12>: env invocation. (line 13217) * -s <29>: timeout invocation. (line 13793) * -s <30>: seq invocation. (line 14370) * -s BYTES: shred invocation. (line 7875) * -S, env and single quotes: env invocation. (line 13293) * -t: cat invocation. (line 1038) * -T: cat invocation. (line 1042) * -t <1>: od invocation. (line 1320) * -t <2>: fmt invocation. (line 1639) * -t <3>: pr invocation. (line 1884) * -T <1>: pr invocation. (line 1894) * -t <4>: split invocation. (line 2420) * -t <5>: cksum common options. (line 2990) * -t <6>: sort invocation. (line 3460) * -T <2>: sort invocation. (line 3480) * -T <3>: Output formatting in ptx. (line 4343) * -t <7>: tr invocation. (line 5168) * -t <8>: expand invocation. (line 5458) * -t <9>: unexpand invocation. (line 5509) * -t <10>: Sorting the output. (line 5976) * -T <4>: General output formatting. (line 6153) * -t <11>: cp invocation. (line 6813) * -T <5>: cp invocation. (line 6817) * -t <12>: install invocation. (line 7450) * -T <6>: install invocation. (line 7455) * -t <13>: mv invocation. (line 7607) * -T <7>: mv invocation. (line 7611) * -t <14>: ln invocation. (line 8172) * -T <8>: ln invocation. (line 8176) * -t <15>: df invocation. (line 9239) * -T <9>: df invocation. (line 9245) * -t <16>: du invocation. (line 9445) * -t <17>: stat invocation. (line 9646) * -t <18>: File type tests. (line 10261) * -t <19>: mktemp invocation. (line 11032) * -t <20>: who invocation. (line 11996) * -T <10>: who invocation. (line 12008) * -t <21>: chcon invocation. (line 12866) * -t <22>: runcon invocation. (line 12916) * -u: cat invocation. (line 1045) * -u <1>: fmt invocation. (line 1652) * -u <2>: split invocation. (line 2427) * -u <3>: sort invocation. (line 3496) * -u <4>: uniq invocation. (line 3914) * -u <5>: Sorting the output. (line 5984) * -U: Sorting the output. (line 6004) * -u <6>: cp invocation. (line 6822) * -u <7>: mv invocation. (line 7563) * -u <8>: shred invocation. (line 7881) * -u <9>: Access permission tests. (line 10278) * -u <10>: mktemp invocation. (line 11008) * -u <11>: id invocation. (line 11808) * -u <12>: who invocation. (line 11999) * -u <13>: Options for date. (line 12461) * -u <14>: chcon invocation. (line 12858) * -u <15>: runcon invocation. (line 12908) * -u <16>: env invocation. (line 13121) * -v: cat invocation. (line 1049) * -v <1>: nl invocation. (line 1223) * -v <2>: od invocation. (line 1380) * -v <3>: pr invocation. (line 1899) * -v <4>: head invocation. (line 2021) * -v <5>: tail invocation. (line 2227) * -V: sort invocation. (line 3305) * -v <6>: Sorting the output. (line 6011) * -v <7>: cp invocation. (line 6857) * -v <8>: install invocation. (line 7460) * -v <9>: mv invocation. (line 7594) * -v <10>: rm invocation. (line 7720) * -v <11>: shred invocation. (line 7896) * -v <12>: ln invocation. (line 8181) * -v <13>: mkdir invocation. (line 8269) * -v <14>: readlink invocation. (line 8437) * -v <15>: rmdir invocation. (line 8480) * -v <16>: chown invocation. (line 8673) * -v <17>: chgrp invocation. (line 8785) * -v <18>: chmod invocation. (line 8889) * -v <19>: uname invocation. (line 12712) * -v <20>: chcon invocation. (line 12854) * -v <21>: env invocation. (line 13202) * -v <22>: timeout invocation. (line 13799) * -w: nl invocation. (line 1228) * -w <1>: od invocation. (line 1387) * -w <2>: base64 invocation. (line 1476) * -w <3>: fmt invocation. (line 1658) * -w <4>: pr invocation. (line 1903) * -W: pr invocation. (line 1913) * -w <5>: fold invocation. (line 1960) * -w <6>: wc invocation. (line 2701) * -w <7>: cksum common options. (line 3000) * -w <8>: uniq invocation. (line 3920) * -W <1>: Input processing in ptx. (line 4214) * -w <9>: Output formatting in ptx. (line 4257) * -w <10>: General output formatting. (line 6173) * -w <11>: Access permission tests. (line 10281) * -w <12>: who invocation. (line 12008) * -w <13>: seq invocation. (line 14375) * -WIDTH: fmt invocation. (line 1658) * -x: od invocation. (line 1427) * -x <1>: split invocation. (line 2403) * -X: Sorting the output. (line 6023) * -x <2>: General output formatting. (line 6149) * -x <3>: cp invocation. (line 6861) * -x <4>: shred invocation. (line 7901) * -x <5>: df invocation. (line 9268) * -x <6>: du invocation. (line 9547) * -x <7>: Access permission tests. (line 10284) * -X FILE: du invocation. (line 9536) * -z: head invocation. (line 2025) * -z <1>: tail invocation. (line 2231) * -z <2>: csplit invocation. (line 2582) * -z <3>: cksum common options. (line 3010) * -z <4>: sort invocation. (line 3511) * -z <5>: shuf invocation. (line 3732) * -z <6>: uniq invocation. (line 3926) * -z <7>: comm invocation. (line 4023) * -z <8>: cut invocation. (line 4664) * -z <9>: paste invocation. (line 4743) * -z <10>: General options in join. (line 4876) * -Z: What information is listed. (line 5939) * -Z <1>: cp invocation. (line 6867) * -Z <2>: install invocation. (line 7464) * -Z <3>: mv invocation. (line 7616) * -z <11>: shred invocation. (line 7912) * -Z <4>: mkdir invocation. (line 8274) * -Z <5>: mkfifo invocation. (line 8309) * -Z <6>: mknod invocation. (line 8369) * -z <12>: readlink invocation. (line 8441) * -z <13>: String tests. (line 10332) * -z <14>: basename invocation. (line 10797) * -z <15>: dirname invocation. (line 10846) * -z <16>: realpath invocation. (line 11112) * -Z <7>: id invocation. (line 11812) * -z <17>: id invocation. (line 11819) * -z <18>: numfmt invocation. (line 14130) * /: Numeric expressions. (line 10523) * 128-bit checksum: md5sum invocation. (line 3018) * 16-bit checksum: sum invocation. (line 2753) * 160-bit checksum: sha1sum invocation. (line 3076) * 224-bit checksum: sha2 utilities. (line 3104) * 256-bit checksum: sha2 utilities. (line 3104) * 32-bit checksum: cksum invocation. (line 2788) * 384-bit checksum: sha2 utilities. (line 3104) * 512-bit checksum: b2sum invocation. (line 3050) * 512-bit checksum <1>: sha2 utilities. (line 3104) * <: Relations for expr. (line 10545) * <=: Relations for expr. (line 10545) * =: Relations for expr. (line 10545) * = <1>: String tests. (line 10339) * ==: Relations for expr. (line 10545) * == <1>: String tests. (line 10342) * >: Relations for expr. (line 10545) * >=: Relations for expr. (line 10545) * \( regexp operator: String expressions. (line 10472) * \+ regexp operator: String expressions. (line 10476) * \? regexp operator: String expressions. (line 10476) * \c: printf invocation. (line 10021) * \OOO: printf invocation. (line 10060) * \uhhhh: printf invocation. (line 10067) * \Uhhhhhhhh: printf invocation. (line 10067) * \xHH: printf invocation. (line 10060) * \| regexp operator: String expressions. (line 10476) * _POSIX2_VERSION: Standards conformance. (line 970) * _POSIX2_VERSION <1>: tail invocation. (line 2246) * _POSIX2_VERSION <2>: sort invocation. (line 3543) * _POSIX2_VERSION <3>: uniq invocation. (line 3833) * _POSIX2_VERSION <4>: touch invocation. (line 9024) * |: Relations for expr. (line 10534) * abbreviations for months: Calendar date items. (line 15105) * access control lists (ACLs): cp invocation. (line 6628) * access control lists (ACLs) <1>: cp invocation. (line 6670) * access permission tests: Access permission tests. (line 10266) * access permissions, changing: chmod invocation. (line 8832) * access time, changing: touch invocation. (line 8965) * access timestamp: dd invocation. (line 7200) * access timestamp, printing or sorting files by: Sorting the output. (line 5984) * access timestamp, show the most recent: du invocation. (line 9494) * across columns: pr invocation. (line 1736) * across, listing files: General output formatting. (line 6149) * adding permissions: Setting Permissions. (line 14571) * addition: Numeric expressions. (line 10519) * ago in date strings: Relative items in date strings. (line 15247) * all lines, grouping: uniq invocation. (line 3887) * all repeated lines, outputting: uniq invocation. (line 3856) * alnum: Character arrays. (line 5278) * alpha: Character arrays. (line 5280) * alternate ebcdic, converting to: dd invocation. (line 7025) * always classify option: General output formatting. (line 6083) * always color option: General output formatting. (line 6056) * always hyperlink option: General output formatting. (line 6101) * always interactive option: rm invocation. (line 7680) * always total option: wc invocation. (line 2722) * am i: who invocation. (line 11935) * am in date strings: Time of day items. (line 15141) * and operator: Connectives for test. (line 10400) * and operator <1>: Relations for expr. (line 10540) * append: dd invocation. (line 7129) * appending to the output file: dd invocation. (line 7129) * appropriate privileges: install invocation. (line 7416) * appropriate privileges <1>: Setting the time. (line 12298) * appropriate privileges <2>: hostname invocation. (line 12720) * appropriate privileges <3>: nice invocation. (line 13506) * arbitrary date strings, debugging: Options for date. (line 12355) * arbitrary date strings, parsing: Options for date. (line 12341) * arbitrary text, displaying: echo invocation. (line 9914) * arch: arch invocation. (line 12597) * arithmetic tests: Numeric tests. (line 10351) * arrays of characters in tr: Character arrays. (line 5176) * ASCII dump of files: od invocation. (line 1236) * ascii, converting to: dd invocation. (line 7014) * atime: File timestamps. (line 14917) * atime, changing: touch invocation. (line 8965) * atime, printing or sorting files by: Sorting the output. (line 5984) * atime, show the most recent: du invocation. (line 9494) * attribute caching: stat invocation. (line 9609) * attributes, file: Changing file attributes. (line 8524) * authors of parse_datetime: Authors of parse_datetime. (line 15403) * auto classify option: General output formatting. (line 6082) * auto color option: General output formatting. (line 6055) * auto hyperlink option: General output formatting. (line 6100) * auto total option: wc invocation. (line 2719) * b for block special file: mknod invocation. (line 8347) * b2sum: b2sum invocation. (line 3050) * background jobs, stopping at terminal write: Local. (line 11475) * backslash escapes: Character arrays. (line 5200) * backslash escapes <1>: echo invocation. (line 9940) * backslash escapes <2>: echo invocation. (line 9976) * backslash sequences for file names: Formatting the file names. (line 6306) * backup files, ignoring: Which files are listed. (line 5613) * backup options: Backup options. (line 402) * backup suffix: Backup options. (line 445) * backups, making: Backup options. (line 409) * backups, making <1>: cp invocation. (line 6529) * backups, making <2>: install invocation. (line 7362) * backups, making <3>: mv invocation. (line 7528) * backups, making <4>: ln invocation. (line 8081) * backups, making only: cp invocation. (line 6501) * base32: base32 invocation. (line 1447) * base32 encoding: base32 invocation. (line 1447) * base32 encoding <1>: basenc invocation. (line 1499) * base64: base64 invocation. (line 1457) * base64 checksum encoding: cksum general options. (line 2864) * Base64 decoding: base64 invocation. (line 1484) * base64 encoding: base64 invocation. (line 1457) * basename: basename invocation. (line 10761) * basenc: basenc invocation. (line 1499) * baud rate, setting: Special. (line 11699) * beeping at input buffer full: Input. (line 11372) * beginning of time: Time conversion specifiers. (line 12091) * beginning of time, for POSIX: Seconds since the Epoch. (line 15318) * Bellovin, Steven M.: Authors of parse_datetime. (line 15403) * Berets, Jim: Authors of parse_datetime. (line 15403) * Berry, K.: Introduction. (line 336) * Berry, K. <1>: Authors of parse_datetime. (line 15416) * binary: dd invocation. (line 7217) * binary I/O: dd invocation. (line 7217) * binary input files: cksum common options. (line 2906) * bind mount: rm invocation. (line 7688) * bind mount <1>: stat invocation. (line 9775) * birth time, printing or sorting files by: Sorting the output. (line 5997) * birthtime: File timestamps. (line 14917) * BLAKE2: b2sum invocation. (line 3050) * BLAKE2 hash length: cksum general options. (line 2880) * BLAKE2 hash length <1>: b2sum invocation. (line 3068) * blank: Character arrays. (line 5282) * blank lines, numbering: nl invocation. (line 1195) * blanks, ignoring leading: sort invocation. (line 3201) * block (space-padding): dd invocation. (line 7035) * block size: Block size. (line 453) * block size <1>: dd invocation. (line 6936) * block size of conversion: dd invocation. (line 6943) * block size of input: dd invocation. (line 6928) * block size of output: dd invocation. (line 6932) * block special check: File type tests. (line 10237) * block special files: mknod invocation. (line 8327) * block special files, creating: mknod invocation. (line 8322) * BLOCKSIZE: Block size. (line 459) * BLOCK_SIZE: Block size. (line 459) * body, numbering: nl invocation. (line 1127) * Bourne shell syntax for color setup: dircolors invocation. (line 6425) * breaks, cause interrupts: Input. (line 11325) * breaks, ignoring: Input. (line 11322) * brkint: Input. (line 11325) * bs: dd invocation. (line 6936) * BSD output: cksum common options. (line 2978) * BSD sum: sum invocation. (line 2770) * BSD tail: tail invocation. (line 2067) * BSD touch compatibility: touch invocation. (line 8983) * bsN: Output. (line 11427) * btrfs file system type: df invocation. (line 9256) * bugs, reporting: Introduction. (line 319) * built-in shell commands, conflicts with: mknod invocation. (line 8336) * built-in shell commands, conflicts with <1>: stat invocation. (line 9591) * built-in shell commands, conflicts with <2>: echo invocation. (line 9919) * built-in shell commands, conflicts with <3>: printf invocation. (line 10009) * built-in shell commands, conflicts with <4>: test invocation. (line 10211) * built-in shell commands, conflicts with <5>: pwd invocation. (line 11205) * built-in shell commands, conflicts with <6>: nice invocation. (line 13538) * built-in shell commands, conflicts with <7>: kill invocation. (line 13860) * built-in shell commands, conflicts with <8>: sleep invocation. (line 13954) * byte count: wc invocation. (line 2652) * byte-swapping: od invocation. (line 1281) * byte-swapping <1>: dd invocation. (line 7071) * c for character special file: mknod invocation. (line 8350) * C shell syntax for color setup: dircolors invocation. (line 6431) * C-s/C-q flow control: Input. (line 11353) * calendar date item: Calendar date items. (line 15075) * calling combined multi-call program: Multi-call invocation. (line 985) * canonical file name: readlink invocation. (line 8382) * canonical file name <1>: realpath invocation. (line 11047) * canonicalize a file name: readlink invocation. (line 8382) * canonicalize a file name <1>: realpath invocation. (line 11047) * case folding: sort invocation. (line 3216) * case translation: Local. (line 11470) * case, ignored in dates: General date syntax. (line 15062) * cat: cat invocation. (line 1003) * cbreak: Combination. (line 11553) * cbs: dd invocation. (line 6943) * CD-ROM file system type: df invocation. (line 9260) * cdfs file system type: df invocation. (line 9260) * cdtrdsr: Control. (line 11314) * change or print terminal settings: stty invocation. (line 11216) * change SELinux context: chcon invocation. (line 12799) * changed files, verbosely describing: chgrp invocation. (line 8740) * changed owners, verbosely describing: chown invocation. (line 8604) * changing access permissions: chmod invocation. (line 8832) * changing file attributes: Changing file attributes. (line 8524) * changing file ownership: chown invocation. (line 8536) * changing file timestamps: touch invocation. (line 8923) * changing group ownership: chown invocation. (line 8536) * changing group ownership <1>: chgrp invocation. (line 8722) * changing security context: chcon invocation. (line 12799) * changing special mode bits: Changing Special Mode Bits. (line 14638) * character classes: Character arrays. (line 5264) * character count: wc invocation. (line 2652) * character size: Control. (line 11294) * character special check: File type tests. (line 10240) * character special files: mknod invocation. (line 8327) * character special files, creating: mknod invocation. (line 8322) * characters, special: Characters. (line 11584) * chcon: chcon invocation. (line 12799) * check file types: test invocation. (line 10189) * checking whether a file is sorted: sort invocation. (line 3161) * checking whether a file is sorted <1>: sort invocation. (line 3169) * checksum, 128-bit: md5sum invocation. (line 3018) * checksum, 16-bit: sum invocation. (line 2753) * checksum, 160-bit: sha1sum invocation. (line 3076) * checksum, 224-bit: sha2 utilities. (line 3104) * checksum, 256-bit: sha2 utilities. (line 3104) * checksum, 32-bit: cksum invocation. (line 2788) * checksum, 384-bit: sha2 utilities. (line 3104) * checksum, 512-bit: b2sum invocation. (line 3050) * checksum, 512-bit <1>: sha2 utilities. (line 3104) * chgrp: chgrp invocation. (line 8722) * chmod: chmod invocation. (line 8832) * chown: chown invocation. (line 8536) * chroot: chroot invocation. (line 12939) * cio: dd invocation. (line 7137) * cksum: cksum invocation. (line 2788) * clocal: Control. (line 11308) * clock skew: Formatting file timestamps. (line 6196) * clock skew <1>: File timestamps. (line 14950) * clone: cp invocation. (line 6725) * cmspar: Control. (line 11286) * cntrl: Character arrays. (line 5284) * color database, printing: dircolors invocation. (line 6436) * color setup: dircolors invocation. (line 6397) * color, distinguishing file types with: General output formatting. (line 6052) * cols: Special. (line 11672) * column to wrap data after: base64 invocation. (line 1476) * COLUMNS: General output formatting. (line 6173) * COLUMNS <1>: Special. (line 11686) * columns: Special. (line 11672) * combination settings: Combination. (line 11507) * combined: Multi-call invocation. (line 985) * combined date and time of day item: Combined date and time of day items. (line 15192) * comm: comm invocation. (line 3940) * command-line operands to shuffle: shuf invocation. (line 3696) * commands for controlling processes: Process control. (line 13850) * commands for delaying: Delaying. (line 13917) * commands for exit status: Conditions. (line 10134) * commands for file name manipulation: File name manipulation. (line 10756) * commands for invoking other commands: Modified command invocation. (line 12932) * commands for printing text: Printing text. (line 9909) * commands for printing the working context: Working context. (line 11174) * commands for printing user information: User information. (line 11761) * commands for redirection: Redirection. (line 10583) * commands for SELinux context: SELinux context. (line 12794) * commands for system context: System context. (line 12025) * commas, outputting between files: General output formatting. (line 6138) * comments, in dates: General date syntax. (line 15062) * common field, joining on: join invocation. (line 4756) * common lines: comm invocation. (line 3952) * common options: Common options. (line 351) * compare values: test invocation. (line 10189) * comparing sorted files: comm invocation. (line 3940) * comparison operators: Relations for expr. (line 10545) * compression and du: du invocation. (line 9563) * concatenate and write files: cat invocation. (line 1003) * concurrent I/O: dd invocation. (line 7137) * conditional executability: Conditional Executability. (line 14689) * conditions: Conditions. (line 10134) * conflicts with shell built-ins: mknod invocation. (line 8336) * conflicts with shell built-ins <1>: stat invocation. (line 9591) * conflicts with shell built-ins <2>: echo invocation. (line 9919) * conflicts with shell built-ins <3>: printf invocation. (line 10009) * conflicts with shell built-ins <4>: test invocation. (line 10211) * conflicts with shell built-ins <5>: pwd invocation. (line 11205) * conflicts with shell built-ins <6>: nice invocation. (line 13538) * conflicts with shell built-ins <7>: kill invocation. (line 13860) * conflicts with shell built-ins <8>: sleep invocation. (line 13954) * connectives, logical: Connectives for test. (line 10377) * connectives, logical <1>: Relations for expr. (line 10529) * constant parity: Control. (line 11286) * context splitting: csplit invocation. (line 2492) * context, system: System context. (line 12025) * control characters, using ^C: Local. (line 11485) * control settings: Control. (line 11276) * controlling terminal: dd invocation. (line 7206) * conv: dd invocation. (line 7008) * conversion block size: dd invocation. (line 6943) * conversion specifiers, date: Date conversion specifiers. (line 12127) * conversion specifiers, literal: Literal conversion specifiers. (line 12207) * conversion specifiers, time: Time conversion specifiers. (line 12065) * converting tabs to spaces: expand invocation. (line 5442) * converting while copying a file: dd invocation. (line 6882) * cooked: Combination. (line 11538) * Coordinated Universal Time: Options for date. (line 12461) * copy on write: cp invocation. (line 6725) * copy-on-write and du: du invocation. (line 9558) * copying directories recursively: cp invocation. (line 6546) * copying directories recursively <1>: cp invocation. (line 6712) * copying existing permissions: Copying Permissions. (line 14622) * copying files: cat invocation. (line 1003) * copying files and directories: cp invocation. (line 6456) * copying files and setting attributes: install invocation. (line 7327) * core utilities: Top. (line 310) * count: dd invocation. (line 6962) * COW: cp invocation. (line 6725) * cp: cp invocation. (line 6456) * crashes and corruption: sync invocation. (line 9817) * CRC checksum: cksum invocation. (line 2788) * cread: Control. (line 11305) * creating directories: mkdir invocation. (line 8221) * creating FIFOs (named pipes): mkfifo invocation. (line 8287) * creating links (hard only): link invocation. (line 7978) * creating links (hard or soft): ln invocation. (line 8005) * creating output file, avoiding: dd invocation. (line 7086) * creating output file, requiring: dd invocation. (line 7082) * creation timestamp, printing or sorting files by: Sorting the output. (line 5997) * crN: Output. (line 11417) * crown margin: fmt invocation. (line 1633) * crt: Combination. (line 11576) * crterase: Local. (line 11456) * crtkill: Local. (line 11490) * crtscts: Control. (line 11311) * csh syntax for color setup: dircolors invocation. (line 6431) * csN: Control. (line 11294) * csplit: csplit invocation. (line 2492) * cstopb: Control. (line 11302) * ctime: File timestamps. (line 14917) * ctime, printing or sorting by: Sorting the output. (line 5953) * ctime, show the most recent: du invocation. (line 9488) * ctlecho: Local. (line 11485) * current working directory, printing: pwd invocation. (line 11181) * cut: cut invocation. (line 4576) * cyclic redundancy check: cksum invocation. (line 2788) * data modification time, printing or sorting files by: Sorting the output. (line 5990) * data, erasing: shred invocation. (line 7743) * database for color setup, printing: dircolors invocation. (line 6436) * date: date invocation. (line 12031) * date and time of day format, ISO 8601: Combined date and time of day items. (line 15192) * date conversion specifiers: Date conversion specifiers. (line 12127) * date format, ISO 8601: Calendar date items. (line 15097) * date input formats: Date input formats. (line 14974) * date options: Options for date. (line 12332) * date strings, debugging: Options for date. (line 12355) * date strings, parsing: Options for date. (line 12341) * day in date strings: Relative items in date strings. (line 15239) * day in date strings <1>: Relative items in date strings. (line 15253) * day of week item: Day of week items. (line 15211) * dd: dd invocation. (line 6882) * ddrescue: dd invocation. (line 7273) * debugging date strings: Options for date. (line 12355) * debugging, copying: cp invocation. (line 6564) * debugging, copying <1>: install invocation. (line 7393) * debugging, copying <2>: mv invocation. (line 7532) * debugging, env -S: env invocation. (line 13302) * dec: Combination. (line 11579) * decctlq: Combination. (line 11564) * Decode base64 data: base64 invocation. (line 1484) * delay for a specified time: sleep invocation. (line 13920) * delaying commands: Delaying. (line 13917) * deleting characters: Squeezing and deleting. (line 5371) * dereferencing symbolic links: ln invocation. (line 8041) * descriptor follow option: tail invocation. (line 2098) * destination directory: Target directory. (line 749) * destination directory <1>: Target directory. (line 765) * destination directory <2>: cp invocation. (line 6813) * destination directory <3>: cp invocation. (line 6817) * destination directory <4>: install invocation. (line 7450) * destination directory <5>: install invocation. (line 7455) * destination directory <6>: mv invocation. (line 7607) * destination directory <7>: mv invocation. (line 7611) * destination directory <8>: ln invocation. (line 8172) * destination directory <9>: ln invocation. (line 8176) * destinations, multiple output: tee invocation. (line 10591) * device file: df invocation. (line 9074) * df: df invocation. (line 9050) * DF_BLOCK_SIZE: Block size. (line 459) * diagnostic: chcon invocation. (line 12854) * dictionary order: sort invocation. (line 3209) * differing lines: comm invocation. (line 3952) * digest: cksum invocation. (line 2788) * digest algorithm: cksum general options. (line 2846) * digit: Character arrays. (line 5286) * dir: dir invocation. (line 6379) * dircolors: dircolors invocation. (line 6397) * direct: dd invocation. (line 7143) * direct I/O: dd invocation. (line 7143) * directories, copying: cp invocation. (line 6456) * directories, copying recursively: cp invocation. (line 6546) * directories, copying recursively <1>: cp invocation. (line 6712) * directories, creating: mkdir invocation. (line 8221) * directories, creating with given attributes: install invocation. (line 7388) * directories, removing: rm invocation. (line 7655) * directories, removing (recursively): rm invocation. (line 7716) * directories, removing empty: rmdir invocation. (line 8456) * directory: dd invocation. (line 7151) * directory check: File type tests. (line 10243) * directory components, printing: dirname invocation. (line 10821) * directory deletion, ignoring failures: rmdir invocation. (line 8467) * directory deletion, reporting: rmdir invocation. (line 8480) * directory I/O: dd invocation. (line 7151) * directory listing: ls invocation. (line 5551) * directory listing, brief: dir invocation. (line 6379) * directory listing, recursive: Which files are listed. (line 5681) * directory listing, verbose: vdir invocation. (line 6388) * directory order, listing by: Sorting the output. (line 5958) * directory, creating temporary: mktemp invocation. (line 10917) * directory, stripping from file names: basename invocation. (line 10761) * dired Emacs mode support: What information is listed. (line 5696) * dirname: dirname invocation. (line 10821) * disabling special characters: Characters. (line 11590) * disambiguating group names and IDs: Disambiguating names and IDs. (line 669) * discard: Characters. (line 11617) * discarding file cache: dd invocation. (line 7168) * disk device file: df invocation. (line 9074) * disk usage: File space usage. (line 9043) * disk usage by file system: df invocation. (line 9050) * disk usage for files: du invocation. (line 9292) * displacement of dates: Relative items in date strings. (line 15230) * displaying text: echo invocation. (line 9914) * displaying value of a symbolic link: readlink invocation. (line 8382) * division: Numeric expressions. (line 10523) * do nothing, successfully: true invocation. (line 10163) * do nothing, unsuccessfully: false invocation. (line 10142) * DOS file system: df invocation. (line 9264) * double spacing: pr invocation. (line 1748) * down columns: pr invocation. (line 1723) * drain: Special. (line 11675) * dsusp: Characters. (line 11636) * dsync: dd invocation. (line 7157) * DTR/DSR flow control: Control. (line 11314) * du: du invocation. (line 9292) * DU_BLOCK_SIZE: Block size. (line 459) * DVD file system type: df invocation. (line 9260) * ebcdic, converting to: dd invocation. (line 7020) * echo: echo invocation. (line 9914) * echo <1>: Local. (line 11452) * echoctl: Local. (line 11485) * echoe: Local. (line 11456) * echok: Local. (line 11460) * echoke: Local. (line 11490) * echonl: Local. (line 11463) * echoprt: Local. (line 11480) * effective user and group IDs, printing: id invocation. (line 11767) * effective user name, printing: whoami invocation. (line 11856) * Eggert, Paul: Authors of parse_datetime. (line 15403) * eight-bit characters: Control. (line 11294) * eight-bit characters <1>: Combination. (line 11556) * eight-bit input: Input. (line 11338) * ek: Combination. (line 11523) * empty files, creating: touch invocation. (line 8928) * empty lines, numbering: nl invocation. (line 1195) * endianness: od invocation. (line 1281) * entire files, output of: Output of entire files. (line 997) * env: env invocation. (line 13031) * env -S, and single quotes: env invocation. (line 13293) * env -S, debugging: env invocation. (line 13302) * env in scripts: env invocation. (line 13217) * environment variables, printing: printenv invocation. (line 11711) * environment, printing: env invocation. (line 13075) * environment, running a program in a modified: env invocation. (line 13031) * eof: Characters. (line 11608) * eol: Characters. (line 11611) * eol2: Characters. (line 11614) * Epoch, for POSIX: Seconds since the Epoch. (line 15318) * Epoch, seconds since: Time conversion specifiers. (line 12091) * equal string check: String tests. (line 10339) * equal string check <1>: String tests. (line 10342) * equivalence classes: Character arrays. (line 5303) * erase: Characters. (line 11602) * erasing data: shred invocation. (line 7743) * error messages, omitting: chown invocation. (line 8610) * error messages, omitting <1>: chgrp invocation. (line 8746) * error messages, omitting <2>: chmod invocation. (line 8875) * evaluation of expressions: expr invocation. (line 10409) * even parity: Control. (line 11283) * evenp: Combination. (line 11510) * exabyte, definition of: Block size. (line 553) * examples of date: Examples of date. (line 12471) * examples of expr: Examples of expr. (line 10555) * exbibyte, definition of: Block size. (line 556) * excl: dd invocation. (line 7082) * excluding files from du: du invocation. (line 9536) * excluding files from du <1>: du invocation. (line 9541) * executable file check: Access permission tests. (line 10284) * executables and file type, marking: General output formatting. (line 6076) * execute/search permission: Mode Structure. (line 14452) * execute/search permission, symbolic: Setting Permissions. (line 14592) * existence-of-file check: File characteristic tests. (line 10299) * existing backup method: Backup options. (line 435) * exit status commands: Conditions. (line 10134) * exit status of chroot: chroot invocation. (line 13011) * exit status of env: env invocation. (line 13222) * exit status of expr: expr invocation. (line 10445) * exit status of false: false invocation. (line 10142) * exit status of ls: ls invocation. (line 5574) * exit status of mktemp: mktemp invocation. (line 11039) * exit status of nice: nice invocation. (line 13563) * exit status of nohup: nohup invocation. (line 13654) * exit status of pathchk: pathchk invocation. (line 10909) * exit status of printenv: printenv invocation. (line 11728) * exit status of realpath: realpath invocation. (line 11116) * exit status of runcon: runcon invocation. (line 12922) * exit status of sort: sort invocation. (line 3180) * exit status of stdbuf: stdbuf invocation. (line 13731) * exit status of test: test invocation. (line 10224) * exit status of timeout: timeout invocation. (line 13811) * exit status of true: true invocation. (line 10163) * exit status of tty: tty invocation. (line 11751) * expand: expand invocation. (line 5442) * expr: expr invocation. (line 10409) * expression evaluation: test invocation. (line 10189) * expression evaluation <1>: expr invocation. (line 10409) * expressions, numeric: Numeric expressions. (line 10513) * expressions, string: String expressions. (line 10455) * ext2 file system type: df invocation. (line 9256) * ext3 file system type: df invocation. (line 9256) * ext4 file system type: df invocation. (line 9256) * extended attributes, xattr: install invocation. (line 7355) * extended attributes, xattr <1>: mv invocation. (line 7502) * extension, sorting files by: Sorting the output. (line 6023) * extproc: Local. (line 11495) * factor: factor invocation. (line 13970) * failure exit status: false invocation. (line 10142) * false: false invocation. (line 10142) * fat file system file: df invocation. (line 9264) * fdatasync: dd invocation. (line 7099) * ffN: Output. (line 11435) * field separator character: sort invocation. (line 3460) * fields, padding numeric: Padding and other flags. (line 12219) * FIFOs, creating: mkfifo invocation. (line 8287) * file attributes, changing: Changing file attributes. (line 8524) * file characteristic tests: File characteristic tests. (line 10296) * file contents, dumping unambiguously: od invocation. (line 1236) * file information, preserving: cp invocation. (line 6695) * file information, preserving, extended attributes, xattr: cp invocation. (line 6623) * file mode bits, numeric: Numeric Modes. (line 14778) * file name manipulation: File name manipulation. (line 10756) * file names, canonicalization: realpath invocation. (line 11047) * file names, checking validity and portability: pathchk invocation. (line 10868) * file names, creating temporary: mktemp invocation. (line 10917) * file names, stripping directory and suffix: basename invocation. (line 10761) * file offset radix: od invocation. (line 1266) * file ownership, changing: chown invocation. (line 8536) * file sizes: du invocation. (line 9340) * File space usage: File space usage. (line 9043) * file space usage: du invocation. (line 9292) * file status: stat invocation. (line 9582) * file system allocation: What information is listed. (line 5916) * file system sizes: df invocation. (line 9098) * file system space, retrieving current data more slowly: df invocation. (line 9219) * file system space, retrieving old data more quickly: df invocation. (line 9129) * file system status: stat invocation. (line 9582) * file system types, limiting output to certain: df invocation. (line 9125) * file system types, limiting output to certain <1>: df invocation. (line 9239) * file system types, printing: df invocation. (line 9245) * file system usage: df invocation. (line 9050) * file systems: stat invocation. (line 9604) * file systems and hard links: ln invocation. (line 8005) * file systems, omitting copying to different: cp invocation. (line 6861) * file timestamp resolution: File timestamps. (line 14956) * file timestamps, changing: touch invocation. (line 8923) * file type and executables, marking: General output formatting. (line 6076) * file type tests: File type tests. (line 10233) * file type, marking: General output formatting. (line 6092) * file type, marking <1>: General output formatting. (line 6144) * file types: Special file types. (line 7963) * file types, special: Special file types. (line 7960) * file utilities: Top. (line 310) * files beginning with -, removing: rm invocation. (line 7722) * files, copying: cp invocation. (line 6456) * files, creating: truncate invocation. (line 9858) * fingerprint, 128-bit: md5sum invocation. (line 3018) * fingerprint, 160-bit: sha1sum invocation. (line 3076) * fingerprint, 224-bit: sha2 utilities. (line 3104) * fingerprint, 256-bit: sha2 utilities. (line 3104) * fingerprint, 384-bit: sha2 utilities. (line 3104) * fingerprint, 512-bit: b2sum invocation. (line 3050) * fingerprint, 512-bit <1>: sha2 utilities. (line 3104) * first in date strings: General date syntax. (line 15024) * first part of files, outputting: head invocation. (line 1976) * fixed-length records, converting to variable-length: dd invocation. (line 6943) * floating point: Floating point. (line 924) * flow control, hardware: Control. (line 11311) * flow control, hardware <1>: Control. (line 11314) * flow control, software: Input. (line 11358) * flush: Characters. (line 11617) * flushing, disabling: Local. (line 11466) * flusho: Local. (line 11501) * fmt: fmt invocation. (line 1605) * fold: fold invocation. (line 1931) * folding long input lines: fold invocation. (line 1931) * footers, numbering: nl invocation. (line 1127) * force deletion: shred invocation. (line 7860) * formatting file contents: Formatting file contents. (line 1600) * formatting of numbers in seq: seq invocation. (line 14354) * formatting times: pr invocation. (line 1752) * formatting times <1>: Date format specifiers. (line 12055) * fortnight in date strings: Relative items in date strings. (line 15239) * fsync: dd invocation. (line 7110) * fullblock: dd invocation. (line 7225) * general date syntax: General date syntax. (line 15008) * general numeric sort: sort invocation. (line 3227) * gibibyte, definition of: Block size. (line 541) * gigabyte, definition of: Block size. (line 538) * giving away permissions: Umask and Protection. (line 14756) * GMT: Options for date. (line 12461) * grand total of file system size, usage and available space: df invocation. (line 9225) * grand total of file system space: du invocation. (line 9349) * graph: Character arrays. (line 5288) * Greenwich Mean Time: Options for date. (line 12461) * group IDs, disambiguating: Disambiguating names and IDs. (line 669) * group names, disambiguating: Disambiguating names and IDs. (line 669) * group owner, default: Mode Structure. (line 14463) * group ownership of installed files, setting: install invocation. (line 7398) * group ownership, changing: chown invocation. (line 8536) * group ownership, changing <1>: chgrp invocation. (line 8722) * group, permissions for: Setting Permissions. (line 14561) * groups: groups invocation. (line 11868) * growing files: tail invocation. (line 2098) * hangups, immunity to: nohup invocation. (line 13612) * hard link check: File characteristic tests. (line 10313) * hard link, defined: ln invocation. (line 8031) * hard links: dd invocation. (line 7214) * hard links to directories: ln invocation. (line 8087) * hard links to symbolic links: ln invocation. (line 8183) * hard links, counting in du: du invocation. (line 9411) * hard links, creating: link invocation. (line 7978) * hard links, creating <1>: ln invocation. (line 8005) * hard links, preserving: cp invocation. (line 6559) * hardware class: uname invocation. (line 12677) * hardware flow control: Control. (line 11311) * hardware flow control <1>: Control. (line 11314) * hardware platform: uname invocation. (line 12670) * hardware type: uname invocation. (line 12677) * hat notation for control characters: Local. (line 11485) * head: head invocation. (line 1976) * head of output: shuf invocation. (line 3708) * headers, numbering: nl invocation. (line 1127) * help, online: Common options. (line 381) * hex dump of files: od invocation. (line 1236) * holes, copying files with: cp invocation. (line 6756) * holes, creating files with: truncate invocation. (line 9860) * horizontal, listing files: General output formatting. (line 6149) * host processor type: uname invocation. (line 12686) * hostid: hostid invocation. (line 12740) * hostname: hostname invocation. (line 12720) * hostname <1>: uname invocation. (line 12682) * hour in date strings: Relative items in date strings. (line 15239) * human numeric sort: sort invocation. (line 3254) * human-readable output: Block size. (line 489) * human-readable output <1>: What information is listed. (line 5798) * human-readable output <2>: df invocation. (line 9103) * human-readable output <3>: du invocation. (line 9384) * hup[cl]: Control. (line 11298) * hurd, author, printing: What information is listed. (line 5690) * hyperlink, linking to files: General output formatting. (line 6097) * ibs: dd invocation. (line 6928) * icanon: Local. (line 11445) * icrnl: Input. (line 11347) * id: id invocation. (line 11767) * idle time: who invocation. (line 11999) * IEEE floating point: Floating point. (line 924) * iexten: Local. (line 11449) * if: dd invocation. (line 6921) * iflag: dd invocation. (line 7119) * ignbrk: Input. (line 11322) * igncr: Input. (line 11344) * ignore file systems: df invocation. (line 9086) * Ignore garbage in base64 stream: base64 invocation. (line 1490) * ignoring case: sort invocation. (line 3216) * ignpar: Input. (line 11328) * imaxbel: Input. (line 11372) * immunity to hangups: nohup invocation. (line 13612) * implementation, hardware: uname invocation. (line 12670) * indenting lines: pr invocation. (line 1854) * index: String expressions. (line 10492) * information, about current users: who invocation. (line 11920) * initial part of files, outputting: head invocation. (line 1976) * initial tabs, converting: expand invocation. (line 5482) * inlcr: Input. (line 11341) * inode number, printing: What information is listed. (line 5805) * inode usage: df invocation. (line 9113) * inode usage, dereferencing in du: du invocation. (line 9390) * inode, and hard links: ln invocation. (line 8031) * inodes, written buffered: sync invocation. (line 9811) * inpck: Input. (line 11335) * input block size: dd invocation. (line 6928) * input encoding, UTF-8: Input. (line 11350) * input range to shuffle: shuf invocation. (line 3700) * input settings: Input. (line 11319) * input tabs: pr invocation. (line 1772) * install: install invocation. (line 7327) * intr: Characters. (line 11596) * invocation of commands, modified: Modified command invocation. (line 12932) * iseek: dd invocation. (line 6949) * isig: Local. (line 11441) * ISO 8601 date and time of day format: Combined date and time of day items. (line 15192) * ISO 8601 date format: Calendar date items. (line 15097) * ISO/IEC 10646: printf invocation. (line 10067) * ISO9660 file system type: df invocation. (line 9260) * iso9660 file system type: df invocation. (line 9260) * ispeed: Special. (line 11661) * istrip: Input. (line 11338) * items in date strings: General date syntax. (line 15008) * iterations, selecting the number of: shred invocation. (line 7864) * iuclc: Input. (line 11363) * iutf8: Input. (line 11350) * ixany: Input. (line 11368) * ixoff: Input. (line 11358) * ixon: Input. (line 11353) * join: join invocation. (line 4756) * kernel name: uname invocation. (line 12701) * kernel release: uname invocation. (line 12697) * kernel version: uname invocation. (line 12712) * kibibyte, definition of: Block size. (line 529) * kibibytes for file sizes: du invocation. (line 9399) * kibibytes for file system sizes: df invocation. (line 9119) * kill: kill invocation. (line 13853) * kill <1>: Characters. (line 11605) * kilobyte, definition of: Block size. (line 525) * Knuth, Donald E.: fmt invocation. (line 1618) * language, in dates: General date syntax. (line 15038) * language, in dates <1>: General date syntax. (line 15042) * last DAY: Day of week items. (line 15220) * last DAY <1>: Options for date. (line 12341) * last in date strings: General date syntax. (line 15024) * last modified dates, displaying in du: du invocation. (line 9481) * last part of files, outputting: tail invocation. (line 2047) * lcase: Combination. (line 11572) * LCASE: Combination. (line 11572) * lcase, converting to: dd invocation. (line 7048) * lchown: chown invocation. (line 8637) * lchown <1>: chown invocation. (line 8649) * lchown <2>: chgrp invocation. (line 8750) * lchown <3>: chgrp invocation. (line 8762) * LC_ALL: sort invocation. (line 3145) * LC_ALL <1>: ls invocation. (line 5562) * LC_COLLATE: sort invocation. (line 3145) * LC_COLLATE <1>: uniq invocation. (line 3807) * LC_COLLATE <2>: comm invocation. (line 3946) * LC_COLLATE <3>: Sorting files for join. (line 4914) * LC_COLLATE <4>: Relations for expr. (line 10545) * LC_CTYPE: sort invocation. (line 3201) * LC_CTYPE <1>: sort invocation. (line 3209) * LC_CTYPE <2>: sort invocation. (line 3216) * LC_CTYPE <3>: sort invocation. (line 3271) * LC_CTYPE <4>: printf invocation. (line 10067) * LC_MESSAGES: pr invocation. (line 1687) * LC_NUMERIC: Block size. (line 504) * LC_NUMERIC <1>: Floating point. (line 947) * LC_NUMERIC <2>: sort invocation. (line 3227) * LC_NUMERIC <3>: sort invocation. (line 3254) * LC_NUMERIC <4>: sort invocation. (line 3288) * LC_NUMERIC <5>: printf invocation. (line 10054) * LC_TIME: pr invocation. (line 1759) * LC_TIME <1>: sort invocation. (line 3278) * LC_TIME <2>: Formatting file timestamps. (line 6213) * LC_TIME <3>: Formatting file timestamps. (line 6258) * LC_TIME <4>: Formatting file timestamps. (line 6282) * LC_TIME <5>: du invocation. (line 9503) * LC_TIME <6>: date invocation. (line 12040) * leading directories, creating missing: install invocation. (line 7388) * leading directory components, stripping: basename invocation. (line 10761) * leap seconds: touch invocation. (line 9016) * leap seconds <1>: Time conversion specifiers. (line 12091) * leap seconds <2>: Time conversion specifiers. (line 12095) * leap seconds <3>: Options for date. (line 12461) * leap seconds <4>: Examples of date. (line 12569) * leap seconds <5>: General date syntax. (line 15067) * leap seconds <6>: Time of day items. (line 15134) * leap seconds <7>: Seconds since the Epoch. (line 15332) * left margin: pr invocation. (line 1854) * length: String expressions. (line 10497) * limiting output of du: du invocation. (line 9362) * line: Special. (line 11693) * line buffered: stdbuf invocation. (line 13667) * line count: wc invocation. (line 2652) * line numbering: nl invocation. (line 1116) * line separator character: split invocation. (line 2420) * line settings of terminal: stty invocation. (line 11216) * line-breaking: fmt invocation. (line 1618) * line-by-line comparison: comm invocation. (line 3940) * LINES: Special. (line 11686) * link: link invocation. (line 7978) * links, creating: link invocation. (line 7978) * links, creating <1>: ln invocation. (line 8005) * Linux file system types: df invocation. (line 9256) * literal conversion specifiers: Literal conversion specifiers. (line 12207) * litout: Combination. (line 11560) * ln: ln invocation. (line 8005) * ln format for nl: nl invocation. (line 1206) * lnext: Characters. (line 11645) * local file system types: df invocation. (line 9256) * local settings: Local. (line 11440) * logging out and continuing to run: nohup invocation. (line 13612) * logical and operator: Connectives for test. (line 10400) * logical and operator <1>: Relations for expr. (line 10540) * logical connectives: Connectives for test. (line 10377) * logical connectives <1>: Relations for expr. (line 10529) * logical or operator: Connectives for test. (line 10404) * logical or operator <1>: Relations for expr. (line 10534) * logical pages, numbering on: nl invocation. (line 1122) * login name, printing: logname invocation. (line 11842) * login sessions, printing users with: users invocation. (line 11894) * login time: who invocation. (line 11925) * logname: logname invocation. (line 11842) * long ls format: What information is listed. (line 5813) * lower: Character arrays. (line 5290) * lowercase, translating to output: Output. (line 11384) * ls: ls invocation. (line 5551) * LS_BLOCK_SIZE: Block size. (line 459) * LS_COLORS: General output formatting. (line 6061) * LS_COLORS <1>: dircolors invocation. (line 6414) * lutimes: touch invocation. (line 8987) * machine type: uname invocation. (line 12677) * machine-readable stty output: stty invocation. (line 11251) * MacKenzie, D.: Introduction. (line 336) * MacKenzie, David: Authors of parse_datetime. (line 15403) * Makefiles, installing programs in: install invocation. (line 7350) * manipulating files: Basic operations. (line 6450) * manipulation of file names: File name manipulation. (line 10756) * mark parity: Control. (line 11286) * match: String expressions. (line 10483) * matching patterns: String expressions. (line 10460) * MD5: md5sum invocation. (line 3018) * md5sum: md5sum invocation. (line 3018) * mebibyte, definition of: Block size. (line 536) * mebibytes for file sizes: du invocation. (line 9415) * megabyte, definition of: Block size. (line 533) * merging files: paste invocation. (line 4677) * merging files in parallel: pr invocation. (line 1680) * merging sorted files: sort invocation. (line 3175) * message status: who invocation. (line 12008) * message-digest, 128-bit: md5sum invocation. (line 3018) * message-digest, 160-bit: sha1sum invocation. (line 3076) * message-digest, 224-bit: sha2 utilities. (line 3104) * message-digest, 256-bit: sha2 utilities. (line 3104) * message-digest, 384-bit: sha2 utilities. (line 3104) * message-digest, 512-bit: b2sum invocation. (line 3050) * message-digest, 512-bit <1>: sha2 utilities. (line 3104) * Meyering, J.: Introduction. (line 336) * Meyering, Jim: Authors of parse_datetime. (line 15403) * midnight in date strings: Time of day items. (line 15141) * min: Special. (line 11652) * minute in date strings: Relative items in date strings. (line 15239) * minutes, time zone correction by: Time of day items. (line 15149) * mkdir: mkdir invocation. (line 8221) * mkfifo: mkfifo invocation. (line 8287) * mknod: mknod invocation. (line 8322) * mktemp: mktemp invocation. (line 10917) * modem control: Control. (line 11308) * modes and umask: Umask and Protection. (line 14750) * modes of created directories, setting: mkdir invocation. (line 8234) * modes of created FIFOs, setting: mkfifo invocation. (line 8302) * modification timestamp, sorting files by: Sorting the output. (line 5976) * modified command invocation: Modified command invocation. (line 12932) * modified environment, running a program in a: env invocation. (line 13031) * modify time, changing: touch invocation. (line 9002) * month in date strings: Relative items in date strings. (line 15239) * month names in date strings: Calendar date items. (line 15105) * months, sorting by: sort invocation. (line 3278) * months, written-out: General date syntax. (line 15034) * MS-DOS file system: df invocation. (line 9264) * MS-Windows file system: df invocation. (line 9264) * mtime: File timestamps. (line 14917) * mtime, changing: touch invocation. (line 9002) * mtime, printing or sorting files by: Sorting the output. (line 5990) * mtime-greater-atime file check: File characteristic tests. (line 10317) * multicall: Multi-call invocation. (line 985) * multicolumn output, generating: pr invocation. (line 1680) * multiple changes to permissions: Multiple Changes. (line 14703) * multiplication: Numeric expressions. (line 10523) * multipliers after numbers: dd invocation. (line 7245) * multithreaded sort: sort invocation. (line 3488) * mv: mv invocation. (line 7478) * name follow option: tail invocation. (line 2098) * name of kernel: uname invocation. (line 12701) * named pipe check: File type tests. (line 10255) * named pipes, creating: mkfifo invocation. (line 8287) * network node name: uname invocation. (line 12682) * networked file systems and du: du invocation. (line 9568) * never interactive option: rm invocation. (line 7677) * newer files, copying only: cp invocation. (line 6822) * newer files, moving only: mv invocation. (line 7563) * newer-than file check: File characteristic tests. (line 10305) * newline echoing after kill: Local. (line 11460) * newline, echoing: Local. (line 11463) * newline, translating to crlf: Output. (line 11391) * newline, translating to return: Input. (line 11341) * next DAY: Day of week items. (line 15220) * next DAY <1>: Options for date. (line 12341) * next in date strings: General date syntax. (line 15024) * NFS file system type: df invocation. (line 9251) * NFS mounts from BSD to HP-UX: What information is listed. (line 5924) * nice: nice invocation. (line 13506) * niceness: nice invocation. (line 13506) * nl: nl invocation. (line 1116) * nl <1>: Combination. (line 11519) * nlN: Output. (line 11411) * no dereference: chcon invocation. (line 12819) * no-op: true invocation. (line 10163) * noatime: dd invocation. (line 7200) * nocache: dd invocation. (line 7168) * nocreat: dd invocation. (line 7086) * noctty: dd invocation. (line 7206) * node name: uname invocation. (line 12682) * noerror: dd invocation. (line 7096) * noflsh: Local. (line 11466) * nofollow: dd invocation. (line 7211) * nohup: nohup invocation. (line 13612) * nohup.out: nohup invocation. (line 13612) * nohup.out <1>: nohup invocation. (line 13626) * nolinks: dd invocation. (line 7214) * non-directories, copying as special files: cp invocation. (line 6546) * non-directories, copying as special files <1>: cp invocation. (line 6712) * non-directory suffix, stripping: dirname invocation. (line 10821) * nonblock: dd invocation. (line 7197) * nonblocking I/O: dd invocation. (line 7197) * nonblocking stty setting: Special. (line 11675) * none backup method: Backup options. (line 427) * none classify option: General output formatting. (line 6081) * none color option: General output formatting. (line 6054) * none dd status=: dd invocation. (line 6977) * none hyperlink option: General output formatting. (line 6099) * none total option: wc invocation. (line 2727) * none, sorting option for ls: Sorting the output. (line 6004) * nonempty file check: File characteristic tests. (line 10302) * nonprinting characters, ignoring: sort invocation. (line 3271) * nonzero-length string check: String tests. (line 10336) * noon in date strings: Time of day items. (line 15141) * not-equal string check: String tests. (line 10346) * notrunc: dd invocation. (line 7093) * now in date strings: Relative items in date strings. (line 15257) * noxfer dd status=: dd invocation. (line 6981) * NO_NEW_PRIVS: runcon invocation. (line 12894) * nproc: nproc invocation. (line 12613) * NTFS file system: df invocation. (line 9264) * ntfs file system file: df invocation. (line 9264) * number of inputs to merge, nmerge: sort invocation. (line 3390) * numbered backup method: Backup options. (line 431) * numbering lines: nl invocation. (line 1116) * numbers, written-out: General date syntax. (line 15024) * numeric expressions: Numeric expressions. (line 10513) * numeric field padding: Padding and other flags. (line 12219) * numeric modes: Numeric Modes. (line 14778) * numeric operations: Numeric operations. (line 13965) * numeric sequences: seq invocation. (line 14331) * numeric sort: sort invocation. (line 3288) * numeric tests: Numeric tests. (line 10351) * numeric uid and gid: What information is listed. (line 5907) * numeric user and group IDs: What information is listed. (line 5907) * numfmt: numfmt invocation. (line 14021) * obs: dd invocation. (line 6932) * ocrnl: Output. (line 11388) * octal dump of files: od invocation. (line 1236) * octal numbers for file modes: Numeric Modes. (line 14778) * od: od invocation. (line 1236) * odd parity: Control. (line 11283) * oddp: Combination. (line 11515) * of: dd invocation. (line 6924) * ofdel: Output. (line 11406) * ofill: Output. (line 11402) * oflag: dd invocation. (line 7123) * olcuc: Output. (line 11384) * older-than file check: File characteristic tests. (line 10309) * once interactive option: rm invocation. (line 7678) * one file system, restricting du to: du invocation. (line 9547) * one file system, restricting rm to: rm invocation. (line 7686) * one-line output format: df invocation. (line 9193) * onlcr: Output. (line 11391) * onlret: Output. (line 11399) * only total option: wc invocation. (line 2724) * onocr: Output. (line 11395) * operating on characters: Operating on characters. (line 5122) * operating on sorted files: Operating on sorted files. (line 3123) * operating system name: uname invocation. (line 12693) * opost: Output. (line 11381) * option delimiter: Common options. (line 388) * options for date: Options for date. (line 12332) * or operator: Connectives for test. (line 10404) * or operator <1>: Relations for expr. (line 10534) * ordinal numbers: General date syntax. (line 15024) * oseek: dd invocation. (line 6956) * ospeed: Special. (line 11664) * other permissions: Setting Permissions. (line 14563) * output block size: dd invocation. (line 6932) * output file name prefix: split invocation. (line 2283) * output file name prefix <1>: csplit invocation. (line 2550) * output file name suffix: csplit invocation. (line 2554) * output format: stat invocation. (line 9626) * output format <1>: stat invocation. (line 9635) * output format, portable: df invocation. (line 9193) * output NUL-byte-terminated lines: cksum common options. (line 3010) * output NUL-byte-terminated lines <1>: General output formatting. (line 6181) * output NUL-byte-terminated lines <2>: readlink invocation. (line 8441) * output NUL-byte-terminated lines <3>: du invocation. (line 9311) * output NUL-byte-terminated lines <4>: basename invocation. (line 10797) * output NUL-byte-terminated lines <5>: dirname invocation. (line 10846) * output NUL-byte-terminated lines <6>: realpath invocation. (line 11112) * output NUL-byte-terminated lines <7>: printenv invocation. (line 11724) * output NUL-byte-terminated lines <8>: env invocation. (line 13115) * output of entire files: Output of entire files. (line 997) * output of parts of files: Output of parts of files. (line 1971) * output settings: Output. (line 11378) * output tabs: pr invocation. (line 1791) * overwriting of input, allowed: sort invocation. (line 3416) * overwriting of input, allowed <1>: shuf invocation. (line 3713) * owned by effective group ID check: Access permission tests. (line 10291) * owned by effective user ID check: Access permission tests. (line 10288) * owner of file, permissions for: Setting Permissions. (line 14559) * owner, default: Mode Structure. (line 14463) * ownership of installed files, setting: install invocation. (line 7416) * p for FIFO file: mknod invocation. (line 8344) * pad character: Output. (line 11406) * pad instead of timing for delaying: Output. (line 11402) * padding of numeric fields: Padding and other flags. (line 12219) * paragraphs, reformatting: fmt invocation. (line 1605) * parenb: Control. (line 11279) * parent directories and cp: cp invocation. (line 6699) * parent directories, creating: mkdir invocation. (line 8251) * parent directories, creating missing: install invocation. (line 7388) * parent directories, removing: rmdir invocation. (line 8471) * parentheses for grouping: expr invocation. (line 10434) * parity: Combination. (line 11511) * parity errors, marking: Input. (line 11331) * parity, ignoring: Input. (line 11328) * parmrk: Input. (line 11331) * parodd: Control. (line 11283) * parse_datetime: Date input formats. (line 14974) * parsing date strings: Options for date. (line 12341) * parts of files, output of: Output of parts of files. (line 1971) * pass8: Combination. (line 11556) * paste: paste invocation. (line 4677) * Paterson, R.: Introduction. (line 336) * PATH: env invocation. (line 13053) * pathchk: pathchk invocation. (line 10868) * pattern matching: String expressions. (line 10460) * pebibyte, definition of: Block size. (line 551) * permission tests: Access permission tests. (line 10266) * permissions of installed files, setting: install invocation. (line 7404) * permissions, changing access: chmod invocation. (line 8832) * permissions, copying existing: Copying Permissions. (line 14622) * permissions, for changing file timestamps: touch invocation. (line 8938) * permissions, output by ls: What information is listed. (line 5864) * petabyte, definition of: Block size. (line 548) * phone directory order: sort invocation. (line 3209) * pieces, splitting a file into: split invocation. (line 2274) * Pinard, F.: Introduction. (line 336) * Pinard, F. <1>: Authors of parse_datetime. (line 15416) * pipe fitting: tee invocation. (line 10591) * Plass, Michael F.: fmt invocation. (line 1618) * platform, hardware: uname invocation. (line 12670) * pm in date strings: Time of day items. (line 15141) * portable file names, checking for: pathchk invocation. (line 10868) * portable output format: df invocation. (line 9193) * POSIX: Introduction. (line 318) * POSIX output format: df invocation. (line 9193) * POSIXLY_CORRECT: Common options. (line 356) * POSIXLY_CORRECT <1>: Standards conformance. (line 956) * POSIXLY_CORRECT <2>: pr invocation. (line 1759) * POSIXLY_CORRECT <3>: sort invocation. (line 3427) * POSIXLY_CORRECT <4>: sort invocation. (line 3543) * POSIXLY_CORRECT <5>: dd invocation. (line 7317) * POSIXLY_CORRECT <6>: echo invocation. (line 9980) * POSIXLY_CORRECT <7>: printf invocation. (line 10046) * POSIXLY_CORRECT <8>: id invocation. (line 11776) * POSIXLY_CORRECT, and block size: Block size. (line 459) * pr: pr invocation. (line 1680) * prime factors: factor invocation. (line 13970) * print: Character arrays. (line 5292) * print machine hardware name: arch invocation. (line 12597) * print name of current directory: pwd invocation. (line 11181) * print system information: uname invocation. (line 12641) * print terminal file name: tty invocation. (line 11737) * Print the number of processors: nproc invocation. (line 12613) * printenv: printenv invocation. (line 11711) * printf: printf invocation. (line 9999) * printing all or some environment variables: printenv invocation. (line 11711) * printing color database: dircolors invocation. (line 6436) * printing current user information: who invocation. (line 11920) * printing current usernames: users invocation. (line 11894) * printing groups a user is in: groups invocation. (line 11868) * printing ls colors: dircolors invocation. (line 6441) * printing real and effective user and group IDs: id invocation. (line 11767) * printing text: echo invocation. (line 9914) * printing text, commands for: Printing text. (line 9909) * printing the current time: date invocation. (line 12031) * printing the effective user ID: whoami invocation. (line 11856) * printing the host identifier: hostid invocation. (line 12740) * printing the hostname: hostname invocation. (line 12720) * printing the system uptime and load: uptime invocation. (line 12761) * printing user’s login name: logname invocation. (line 11842) * printing, preparing files for: pr invocation. (line 1680) * process zero-terminated items: head invocation. (line 2025) * process zero-terminated items <1>: tail invocation. (line 2231) * process zero-terminated items <2>: sort invocation. (line 3511) * process zero-terminated items <3>: shuf invocation. (line 3732) * process zero-terminated items <4>: uniq invocation. (line 3926) * process zero-terminated items <5>: comm invocation. (line 4023) * process zero-terminated items <6>: cut invocation. (line 4664) * process zero-terminated items <7>: paste invocation. (line 4743) * process zero-terminated items <8>: General options in join. (line 4876) * process zero-terminated items <9>: numfmt invocation. (line 14130) * processes, commands for controlling: Process control. (line 13850) * progress dd status=: dd invocation. (line 6985) * prompting, and ln: ln invocation. (line 8097) * prompting, and mv: mv invocation. (line 7506) * prompting, and rm: rm invocation. (line 7631) * prompts, forcing: mv invocation. (line 7543) * prompts, omitting: mv invocation. (line 7537) * prompts, omitting <1>: mv invocation. (line 7550) * prterase: Local. (line 11480) * ptx: ptx invocation. (line 4033) * punct: Character arrays. (line 5294) * pure numbers in date strings: Pure numbers in date strings. (line 15291) * pwd: pwd invocation. (line 11181) * quettabyte, definition of: Block size. (line 573) * quit: Characters. (line 11599) * quoting style: Formatting the file names. (line 6329) * radix for file offsets: od invocation. (line 1266) * random seed: Random sources. (line 724) * random sort: sort invocation. (line 3317) * random source for shredding: shred invocation. (line 7870) * random source for shuffling: shuf invocation. (line 3719) * random source for sorting: sort invocation. (line 3432) * random sources: Random sources. (line 699) * ranges: Character arrays. (line 5235) * raw: Combination. (line 11544) * raw binary checksum: cksum general options. (line 2886) * read errors, ignoring: dd invocation. (line 7096) * read from standard input and write to standard output and files: tee invocation. (line 10591) * read permission: Mode Structure. (line 14448) * read permission, symbolic: Setting Permissions. (line 14588) * read system call, and holes: cp invocation. (line 6756) * readable file check: Access permission tests. (line 10275) * readlink: readlink invocation. (line 8382) * real user and group IDs, printing: id invocation. (line 11767) * realpath: realpath invocation. (line 11047) * realpath <1>: realpath invocation. (line 11047) * realpath <2>: realpath invocation. (line 11047) * realpath <3>: readlink invocation. (line 8382) * record separator character: split invocation. (line 2420) * recursive directory listing: Which files are listed. (line 5681) * recursively changing access permissions: chmod invocation. (line 8899) * recursively changing file ownership: chown invocation. (line 8681) * recursively changing group ownership: chgrp invocation. (line 8793) * recursively copying directories: cp invocation. (line 6546) * recursively copying directories <1>: cp invocation. (line 6712) * redirection: Redirection. (line 10583) * reference file: chcon invocation. (line 12823) * reformatting paragraph text: fmt invocation. (line 1605) * regular expression matching: String expressions. (line 10460) * regular file check: File type tests. (line 10246) * relations, numeric or string: Relations for expr. (line 10529) * relative items in date strings: Relative items in date strings. (line 15230) * release of kernel: uname invocation. (line 12697) * relpath: realpath invocation. (line 11090) * remainder: Numeric expressions. (line 10523) * remote hostname: who invocation. (line 11925) * removing characters: Squeezing and deleting. (line 5371) * removing empty directories: rmdir invocation. (line 8456) * removing files after shredding: shred invocation. (line 7881) * removing files or directories: rm invocation. (line 7626) * removing files or directories (via the unlink syscall): unlink invocation. (line 8502) * removing permissions: Setting Permissions. (line 14574) * renaming files without copying them: mv invocation. (line 7557) * repeat output values: shuf invocation. (line 3724) * repeated characters: Character arrays. (line 5255) * repeated lines, outputting: uniq invocation. (line 3850) * repeated output of a string: yes invocation. (line 10121) * restricted deletion flag: Mode Structure. (line 14488) * restricted security context: runcon invocation. (line 12894) * return, ignoring: Input. (line 11344) * return, translating to newline: Input. (line 11347) * return, translating to newline <1>: Output. (line 11388) * reverse sorting: sort invocation. (line 3311) * reverse sorting <1>: Sorting the output. (line 5965) * reversing files: tac invocation. (line 1074) * rm: rm invocation. (line 7626) * rmdir: rmdir invocation. (line 8456) * rn format for nl: nl invocation. (line 1208) * ronnabyte, definition of: Block size. (line 568) * root as default owner: install invocation. (line 7416) * root directory, allow recursive destruction: rm invocation. (line 7709) * root directory, allow recursive modification: chown invocation. (line 8662) * root directory, allow recursive modification <1>: chgrp invocation. (line 8775) * root directory, allow recursive modification <2>: chmod invocation. (line 8884) * root directory, disallow recursive destruction: rm invocation. (line 7702) * root directory, disallow recursive modification: chown invocation. (line 8657) * root directory, disallow recursive modification <1>: chgrp invocation. (line 8770) * root directory, disallow recursive modification <2>: chmod invocation. (line 8879) * root directory, running a program in a specified: chroot invocation. (line 12939) * rows: Special. (line 11667) * rprnt: Characters. (line 11639) * RTS/CTS flow control: Control. (line 11311) * run commands with bounded time: timeout invocation. (line 13741) * run with security context: runcon invocation. (line 12878) * runcon: runcon invocation. (line 12878) * running a program in a modified environment: env invocation. (line 13031) * running a program in a specified root directory: chroot invocation. (line 12939) * rz format for nl: nl invocation. (line 1210) * Salz, Rich: Authors of parse_datetime. (line 15403) * same file check: File characteristic tests. (line 10313) * sane: Combination. (line 11527) * scheduling, affecting: nice invocation. (line 13506) * screen columns: fold invocation. (line 1939) * scripts arguments: env invocation. (line 13217) * seconds since the Epoch: Time conversion specifiers. (line 12091) * section delimiters of pages: nl invocation. (line 1173) * security context: What information is listed. (line 5939) * security context <1>: cp invocation. (line 6867) * security context <2>: install invocation. (line 7421) * security context <3>: install invocation. (line 7464) * security context <4>: mv invocation. (line 7616) * security context <5>: mkdir invocation. (line 8274) * security context <6>: mkfifo invocation. (line 8309) * security context <7>: mknod invocation. (line 8369) * security context <8>: id invocation. (line 11812) * seek: dd invocation. (line 6956) * self-backups: cp invocation. (line 6501) * SELinux: What information is listed. (line 5939) * SELinux <1>: install invocation. (line 7421) * SELinux <2>: id invocation. (line 11812) * SELinux context: SELinux context. (line 12794) * SELinux, context: SELinux context. (line 12794) * SELinux, restoring security context: mv invocation. (line 7616) * SELinux, setting/restoring security context: cp invocation. (line 6867) * SELinux, setting/restoring security context <1>: install invocation. (line 7464) * SELinux, setting/restoring security context <2>: mkdir invocation. (line 8274) * SELinux, setting/restoring security context <3>: mkfifo invocation. (line 8309) * SELinux, setting/restoring security context <4>: mknod invocation. (line 8369) * send a signal to processes: kill invocation. (line 13853) * sentences and line-breaking: fmt invocation. (line 1618) * separator for numbers in seq: seq invocation. (line 14370) * seq: seq invocation. (line 14331) * sequence of numbers: seq invocation. (line 14331) * set-group-ID: Mode Structure. (line 14481) * set-group-ID check: Access permission tests. (line 10269) * set-user-ID: Mode Structure. (line 14475) * set-user-ID check: Access permission tests. (line 10278) * setgid: Mode Structure. (line 14481) * setting permissions: Setting Permissions. (line 14577) * setting the hostname: hostname invocation. (line 12720) * setting the time: Setting the time. (line 12298) * setuid: Mode Structure. (line 14475) * setup for color: dircolors invocation. (line 6397) * sh syntax for color setup: dircolors invocation. (line 6425) * SHA-1: sha1sum invocation. (line 3076) * SHA-2: sha2 utilities. (line 3104) * sha1sum: sha1sum invocation. (line 3076) * sha224sum: sha2 utilities. (line 3104) * sha256sum: sha2 utilities. (line 3104) * sha384sum: sha2 utilities. (line 3104) * sha512sum: sha2 utilities. (line 3104) * shebang arguments: env invocation. (line 13217) * SHELL environment variable, and color: General output formatting. (line 6061) * SHELL environment variable, and color <1>: dircolors invocation. (line 6414) * shell utilities: Top. (line 310) * shred: shred invocation. (line 7743) * shuf: shuf invocation. (line 3683) * shuffling files: shuf invocation. (line 3683) * SI output: Block size. (line 489) * SI output <1>: What information is listed. (line 5931) * SI output <2>: df invocation. (line 9212) * SI output <3>: du invocation. (line 9433) * signals, specifying: Signal specifications. (line 590) * simple backup method: Backup options. (line 440) * SIMPLE_BACKUP_SUFFIX: Backup options. (line 445) * single quotes, and env -S: env invocation. (line 13293) * single-column output of files: General output formatting. (line 6033) * size: Special. (line 11686) * size for main memory sorting: sort invocation. (line 3444) * size of file to shred: shred invocation. (line 7875) * size of files, reporting: What information is listed. (line 5916) * size of files, sorting files by: Sorting the output. (line 5972) * skip: dd invocation. (line 6949) * sleep: sleep invocation. (line 13920) * socket check: File type tests. (line 10258) * software flow control: Input. (line 11358) * sort: sort invocation. (line 3128) * sort field: sort invocation. (line 3359) * sort stability: sort invocation. (line 3134) * sort stability <1>: sort invocation. (line 3437) * sort’s last-resort comparison: sort invocation. (line 3134) * sort’s last-resort comparison <1>: sort invocation. (line 3437) * sorted files, operations on: Operating on sorted files. (line 3123) * sorting files: sort invocation. (line 3128) * sorting ls output: Sorting the output. (line 5946) * space: Character arrays. (line 5296) * space parity: Control. (line 11286) * sparse: dd invocation. (line 7056) * sparse files, copying: cp invocation. (line 6756) * sparse files, creating: truncate invocation. (line 9860) * special characters: Characters. (line 11584) * special file types: Special file types. (line 7960) * special file types <1>: Special file types. (line 7963) * special files: mknod invocation. (line 8327) * special settings: Special. (line 11651) * speed: Special. (line 11696) * split: split invocation. (line 2274) * splitting a file into pieces: split invocation. (line 2274) * splitting a file into pieces by context: csplit invocation. (line 2492) * squeezing blank lines: cat invocation. (line 1034) * squeezing empty lines: cat invocation. (line 1034) * squeezing repeat characters: Squeezing and deleting. (line 5371) * Stallman, R.: Introduction. (line 336) * standard input: Common options. (line 392) * standard output: Common options. (line 392) * standard streams, buffering: stdbuf invocation. (line 13667) * start: Characters. (line 11627) * stat: stat invocation. (line 9582) * status: dd invocation. (line 6972) * status <1>: Characters. (line 11623) * status time, printing or sorting by: Sorting the output. (line 5953) * status time, show the most recent: du invocation. (line 9488) * stdbuf: stdbuf invocation. (line 13667) * stick parity: Control. (line 11286) * sticky: Mode Structure. (line 14488) * sticky bit check: Access permission tests. (line 10272) * stop: Characters. (line 11630) * stop bits: Control. (line 11302) * storage devices, failing: dd invocation. (line 7273) * strftime and date: Date format specifiers. (line 12055) * string constants, outputting: od invocation. (line 1311) * string expressions: String expressions. (line 10455) * string tests: String tests. (line 10323) * strip directory and suffix from file names: basename invocation. (line 10761) * stripping non-directory suffix: dirname invocation. (line 10821) * stripping symbol table information: install invocation. (line 7438) * stripping trailing slashes: cp invocation. (line 6795) * stripping trailing slashes <1>: mv invocation. (line 7597) * stty: stty invocation. (line 11216) * substr: String expressions. (line 10487) * subtracting permissions: Setting Permissions. (line 14574) * subtraction: Numeric expressions. (line 10519) * successful exit: true invocation. (line 10163) * suffix, stripping from file names: basename invocation. (line 10761) * sum: sum invocation. (line 2753) * summarizing files: Summarizing files. (line 2646) * superblock, writing: sync invocation. (line 9811) * supplementary groups, printing: groups invocation. (line 11868) * susp: Characters. (line 11633) * swab (byte-swapping): dd invocation. (line 7071) * swap space, saving text image in: Mode Structure. (line 14488) * swtch: Characters. (line 11620) * symbol table information, stripping: install invocation. (line 7438) * symbol table information, stripping, program: install invocation. (line 7441) * symbolic (soft) links, creating: ln invocation. (line 8005) * symbolic link check: File type tests. (line 10250) * symbolic link to directory, controlling traversal of: Traversing symlinks. (line 831) * symbolic link to directory, never traverse: Traversing symlinks. (line 851) * symbolic link to directory, never traverse <1>: chown invocation. (line 8702) * symbolic link to directory, never traverse <2>: chgrp invocation. (line 8815) * symbolic link to directory, never traverse <3>: chcon invocation. (line 12849) * symbolic link to directory, traverse each that is encountered: Traversing symlinks. (line 847) * symbolic link to directory, traverse each that is encountered <1>: chown invocation. (line 8689) * symbolic link to directory, traverse each that is encountered <2>: chgrp invocation. (line 8802) * symbolic link to directory, traverse each that is encountered <3>: chcon invocation. (line 12845) * symbolic link to directory, traverse if on the command line: Traversing symlinks. (line 843) * symbolic link to directory, traverse if on the command line <1>: chown invocation. (line 8684) * symbolic link to directory, traverse if on the command line <2>: chgrp invocation. (line 8797) * symbolic link to directory, traverse if on the command line <3>: chcon invocation. (line 12840) * symbolic link, defined: ln invocation. (line 8041) * symbolic links and ln: ln invocation. (line 8183) * symbolic links and pwd: pwd invocation. (line 11201) * symbolic links, changing group: chgrp invocation. (line 8762) * symbolic links, changing owner: chown invocation. (line 8614) * symbolic links, changing owner <1>: chown invocation. (line 8637) * symbolic links, changing owner <2>: chown invocation. (line 8649) * symbolic links, changing owner <3>: chgrp invocation. (line 8750) * symbolic links, changing time: touch invocation. (line 8987) * symbolic links, copying: cp invocation. (line 6559) * symbolic links, copying <1>: cp invocation. (line 6616) * symbolic links, copying with: cp invocation. (line 6800) * symbolic links, dereferencing: Which files are listed. (line 5626) * symbolic links, dereferencing <1>: Which files are listed. (line 5631) * symbolic links, dereferencing <2>: Which files are listed. (line 5674) * symbolic links, dereferencing in du: du invocation. (line 9405) * symbolic links, dereferencing in du <1>: du invocation. (line 9421) * symbolic links, dereferencing in stat: stat invocation. (line 9598) * symbolic links, following: dd invocation. (line 7211) * symbolic links, permissions of: chmod invocation. (line 8836) * symbolic modes: Symbolic Modes. (line 14523) * symlinks, resolution: realpath invocation. (line 11047) * sync: sync invocation. (line 9806) * sync <1>: dd invocation. (line 7165) * sync (padding with ASCII NULs): dd invocation. (line 7074) * Synchronize cached writes to persistent storage: sync invocation. (line 9806) * synchronize file system and memory: sync invocation. (line 9806) * synchronized data and metadata I/O: dd invocation. (line 7165) * synchronized data and metadata writes, before finishing: dd invocation. (line 7110) * synchronized data reads: dd invocation. (line 7157) * synchronized data writes, before finishing: dd invocation. (line 7099) * system context: System context. (line 12025) * system information, printing: arch invocation. (line 12597) * system information, printing <1>: nproc invocation. (line 12613) * system information, printing <2>: uname invocation. (line 12641) * system name, printing: hostname invocation. (line 12720) * System V sum: sum invocation. (line 2776) * tab stops, setting: expand invocation. (line 5458) * tabN: Output. (line 11423) * tabs: Combination. (line 11567) * tabs to spaces, converting: expand invocation. (line 5442) * tac: tac invocation. (line 1074) * tagged paragraphs: fmt invocation. (line 1639) * tail: tail invocation. (line 2047) * tandem: Input. (line 11358) * target directory: Target directory. (line 740) * target directory <1>: Target directory. (line 749) * target directory <2>: Target directory. (line 765) * target directory <3>: cp invocation. (line 6813) * target directory <4>: cp invocation. (line 6817) * target directory <5>: install invocation. (line 7450) * target directory <6>: install invocation. (line 7455) * target directory <7>: mv invocation. (line 7607) * target directory <8>: mv invocation. (line 7611) * target directory <9>: ln invocation. (line 8172) * target directory <10>: ln invocation. (line 8176) * tebibyte, definition of: Block size. (line 546) * tee: tee invocation. (line 10591) * telephone directory order: sort invocation. (line 3209) * temporary directory: sort invocation. (line 3480) * temporary files and directories: mktemp invocation. (line 10917) * terabyte, definition of: Block size. (line 543) * terminal check: File type tests. (line 10261) * terminal file name, printing: tty invocation. (line 11737) * terminal lines, currently used: who invocation. (line 11925) * terminal settings: stty invocation. (line 11216) * terminal, using classify iff: General output formatting. (line 6082) * terminal, using color iff: General output formatting. (line 6055) * terminal, using hyperlink iff: General output formatting. (line 6100) * terse output: stat invocation. (line 9646) * test: test invocation. (line 10189) * text: dd invocation. (line 7221) * text I/O: dd invocation. (line 7221) * text image, saving in swap space: Mode Structure. (line 14488) * text input files: cksum common options. (line 2990) * text utilities: Top. (line 310) * text, displaying: echo invocation. (line 9914) * text, reformatting: fmt invocation. (line 1605) * this in date strings: Relative items in date strings. (line 15257) * time: touch invocation. (line 8973) * time <1>: Special. (line 11656) * time conversion specifiers: Time conversion specifiers. (line 12065) * time formats: pr invocation. (line 1752) * time formats <1>: Date format specifiers. (line 12055) * time limit: timeout invocation. (line 13741) * time of day item: Time of day items. (line 15126) * time setting: Setting the time. (line 12298) * time style: Formatting file timestamps. (line 6209) * time style <1>: du invocation. (line 9498) * time units: timeout invocation. (line 13801) * time units <1>: sleep invocation. (line 13925) * time zone correction: Time of day items. (line 15149) * time zone item: General date syntax. (line 15042) * time zone item <1>: Time zone items. (line 15168) * time, printing or setting: date invocation. (line 12031) * timeout: timeout invocation. (line 13741) * timestamps of installed files, preserving: install invocation. (line 7428) * timestamps, changing file: touch invocation. (line 8923) * TIME_STYLE: Formatting file timestamps. (line 6288) * TIME_STYLE <1>: du invocation. (line 9526) * TMPDIR: sort invocation. (line 3186) * TMPDIR <1>: sort invocation. (line 3480) * today in date strings: Relative items in date strings. (line 15257) * tomorrow: Options for date. (line 12341) * tomorrow in date strings: Relative items in date strings. (line 15253) * topological sort: tsort invocation. (line 4434) * tostop: Local. (line 11475) * total counts: wc invocation. (line 2659) * touch: touch invocation. (line 8923) * tr: tr invocation. (line 5127) * trailing slashes: Trailing slashes. (line 813) * translating characters: Translating. (line 5318) * true: true invocation. (line 10163) * truncate: truncate invocation. (line 9853) * truncating output file, avoiding: dd invocation. (line 7093) * truncating, file sizes: truncate invocation. (line 9853) * tsort: tsort invocation. (line 4434) * tty: tty invocation. (line 11737) * two-way parity: Control. (line 11279) * type size: od invocation. (line 1352) * TZ: pr invocation. (line 1765) * TZ <1>: Formatting file timestamps. (line 6202) * TZ <2>: touch invocation. (line 8952) * TZ <3>: stat invocation. (line 9796) * TZ <4>: who invocation. (line 11940) * TZ <5>: date invocation. (line 12045) * TZ <6>: Options for date. (line 12461) * TZ <7>: Specifying time zone rules. (line 15340) * u, and disabling special characters: Characters. (line 11590) * ucase, converting to: dd invocation. (line 7051) * umask and modes: Umask and Protection. (line 14750) * uname: uname invocation. (line 12641) * unblock: dd invocation. (line 7040) * unexpand: unexpand invocation. (line 5491) * Unicode: printf invocation. (line 10067) * uniq: uniq invocation. (line 3792) * unique lines, outputting: uniq invocation. (line 3914) * uniquify files: uniq invocation. (line 3792) * uniquifying output: sort invocation. (line 3496) * Universal Time: Options for date. (line 12461) * unlink: unlink invocation. (line 8502) * unprintable characters, ignoring: sort invocation. (line 3271) * unsorted directory listing: Sorting the output. (line 5958) * upper: Character arrays. (line 5298) * uppercase, translating to lowercase: Input. (line 11363) * uptime: uptime invocation. (line 12761) * use time, changing: touch invocation. (line 8965) * use time, printing or sorting files by: Sorting the output. (line 5953) * use time, printing or sorting files by <1>: Sorting the output. (line 5984) * use time, show the most recent: du invocation. (line 9488) * user IDs, disambiguating: Disambiguating names and IDs. (line 669) * user information, commands for: User information. (line 11761) * user name, printing: logname invocation. (line 11842) * user names, disambiguating: Disambiguating names and IDs. (line 669) * usernames, printing current: users invocation. (line 11894) * users: users invocation. (line 11894) * UTC: Options for date. (line 12461) * utmp: logname invocation. (line 11842) * utmp <1>: users invocation. (line 11902) * utmp <2>: who invocation. (line 11929) * valid file names, checking for: pathchk invocation. (line 10868) * variable-length records, converting to fixed-length: dd invocation. (line 6943) * vdir: vdir invocation. (line 6388) * verbose ls format: What information is listed. (line 5813) * verifying checksums: cksum common options. (line 2955) * verifying checksums <1>: cksum common options. (line 2961) * verifying checksums <2>: cksum common options. (line 2969) * verifying checksums <3>: cksum common options. (line 3000) * verifying checksums <4>: cksum common options. (line 3005) * version number sort: sort invocation. (line 3305) * version number, finding: Common options. (line 385) * version of kernel: uname invocation. (line 12712) * version, sorting option for ls: Sorting the output. (line 6011) * version-control Emacs variable: Backup options. (line 420) * VERSION_CONTROL: Backup options. (line 409) * VERSION_CONTROL <1>: cp invocation. (line 6529) * VERSION_CONTROL <2>: install invocation. (line 7362) * VERSION_CONTROL <3>: mv invocation. (line 7528) * VERSION_CONTROL <4>: ln invocation. (line 8081) * vertical sorted files in columns: General output formatting. (line 6045) * vtN: Output. (line 11431) * wc: wc invocation. (line 2652) * week in date strings: Relative items in date strings. (line 15239) * werase: Characters. (line 11642) * who: who invocation. (line 11920) * who am i: who invocation. (line 11935) * whoami: whoami invocation. (line 11856) * width, sorting option for ls: Sorting the output. (line 6017) * word count: wc invocation. (line 2652) * working context: Working context. (line 11174) * working directory, printing: pwd invocation. (line 11181) * wrap data: base64 invocation. (line 1476) * wrapping long input lines: fold invocation. (line 1931) * writable file check: Access permission tests. (line 10281) * write permission: Mode Structure. (line 14450) * write permission, symbolic: Setting Permissions. (line 14590) * write, allowed: who invocation. (line 12008) * wtmp: users invocation. (line 11902) * wtmp <1>: who invocation. (line 11929) * xcase: Local. (line 11470) * xdigit: Character arrays. (line 5300) * xfs file system type: df invocation. (line 9256) * XON/XOFF flow control: Input. (line 11353) * year in date strings: Relative items in date strings. (line 15239) * yes: yes invocation. (line 10121) * yesterday: Options for date. (line 12341) * yesterday in date strings: Relative items in date strings. (line 15253) * yottabyte, definition of: Block size. (line 563) * Youmans, B.: Introduction. (line 336) * zero-length string check: String tests. (line 10332) * zettabyte, definition of: Block size. (line 558)