GNU Grep 2.18

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grep

grep prints lines that contain a match for a pattern.

This manual is for version 2.18 of GNU Grep.

This manual is for grep, a pattern matching engine.

Copyright © 1999-2002, 2005, 2008-2014 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”.


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1 Introduction

grep searches input files for lines containing a match to a given pattern list. When it finds a match in a line, it copies the line to standard output (by default), or produces whatever other sort of output you have requested with options.

Though grep expects to do the matching on text, it has no limits on input line length other than available memory, and it can match arbitrary characters within a line. If the final byte of an input file is not a newline, grep silently supplies one. Since newline is also a separator for the list of patterns, there is no way to match newline characters in a text.


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2 Invoking grep

The general synopsis of the grep command line is

     grep options pattern input_file_names

There can be zero or more options. pattern will only be seen as such (and not as an input_file_name) if it wasn't already specified within options (by using the ‘-e pattern’ or ‘-f file’ options). There can be zero or more input_file_names.


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2.1 Command-line Options

grep comes with a rich set of options: some from POSIX and some being GNU extensions. Long option names are always a GNU extension, even for options that are from POSIX specifications. Options that are specified by POSIX, under their short names, are explicitly marked as such to facilitate POSIX-portable programming. A few option names are provided for compatibility with older or more exotic implementations.

Several additional options control which variant of the grep matching engine is used. See grep Programs.


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2.1.1 Generic Program Information

--help
Print a usage message briefly summarizing the command-line options and the bug-reporting address, then exit.
-V
--version
Print the version number of grep to the standard output stream. This version number should be included in all bug reports.


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2.1.2 Matching Control

-e pattern
--regexp=pattern
Use pattern as the pattern. This can be used to specify multiple search patterns, or to protect a pattern beginning with a ‘-’. (-e is specified by POSIX.)
-f file
--file=file
Obtain patterns from file, one per line. The empty file contains zero patterns, and therefore matches nothing. (-f is specified by POSIX.)
-i
-y
--ignore-case
Ignore case distinctions in both the pattern and the input files. -y is an obsolete synonym that is provided for compatibility. (-i is specified by POSIX.)
-v
--invert-match
Invert the sense of matching, to select non-matching lines. (-v is specified by POSIX.)
-w
--word-regexp
Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. Similarly, it must be either at the end of the line or followed by a non-word constituent character. Word-constituent characters are letters, digits, and the underscore.
-x
--line-regexp
Select only those matches that exactly match the whole line. (-x is specified by POSIX.)


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2.1.3 General Output Control

-c
--count
Suppress normal output; instead print a count of matching lines for each input file. With the -v (--invert-match) option, count non-matching lines. (-c is specified by POSIX.)
--color[=WHEN]
--colour[=WHEN]
Surround the matched (non-empty) strings, matching lines, context lines, file names, line numbers, byte offsets, and separators (for fields and groups of context lines) with escape sequences to display them in color on the terminal. The colors are defined by the environment variable GREP_COLORS and default to ‘ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36’ for bold red matched text, magenta file names, green line numbers, green byte offsets, cyan separators, and default terminal colors otherwise. The deprecated environment variable GREP_COLOR is still supported, but its setting does not have priority; it defaults to ‘01;31’ (bold red) which only covers the color for matched text. WHEN is ‘never’, ‘always’, or ‘auto’.
-L
--files-without-match
Suppress normal output; instead print the name of each input file from which no output would normally have been printed. The scanning of each file stops on the first match.
-l
--files-with-matches
Suppress normal output; instead print the name of each input file from which output would normally have been printed. The scanning of each file stops on the first match. (-l is specified by POSIX.)
-m num
--max-count=num
Stop reading a file after num matching lines. If the input is standard input from a regular file, and num matching lines are output, grep ensures that the standard input is positioned just after the last matching line before exiting, regardless of the presence of trailing context lines. This enables a calling process to resume a search. For example, the following shell script makes use of it:
          while grep -m 1 PATTERN
          do
            echo xxxx
          done < FILE

But the following probably will not work because a pipe is not a regular file:

          # This probably will not work.
          cat FILE |
          while grep -m 1 PATTERN
          do
            echo xxxx
          done

When grep stops after num matching lines, it outputs any trailing context lines. Since context does not include matching lines, grep will stop when it encounters another matching line. When the -c or --count option is also used, grep does not output a count greater than num. When the -v or --invert-match option is also used, grep stops after outputting num non-matching lines.

-o
--only-matching
Print only the matched (non-empty) parts of matching lines, with each such part on a separate output line.
-q
--quiet
--silent
Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected. Also see the -s or --no-messages option. (-q is specified by POSIX.)
-s
--no-messages
Suppress error messages about nonexistent or unreadable files. Portability note: unlike GNU grep, 7th Edition Unix grep did not conform to POSIX, because it lacked -q and its -s option behaved like GNU grep's -q option.1 USG-style grep also lacked -q but its -s option behaved like GNU grep's. Portable shell scripts should avoid both -q and -s and should redirect standard and error output to /dev/null instead. (-s is specified by POSIX.)


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2.1.4 Output Line Prefix Control

When several prefix fields are to be output, the order is always file name, line number, and byte offset, regardless of the order in which these options were specified.

-b
--byte-offset
Print the 0-based byte offset within the input file before each line of output. If -o (--only-matching) is specified, print the offset of the matching part itself. When grep runs on MS-DOS or MS-Windows, the printed byte offsets depend on whether the -u (--unix-byte-offsets) option is used; see below.
-H
--with-filename
Print the file name for each match. This is the default when there is more than one file to search.
-h
--no-filename
Suppress the prefixing of file names on output. This is the default when there is only one file (or only standard input) to search.
--label=LABEL
Display input actually coming from standard input as input coming from file LABEL. This is especially useful when implementing tools like zgrep; e.g.:
          gzip -cd foo.gz | grep --label=foo -H something

-n
--line-number
Prefix each line of output with the 1-based line number within its input file. (-n is specified by POSIX.)
-T
--initial-tab
Make sure that the first character of actual line content lies on a tab stop, so that the alignment of tabs looks normal. This is useful with options that prefix their output to the actual content: -H, -n, and -b. In order to improve the probability that lines from a single file will all start at the same column, this also causes the line number and byte offset (if present) to be printed in a minimum-size field width.
-u
--unix-byte-offsets
Report Unix-style byte offsets. This option causes grep to report byte offsets as if the file were a Unix-style text file, i.e., the byte offsets ignore the CR characters that were stripped. This will produce results identical to running grep on a Unix machine. This option has no effect unless the -b option is also used; it has no effect on platforms other than MS-DOS and MS-Windows.
-Z
--null
Output a zero byte (the ASCII NUL character) instead of the character that normally follows a file name. For example, ‘grep -lZ’ outputs a zero byte after each file name instead of the usual newline. This option makes the output unambiguous, even in the presence of file names containing unusual characters like newlines. This option can be used with commands like ‘find -print0’, ‘perl -0’, ‘sort -z’, and ‘xargs -0’ to process arbitrary file names, even those that contain newline characters.


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2.1.5 Context Line Control

Regardless of how these options are set, grep will never print any given line more than once. If the -o (--only-matching) option is specified, these options have no effect and a warning is given upon their use.

-A num
--after-context=num
Print num lines of trailing context after matching lines.
-B num
--before-context=num
Print num lines of leading context before matching lines.
-C num
-num
--context=num
Print num lines of leading and trailing output context.
--group-separator=string
When -A, -B or -C are in use, print string instead of -- around disjoint groups of lines.
--no-group-separator
When -A, -B or -C are in use, print disjoint groups of lines adjacent to each other.

Here are some points about how grep chooses the separator to print between prefix fields and line content:


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2.1.6 File and Directory Selection

-a
--text
Process a binary file as if it were text; this is equivalent to the ‘--binary-files=text’ option.
--binary-files=type
If a file's allocation metadata or its first few bytes indicate that the file contains binary data, assume that the file is of type type. By default, type is ‘binary’, and grep normally outputs either a one-line message saying that a binary file matches, or no message if there is no match.

If type is ‘without-match’, grep assumes that a binary file does not match; this is equivalent to the -I option.

If type is ‘text’, grep processes a binary file as if it were text; this is equivalent to the -a option.

Warning:--binary-files=text’ might output binary garbage, which can have nasty side effects if the output is a terminal and if the terminal driver interprets some of it as commands.

-D action
--devices=action
If an input file is a device, FIFO, or socket, use action to process it. If action is ‘read’, all devices are read just as if they were ordinary files. If action is ‘skip’, devices, FIFOs, and sockets are silently skipped. By default, devices are read if they are on the command line or if the -R (--dereference-recursive) option is used, and are skipped if they are encountered recursively and the -r (--recursive) option is used. This option has no effect on a file that is read via standard input.
-d action
--directories=action
If an input file is a directory, use action to process it. By default, action is ‘read’, which means that directories are read just as if they were ordinary files (some operating systems and file systems disallow this, and will cause grep to print error messages for every directory or silently skip them). If action is ‘skip’, directories are silently skipped. If action is ‘recurse’, grep reads all files under each directory, recursively, following command-line symbolic links and skipping other symlinks; this is equivalent to the -r option.
--exclude=glob
Skip files whose base name matches glob (using wildcard matching). A file-name glob can use ‘*’, ‘?’, and ‘[’...‘]’ as wildcards, and \ to quote a wildcard or backslash character literally.
--exclude-from=file
Skip files whose base name matches any of the file-name globs read from file (using wildcard matching as described under --exclude).
--exclude-dir=dir
Exclude directories matching the pattern dir from recursive directory searches.
-I
Process a binary file as if it did not contain matching data; this is equivalent to the ‘--binary-files=without-match’ option.
--include=glob
Search only files whose base name matches glob (using wildcard matching as described under --exclude).
-r
--recursive
For each directory operand, read and process all files in that directory, recursively. Follow symbolic links on the command line, but skip symlinks that are encountered recursively. This is the same as the ‘--directories=recurse’ option.
-R
--dereference-recursive
For each directory operand, read and process all files in that directory, recursively, following all symbolic links.


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2.1.7 Other Options

--line-buffered
Use line buffering on output. This can cause a performance penalty.
-U
--binary
Treat the file(s) as binary. By default, under MS-DOS and MS-Windows, grep guesses whether a file is text or binary as described for the --binary-files option. If grep decides the file is a text file, it strips the CR characters from the original file contents (to make regular expressions with ^ and $ work correctly). Specifying -U overrules this guesswork, causing all files to be read and passed to the matching mechanism verbatim; if the file is a text file with CR/LF pairs at the end of each line, this will cause some regular expressions to fail. This option has no effect on platforms other than MS-DOS and MS-Windows.
-z
--null-data
Treat the input as a set of lines, each terminated by a zero byte (the ASCII NUL character) instead of a newline. Like the -Z or --null option, this option can be used with commands like ‘sort -z’ to process arbitrary file names.


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2.2 Environment Variables

The behavior of grep is affected by the following environment variables.

The locale for category LC_foo is specified by examining the three environment variables LC_ALL, LC_foo, and LANG, in that order. The first of these variables that is set specifies the locale. For example, if LC_ALL is not set, but LC_MESSAGES is set to ‘pt_BR’, then the Brazilian Portuguese locale is used for the LC_MESSAGES category. The ‘C’ locale is used if none of these environment variables are set, if the locale catalog is not installed, or if grep was not compiled with national language support (NLS).

Many of the environment variables in the following list let you control highlighting using Select Graphic Rendition (SGR) commands interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. grep takes care of assembling the result into a complete SGR sequence (‘\33[’...‘m’). Common values to concatenate include ‘1’ for bold, ‘4’ for underline, ‘5’ for blink, ‘7’ for inverse, ‘39’ for default foreground color, ‘30’ to ‘37’ for foreground colors, ‘90’ to ‘97’ for 16-color mode foreground colors, ‘38;5;0’ to ‘38;5;255’ for 88-color and 256-color modes foreground colors, ‘49’ for default background color, ‘40’ to ‘47’ for background colors, ‘100’ to ‘107’ for 16-color mode background colors, and ‘48;5;0’ to ‘48;5;255’ for 88-color and 256-color modes background colors.

The two-letter names used in the GREP_COLORS environment variable (and some of the others) refer to terminal “capabilities,” the ability of a terminal to highlight text, or change its color, and so on. These capabilities are stored in an online database and accessed by the terminfo library.

GREP_OPTIONS
This variable specifies default options to be placed in front of any explicit options. For example, if GREP_OPTIONS is ‘--binary-files=without-match --directories=skip’, grep behaves as if the two options ‘--binary-files=without-match’ and ‘--directories=skip’ had been specified before any explicit options. Option specifications are separated by whitespace. A backslash escapes the next character, so it can be used to specify an option containing whitespace or a backslash.

The GREP_OPTIONS value does not affect whether grep without file operands searches standard input or the working directory; that is affected only by command-line options. For example, the command ‘grep PAT’ searches standard input and the command ‘grep -r PAT’ searches the working directory, regardless of whether GREP_OPTIONS contains -r.

GREP_COLOR
This variable specifies the color used to highlight matched (non-empty) text. It is deprecated in favor of GREP_COLORS, but still supported. The ‘mt’, ‘ms’, and ‘mc’ capabilities of GREP_COLORS have priority over it. It can only specify the color used to highlight the matching non-empty text in any matching line (a selected line when the -v command-line option is omitted, or a context line when -v is specified). The default is ‘01;31’, which means a bold red foreground text on the terminal's default background.
GREP_COLORS
This variable specifies the colors and other attributes used to highlight various parts of the output. Its value is a colon-separated list of terminfo capabilities that defaults to ‘ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36’ with the ‘rv’ and ‘ne’ boolean capabilities omitted (i.e., false). Supported capabilities are as follows.
sl=
SGR substring for whole selected lines (i.e., matching lines when the -v command-line option is omitted, or non-matching lines when -v is specified). If however the boolean ‘rv’ capability and the -v command-line option are both specified, it applies to context matching lines instead. The default is empty (i.e., the terminal's default color pair).
cx=
SGR substring for whole context lines (i.e., non-matching lines when the -v command-line option is omitted, or matching lines when -v is specified). If however the boolean ‘rv’ capability and the -v command-line option are both specified, it applies to selected non-matching lines instead. The default is empty (i.e., the terminal's default color pair).
rv
Boolean value that reverses (swaps) the meanings of the ‘sl=’ and ‘cx=’ capabilities when the -v command-line option is specified. The default is false (i.e., the capability is omitted).
mt=01;31
SGR substring for matching non-empty text in any matching line (i.e., a selected line when the -v command-line option is omitted, or a context line when -v is specified). Setting this is equivalent to setting both ‘ms=’ and ‘mc=’ at once to the same value. The default is a bold red text foreground over the current line background.
ms=01;31
SGR substring for matching non-empty text in a selected line. (This is used only when the -v command-line option is omitted.) The effect of the ‘sl=’ (or ‘cx=’ if ‘rv’) capability remains active when this takes effect. The default is a bold red text foreground over the current line background.
mc=01;31
SGR substring for matching non-empty text in a context line. (This is used only when the -v command-line option is specified.) The effect of the ‘cx=’ (or ‘sl=’ if ‘rv’) capability remains active when this takes effect. The default is a bold red text foreground over the current line background.
fn=35
SGR substring for file names prefixing any content line. The default is a magenta text foreground over the terminal's default background.
ln=32
SGR substring for line numbers prefixing any content line. The default is a green text foreground over the terminal's default background.
bn=32
SGR substring for byte offsets prefixing any content line. The default is a green text foreground over the terminal's default background.
se=36
SGR substring for separators that are inserted between selected line fields (‘:’), between context line fields (‘-’), and between groups of adjacent lines when nonzero context is specified (‘--’). The default is a cyan text foreground over the terminal's default background.
ne
Boolean value that prevents clearing to the end of line using Erase in Line (EL) to Right (‘\33[K’) each time a colorized item ends. This is needed on terminals on which EL is not supported. It is otherwise useful on terminals for which the back_color_erase (bce) boolean terminfo capability does not apply, when the chosen highlight colors do not affect the background, or when EL is too slow or causes too much flicker. The default is false (i.e., the capability is omitted).

Note that boolean capabilities have no ‘=’... part. They are omitted (i.e., false) by default and become true when specified.

LC_ALL
LC_COLLATE
LANG
These variables specify the locale for the LC_COLLATE category, which might affect how range expressions like ‘[a-z]’ are interpreted.
LC_ALL
LC_CTYPE
LANG
These variables specify the locale for the LC_CTYPE category, which determines the type of characters, e.g., which characters are whitespace.
LC_ALL
LC_MESSAGES
LANG
These variables specify the locale for the LC_MESSAGES category, which determines the language that grep uses for messages. The default ‘C’ locale uses American English messages.
POSIXLY_CORRECT
If set, grep behaves as POSIX requires; otherwise, grep behaves more like other GNU programs. POSIX requires that options that follow file names must be treated as file names; by default, such options are permuted to the front of the operand list and are treated as options. Also, POSIXLY_CORRECT disables special handling of an invalid bracket expression. See invalid-bracket-expr.
_N_GNU_nonoption_argv_flags_
(Here N is grep's numeric process ID.) If the ith character of this environment variable's value is ‘1’, do not consider the ith operand of grep to be an option, even if it appears to be one. A shell can put this variable in the environment for each command it runs, specifying which operands are the results of file name wildcard expansion and therefore should not be treated as options. This behavior is available only with the GNU C library, and only when POSIXLY_CORRECT is not set.


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2.3 Exit Status

Normally, the exit status is 0 if selected lines are found and 1 otherwise. But the exit status is 2 if an error occurred, unless the -q or --quiet or --silent option is used and a selected line is found. Note, however, that POSIX only mandates, for programs such as grep, cmp, and diff, that the exit status in case of error be greater than 1; it is therefore advisable, for the sake of portability, to use logic that tests for this general condition instead of strict equality with 2.


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2.4 grep Programs

grep searches the named input files for lines containing a match to the given pattern. By default, grep prints the matching lines. A file named - stands for standard input. If no input is specified, grep searches the working directory . if given a command-line option specifying recursion; otherwise, grep searches standard input. There are four major variants of grep, controlled by the following options.

-G
--basic-regexp
Interpret the pattern as a basic regular expression (BRE). This is the default.
-E
--extended-regexp
Interpret the pattern as an extended regular expression (ERE). (-E is specified by POSIX.)
-F
--fixed-strings
Interpret the pattern as a list of fixed strings, separated by newlines, any of which is to be matched. (-F is specified by POSIX.)
-P
--perl-regexp
Interpret the pattern as a Perl regular expression. This is highly experimental and ‘grep -P’ may warn of unimplemented features.

In addition, two variant programs egrep and fgrep are available. egrep is the same as ‘grep -E’. fgrep is the same as ‘grep -F’. Direct invocation as either egrep or fgrep is deprecated, but is provided to allow historical applications that rely on them to run unmodified.


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3 Regular Expressions

A regular expression is a pattern that describes a set of strings. Regular expressions are constructed analogously to arithmetic expressions, by using various operators to combine smaller expressions. grep understands three different versions of regular expression syntax: “basic,” (BRE) “extended” (ERE) and “perl”. In GNU grep, there is no difference in available functionality between the basic and extended syntaxes. In other implementations, basic regular expressions are less powerful. The following description applies to extended regular expressions; differences for basic regular expressions are summarized afterwards. Perl regular expressions give additional functionality, and are documented in the pcresyntax(3) and pcrepattern(3) manual pages, but may not be available on every system.


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3.1 Fundamental Structure

The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any meta-character with special meaning may be quoted by preceding it with a backslash.

A regular expression may be followed by one of several repetition operators:

.
The period ‘.’ matches any single character.
?
The preceding item is optional and will be matched at most once.
*
The preceding item will be matched zero or more times.
+
The preceding item will be matched one or more times.
{n}
The preceding item is matched exactly n times.
{n,}
The preceding item is matched n or more times.
{,m}
The preceding item is matched at most m times. This is a GNU extension.
{n,m}
The preceding item is matched at least n times, but not more than m times.

The empty regular expression matches the empty string. Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated expressions.

Two regular expressions may be joined by the infix operator ‘|’; the resulting regular expression matches any string matching either alternate expression.

Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole expression may be enclosed in parentheses to override these precedence rules and form a subexpression.


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3.2 Character Classes and Bracket Expressions

A bracket expression is a list of characters enclosed by ‘[’ and ‘]’. It matches any single character in that list; if the first character of the list is the caret ‘^’, then it matches any character not in the list. For example, the regular expression ‘[0123456789]’ matches any single digit.

Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, inclusive. In the default C locale, the sorting sequence is the native character order; for example, ‘[a-d]’ is equivalent to ‘[abcd]’. In other locales, the sorting sequence is not specified, and ‘[a-d]’ might be equivalent to ‘[abcd]’ or to ‘[aBbCcDd]’, or it might fail to match any character, or the set of characters that it matches might even be erratic. To obtain the traditional interpretation of bracket expressions, you can use the ‘C’ locale by setting the LC_ALL environment variable to the value ‘C’.

Finally, certain named classes of characters are predefined within bracket expressions, as follows. Their interpretation depends on the LC_CTYPE locale; for example, ‘[[:alnum:]]’ means the character class of numbers and letters in the current locale.

[:alnum:]
Alphanumeric characters: ‘[:alpha:]’ and ‘[:digit:]’; in the ‘C’ locale and ASCII character encoding, this is the same as ‘[0-9A-Za-z]’.
[:alpha:]
Alphabetic characters: ‘[:lower:]’ and ‘[:upper:]’; in the ‘C’ locale and ASCII character encoding, this is the same as ‘[A-Za-z]’.
[:blank:]
Blank characters: space and tab.
[:cntrl:]
Control characters. In ASCII, these characters have octal codes 000 through 037, and 177 (DEL). In other character sets, these are the equivalent characters, if any.
[:digit:]
Digits: 0 1 2 3 4 5 6 7 8 9.
[:graph:]
Graphical characters: ‘[:alnum:]’ and ‘[:punct:]’.
[:lower:]
Lower-case letters; in the ‘C’ locale and ASCII character encoding, this is a b c d e f g h i j k l m n o p q r s t u v w x y z.
[:print:]
Printable characters: ‘[:alnum:]’, ‘[:punct:]’, and space.
[:punct:]
Punctuation characters; in the ‘C’ locale and ASCII character encoding, this is ! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~.
[:space:]
Space characters: in the ‘C’ locale, this is tab, newline, vertical tab, form feed, carriage return, and space. See Usage, for more discussion of matching newlines.
[:upper:]
Upper-case letters: in the ‘C’ locale and ASCII character encoding, this is A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.
[:xdigit:]
Hexadecimal digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f.
Note that the brackets in these class names are part of the symbolic names, and must be included in addition to the brackets delimiting the bracket expression.

If you mistakenly omit the outer brackets, and search for say, ‘[:upper:]’, GNU grep prints a diagnostic and exits with status 2, on the assumption that you did not intend to search for the nominally equivalent regular expression: ‘[:epru]’. Set the POSIXLY_CORRECT environment variable to disable this feature.

Most meta-characters lose their special meaning inside bracket expressions.

]
ends the bracket expression if it's not the first list item. So, if you want to make the ‘]’ character a list item, you must put it first.
[.
represents the open collating symbol.
.]
represents the close collating symbol.
[=
represents the open equivalence class.
=]
represents the close equivalence class.
[:
represents the open character class symbol, and should be followed by a valid character class name.
:]
represents the close character class symbol.
-
represents the range if it's not first or last in a list or the ending point of a range.
^
represents the characters not in the list. If you want to make the ‘^’ character a list item, place it anywhere but first.


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3.3 The Backslash Character and Special Expressions

The ‘\’ character, when followed by certain ordinary characters, takes a special meaning:

\b
Match the empty string at the edge of a word.
\B
Match the empty string provided it's not at the edge of a word.
\<
Match the empty string at the beginning of word.
\>
Match the empty string at the end of word.
\w
Match word constituent, it is a synonym for ‘[_[:alnum:]]’.
\W
Match non-word constituent, it is a synonym for ‘[^_[:alnum:]]’.
\s
Match whitespace, it is a synonym for ‘[[:space:]]’.
\S
Match non-whitespace, it is a synonym for ‘[^[:space:]]’.

For example, ‘\brat\b’ matches the separate word ‘rat’, ‘\Brat\B’ matches ‘crate’ but not ‘furry rat’.


Next: , Previous: The Backslash Character and Special Expressions, Up: Regular Expressions

3.4 Anchoring

The caret ‘^’ and the dollar sign ‘$’ are meta-characters that respectively match the empty string at the beginning and end of a line. They are termed anchors, since they force the match to be “anchored” to beginning or end of a line, respectively.


Next: , Previous: Anchoring, Up: Regular Expressions

3.5 Back-references and Subexpressions

The back-reference ‘\n’, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression of the regular expression. For example, ‘(a)\1’ matches ‘aa’. When used with alternation, if the group does not participate in the match then the back-reference makes the whole match fail. For example, ‘a(.)|b\1’ will not match ‘ba’. When multiple regular expressions are given with -e or from a file (‘-f file’), back-references are local to each expression.


Previous: Back-references and Subexpressions, Up: Regular Expressions

3.6 Basic vs Extended Regular Expressions

In basic regular expressions the meta-characters ‘?’, ‘+’, ‘{’, ‘|’, ‘(’, and ‘)’ lose their special meaning; instead use the backslashed versions ‘\?’, ‘\+’, ‘\{’, ‘\|’, ‘\(’, and ‘\)’.

Traditional egrep did not support the ‘{’ meta-character, and some egrep implementations support ‘\{’ instead, so portable scripts should avoid ‘{’ in ‘grep -E’ patterns and should use ‘[{]’ to match a literal ‘{’.

GNU grep -E attempts to support traditional usage by assuming that ‘{’ is not special if it would be the start of an invalid interval specification. For example, the command ‘grep -E '{1'’ searches for the two-character string ‘{1’ instead of reporting a syntax error in the regular expression. POSIX allows this behavior as an extension, but portable scripts should avoid it.


Next: , Previous: Regular Expressions, Up: Top

4 Usage

Here is an example command that invokes GNU grep:

     grep -i 'hello.*world' menu.h main.c

This lists all lines in the files menu.h and main.c that contain the string ‘hello’ followed by the string ‘world’; this is because ‘.*’ matches zero or more characters within a line. See Regular Expressions. The -i option causes grep to ignore case, causing it to match the line ‘Hello, world!’, which it would not otherwise match. See Invoking, for more details about how to invoke grep.

Here are some common questions and answers about grep usage.

  1. How can I list just the names of matching files?
              grep -l 'main' *.c
    

    lists the names of all C files in the current directory whose contents mention ‘main’.

  2. How do I search directories recursively?
              grep -r 'hello' /home/gigi
    

    searches for ‘hello’ in all files under the /home/gigi directory. For more control over which files are searched, use find, grep, and xargs. For example, the following command searches only C files:

              find /home/gigi -name '*.c' -print0 | xargs -0r grep -H 'hello'
    

    This differs from the command:

              grep -H 'hello' *.c
    

    which merely looks for ‘hello’ in all files in the current directory whose names end in ‘.c’. The ‘find ...’ command line above is more similar to the command:

              grep -rH --include='*.c' 'hello' /home/gigi
    
  3. What if a pattern has a leading ‘-’?
              grep -e '--cut here--' *
    

    searches for all lines matching ‘--cut here--’. Without -e, grep would attempt to parse ‘--cut here--’ as a list of options.

  4. Suppose I want to search for a whole word, not a part of a word?
              grep -w 'hello' *
    

    searches only for instances of ‘hello’ that are entire words; it does not match ‘Othello’. For more control, use ‘\<’ and ‘\>’ to match the start and end of words. For example:

              grep 'hello\>' *
    

    searches only for words ending in ‘hello’, so it matches the word ‘Othello’.

  5. How do I output context around the matching lines?
              grep -C 2 'hello' *
    

    prints two lines of context around each matching line.

  6. How do I force grep to print the name of the file?

    Append /dev/null:

              grep 'eli' /etc/passwd /dev/null
    

    gets you:

              /etc/passwd:eli:x:2098:1000:Eli Smith:/home/eli:/bin/bash
    

    Alternatively, use -H, which is a GNU extension:

              grep -H 'eli' /etc/passwd
    
  7. Why do people use strange regular expressions on ps output?
              ps -ef | grep '[c]ron'
    

    If the pattern had been written without the square brackets, it would have matched not only the ps output line for cron, but also the ps output line for grep. Note that on some platforms, ps limits the output to the width of the screen; grep does not have any limit on the length of a line except the available memory.

  8. Why does grep report “Binary file matches”?

    If grep listed all matching “lines” from a binary file, it would probably generate output that is not useful, and it might even muck up your display. So GNU grep suppresses output from files that appear to be binary files. To force GNU grep to output lines even from files that appear to be binary, use the -a or ‘--binary-files=text’ option. To eliminate the “Binary file matches” messages, use the -I or ‘--binary-files=without-match’ option.

  9. Why doesn't ‘grep -lv’ print non-matching file names?

    grep -lv’ lists the names of all files containing one or more lines that do not match. To list the names of all files that contain no matching lines, use the -L or --files-without-match option.

  10. I can do “OR” with ‘|’, but what about “AND”?
              grep 'paul' /etc/motd | grep 'franc,ois'
    

    finds all lines that contain both ‘paul’ and ‘franc,ois’.

  11. Why does the empty pattern match every input line?

    The grep command searches for lines that contain strings that match a pattern. Every line contains the empty string, so an empty pattern causes grep to find a match on each line. It is not the only such pattern: ‘^’, ‘$’, ‘.*’, and many other patterns cause grep to match every line.

    To match empty lines, use the pattern ‘^$’. To match blank lines, use the pattern ‘^[[:blank:]]*$’. To match no lines at all, use the command ‘grep -f /dev/null’.

  12. How can I search in both standard input and in files?

    Use the special file name ‘-’:

              cat /etc/passwd | grep 'alain' - /etc/motd
    
  13. How to express palindromes in a regular expression?

    It can be done by using back-references; for example, a palindrome of 4 characters can be written with a BRE:

              grep -w -e '\(.\)\(.\).\2\1' file
    

    It matches the word “radar” or “civic.”

    Guglielmo Bondioni proposed a single RE that finds all palindromes up to 19 characters long using 9 subexpressions and 9 back-references:

              grep -E -e '^(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?)(.?).?\9\8\7\6\5\4\3\2\1$' file
    

    Note this is done by using GNU ERE extensions; it might not be portable to other implementations of grep.

  14. Why is this back-reference failing?
              echo 'ba' | grep -E '(a)\1|b\1'
    

    This gives no output, because the first alternate ‘(a)\1’ does not match, as there is no ‘aa’ in the input, so the ‘\1’ in the second alternate has nothing to refer back to, meaning it will never match anything. (The second alternate in this example can only match if the first alternate has matched—making the second one superfluous.)

  15. How can I match across lines?

    Standard grep cannot do this, as it is fundamentally line-based. Therefore, merely using the [:space:] character class does not match newlines in the way you might expect. However, if your grep is compiled with Perl patterns enabled, the Perl ‘s’ modifier (which makes . match newlines) can be used:

              printf 'foo\nbar\n' | grep -P '(?s)foo.*?bar'
    

    With the GNU grep option -z (see File and Directory Selection), the input is terminated by null bytes. Thus, you can match newlines in the input, but the output will be the whole file, so this is really only useful to determine if the pattern is present:

              printf 'foo\nbar\n' | grep -z -q 'foo[[:space:]]\+bar'
    

    Failing either of those options, you need to transform the input before giving it to grep, or turn to awk, sed, perl, or many other utilities that are designed to operate across lines.

  16. What do grep, fgrep, and egrep stand for?

    The name grep comes from the way line editing was done on Unix. For example, ed uses the following syntax to print a list of matching lines on the screen:

              global/regular expression/print
              g/re/p
    

    fgrep stands for Fixed grep; egrep stands for Extended grep.


Next: , Previous: Usage, Up: Top

5 Reporting bugs

Email bug reports to bug-grep@gnu.org, a mailing list whose web page is http://lists.gnu.org/mailman/listinfo/bug-grep. The Savannah bug tracker for grep is located at http://savannah.gnu.org/bugs/?group=grep.

5.1 Known Bugs

Large repetition counts in the ‘{n,m}’ construct may cause grep to use lots of memory. In addition, certain other obscure regular expressions require exponential time and space, and may cause grep to run out of memory.

Back-references are very slow, and may require exponential time.


Next: , Previous: Reporting Bugs, Up: Top

6 Copying

GNU grep is licensed under the GNU GPL, which makes it free software.

The “free” in “free software” refers to liberty, not price. As some GNU project advocates like to point out, think of “free speech” rather than “free beer”. In short, you have the right (freedom) to run and change grep and distribute it to other people, and—if you want—charge money for doing either. The important restriction is that you have to grant your recipients the same rights and impose the same restrictions.

This general method of licensing software is sometimes called open source. The GNU project prefers the term “free software” for reasons outlined at http://www.gnu.org/philosophy/open-source-misses-the-point.html.

This manual is free documentation in the same sense. The documentation license is included below. The license for the program is available with the source code, or at http://www.gnu.org/licenses/gpl.html.


Up: Copying

6.1 GNU Free Documentation License

Version 1.3, 3 November 2008
     Copyright © 2000-2002, 2007-2008, 2010-2014 Free Software
     Foundation, Inc.
     http://fsf.org/
     
     Everyone is permitted to copy and distribute verbatim copies
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    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.

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  4. COPYING IN QUANTITY

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  5. MODIFICATIONS

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    1. 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.
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ADDENDUM: How to use this License for your documents

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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.


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Index


Footnotes

[1] Of course, 7th Edition Unix predated POSIX by several years!