GNU Gzip: General file (de)compression 1 Overview 2 Sample output 3 Invoking ‘gzip’ 4 Advanced usage 5 Environment 6 Using ‘gzip’ on tapes 7 Reporting Bugs Appendix A GNU Free Documentation License Appendix B Concept index GNU Gzip: General file (de)compression ************************************** This manual is for GNU Gzip (version 1.13, 5 February 2023), and documents commands for compressing and decompressing data. Copyright © 1998–1999, 2001–2002, 2006–2007, 2009–2023 Free Software Foundation, Inc. Copyright © 1992, 1993 Jean-loup Gailly 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 Overview ********** ‘gzip’ reduces the size of the named files using Lempel–Ziv coding (LZ77). Whenever possible, each file is replaced by one with the extension ‘.gz’, while keeping the same ownership modes, access and modification times. (The default extension is ‘z’ for MSDOS, OS/2 FAT and Atari.) If no files are specified or if a file name is ‘-’, the standard input is compressed to the standard output. ‘gzip’ will only attempt to compress regular files. In particular, it will ignore symbolic links. If the new file name is too long for its file system, ‘gzip’ truncates it. ‘gzip’ attempts to truncate only the parts of the file name longer than 3 characters. (A part is delimited by dots.) If the name consists of small parts only, the longest parts are truncated. For example, if file names are limited to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz. Names are not truncated on systems which do not have a limit on file name length. By default, ‘gzip’ keeps the original file name in the compressed file. This can be useful when decompressing the file with ‘-N’ if the compressed file name was truncated after a file transfer. If the original is a regular file, ‘gzip’ by default keeps its timestamp in the compressed file. This can be useful when decompressing the file with ‘-N’ if the timestamp was not preserved after a file transfer. However, due to limitations in the current ‘gzip’ file format, fractional seconds are discarded. Also, timestamps must fall within the range 1970-01-01 00:00:01 through 2106-02-07 06:28:15 UTC, and hosts whose operating systems use 32-bit timestamps are further restricted to timestamps no later than 2038-01-19 03:14:07 UTC. The upper bounds assume the typical case where leap seconds are ignored. Compressed files can be restored to their original form using ‘gzip -d’ or ‘gunzip’ or ‘zcat’. If the original name saved in the compressed file is not suitable for its file system, a new name is constructed from the original one to make it legal. ‘gunzip’ takes a list of files on its command line and replaces each file whose name ends with ‘.gz’, ‘.z’ ‘-gz’, ‘-z’, or ‘_z’ (ignoring case) and which begins with the correct magic number with an uncompressed file without the original extension. ‘gunzip’ also recognizes the special extensions ‘.tgz’ and ‘.taz’ as shorthands for ‘.tar.gz’ and ‘.tar.Z’ respectively. When compressing, ‘gzip’ uses the ‘.tgz’ extension if necessary instead of truncating a file with a ‘.tar’ extension. ‘gunzip’ can currently decompress files created by ‘gzip’, ‘zip’, ‘compress’ or ‘pack’. The detection of the input format is automatic. When using the first two formats, ‘gunzip’ checks a 32 bit CRC (cyclic redundancy check). For ‘pack’, ‘gunzip’ checks the uncompressed length. The ‘compress’ format was not designed to allow consistency checks. However ‘gunzip’ is sometimes able to detect a bad ‘.Z’ file. If you get an error when uncompressing a ‘.Z’ file, do not assume that the ‘.Z’ file is correct simply because the standard ‘uncompress’ does not complain. This generally means that the standard ‘uncompress’ does not check its input, and happily generates garbage output. The SCO ‘compress -H’ format (LZH compression method) does not include a CRC but also allows some consistency checks. Files created by ‘zip’ can be uncompressed by ‘gzip’ only if they have a single member compressed with the “deflation” method. This feature is only intended to help conversion of ‘tar.zip’ files to the ‘tar.gz’ format. To extract a ‘zip’ file with a single member, use a command like ‘gunzip . This is the output of the command ‘gzip -v texinfo.tex’: texinfo.tex: 69.3% -- replaced with texinfo.tex.gz The following command will find all regular ‘.gz’ files in the current directory and subdirectories (skipping file names that contain newlines), and extract them in place without destroying the original, stopping on the first failure: find . -name '* *' -prune -o -name '*.gz' -type f -print | sed " s/'/'\\\\''/g s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/ " | sh -e 3 Invoking ‘gzip’ ***************** The format for running the ‘gzip’ program is: gzip OPTION ... ‘gzip’ supports the following options: ‘--stdout’ ‘--to-stdout’ ‘-c’ Write output on standard output; keep original files unchanged. If there are several input files, the output consists of a sequence of independently compressed members. To obtain better compression, concatenate all input files before compressing them. ‘--decompress’ ‘--uncompress’ ‘-d’ Decompress. ‘--force’ ‘-f’ Force compression or decompression even if the file has multiple links or the corresponding file already exists, or if the compressed data is read from or written to a terminal. If the input data is not in a format recognized by ‘gzip’, and if the option ‘--stdout’ is also given, copy the input data without change to the standard output: let ‘zcat’ behave as ‘cat’. If ‘-f’ is not given, and when not running in the background, ‘gzip’ prompts to verify whether an existing file should be overwritten. ‘--help’ ‘-h’ Print an informative help message describing the options then quit. ‘--keep’ ‘-k’ Keep (don’t delete) input files during compression or decompression. ‘--list’ ‘-l’ For each compressed file, list the following fields: compressed size: size of the compressed file uncompressed size: size of the uncompressed file ratio: compression ratio (0.0% if unknown) uncompressed_name: name of the uncompressed file In combination with the ‘--verbose’ option, the following fields are also displayed: method: compression method (deflate,compress,lzh,pack) crc: the 32-bit CRC of the uncompressed data date & time: timestamp for the uncompressed file The CRC is given as ffffffff for a file not in gzip format. With ‘--verbose’, the size totals and compression ratio for all files is also displayed, unless some sizes are unknown. With ‘--quiet’, the title and totals lines are not displayed. ‘--license’ ‘-L’ Display the ‘gzip’ license then quit. ‘--no-name’ ‘-n’ When compressing, do not save the original file name and timestamp by default. (The original name is always saved if the name had to be truncated.) When decompressing, do not restore the original file name if present (remove only the ‘gzip’ suffix from the compressed file name) and do not restore the original timestamp if present (copy it from the compressed file). This option is the default when decompressing. ‘--name’ ‘-N’ When compressing, always save the original file name, and save the seconds part of the original modification timestamp if the original is a regular file and its timestamp is at least 1 (1970-01-01 00:00:01 UTC) and is less than 2^{32} (2106-02-07 06:28:16 UTC, assuming leap seconds are not counted); this is the default. When decompressing, restore from the saved file name and timestamp if present. This option is useful on systems which have a limit on file name length or when the timestamp has been lost after a file transfer. ‘--quiet’ ‘-q’ Suppress all warning messages. ‘--recursive’ ‘-r’ Travel the directory structure recursively. If any of the file names specified on the command line are directories, ‘gzip’ will descend into the directory and compress all the files it finds there (or decompress them in the case of ‘gunzip’). ‘--rsyncable’ Cater better to the ‘rsync’ program by periodically resetting the internal structure of the compressed data stream. This lets the ‘rsync’ program take advantage of similarities in the uncompressed input when synchronizing two files compressed with this flag. The cost: the compressed output is usually about one percent larger. ‘--suffix SUF’ ‘-S SUF’ Use suffix SUF instead of ‘.gz’. Any suffix can be given, but suffixes other than ‘.z’ and ‘.gz’ should be avoided to avoid confusion when files are transferred to other systems. A null suffix forces gunzip to try decompression on all given files regardless of suffix, as in: gunzip -S "" * (*.* for MSDOS) Previous versions of gzip used the ‘.z’ suffix. This was changed to avoid a conflict with ‘pack’. ‘--synchronous’ Use synchronous output, by transferring output data to the output file’s storage device when the file system supports this. Because file system data can be cached, without this option if the system crashes around the time a command like ‘gzip FOO’ is run the user might lose both ‘FOO’ and ‘FOO.gz’; this is the default with ‘gzip’, just as it is the default with most applications that move data. When this option is used, ‘gzip’ is safer but can be considerably slower. ‘--test’ ‘-t’ Test. Check the compressed file integrity. ‘--verbose’ ‘-v’ Verbose. Display the name and percentage reduction for each file compressed. ‘--version’ ‘-V’ Version. Display the version number and compilation options, then quit. ‘--fast’ ‘--best’ ‘-N’ Regulate the speed of compression using the specified digit N, where ‘-1’ or ‘--fast’ indicates the fastest compression method (less compression) and ‘--best’ or ‘-9’ indicates the slowest compression method (optimal compression). The default compression level is ‘-6’ (that is, biased towards high compression at expense of speed). An exit status of 0 indicates success, 1 indicates failure, and 2 indicates a warning but not failure. 4 Advanced usage **************** Multiple compressed files can be concatenated. In this case, ‘gunzip’ will extract all members at once. If one member is damaged, other members might still be recovered after removal of the damaged member. Better compression can be usually obtained if all members are decompressed and then recompressed in a single step. This is an example of concatenating ‘gzip’ files: gzip -c file1 > foo.gz gzip -c file2 >> foo.gz Then gunzip -c foo is equivalent to cat file1 file2 In case of damage to one member of a ‘.gz’ file, other members can still be recovered (if the damaged member is removed). However, you can get better compression by compressing all members at once: cat file1 file2 | gzip > foo.gz compresses better than gzip -c file1 file2 > foo.gz If you want to recompress concatenated files to get better compression, do: zcat old.gz | gzip > new.gz If a compressed file consists of several members, the uncompressed size and CRC reported by the ‘--list’ option applies to the last member only. If you need the uncompressed size for all members, you can use: zcat file.gz | wc -c If you wish to create a single archive file with multiple members so that members can later be extracted independently, use an archiver such as ‘tar’ or ‘zip’. GNU ‘tar’ supports the ‘-z’ option to invoke ‘gzip’ transparently. ‘gzip’ is designed as a complement to ‘tar’, not as a replacement. 5 Environment ************* The obsolescent environment variable ‘GZIP’ can hold a set of default options for ‘gzip’. These options are interpreted first and can be overwritten by explicit command line parameters. As this can cause problems when using scripts, this feature is supported only for options that are reasonably likely to not cause too much harm, and ‘gzip’ warns if it is used. This feature will be removed in a future release of ‘gzip’. You can use an alias or script instead. For example, if ‘gzip’ is in the directory ‘/usr/bin’ you can prepend ‘$HOME/bin’ to your ‘PATH’ and create an executable script ‘$HOME/bin/gzip’ containing the following: #! /bin/sh export PATH=/usr/bin exec gzip -9 "$@" The following environment variables are applicable only when using ‘gzip’ on IBM Z mainframes supporting DEFLATE COMPRESSION CALL instruction: ‘DFLTCC’ Whether DEFLATE COMPRESSION CALL should be used. Default value is ‘1’. Set this to ‘0’ to disable DEFLATE COMPRESSION CALL altogether. ‘DFLTCC_LEVEL_MASK’ Compression levels on which DEFLATE COMPRESSION CALL should be used. Represented as a bit mask in decimal or hexadecimal form, where each bit corresponds to a compression level. Default value is ‘2’, which means level 1 only. In order to make use of DEFLATE COMPRESSION CALL by default, that is, on levels 1-6, set this to ‘0x7e’. ‘DFLTCC_BLOCK_SIZE’ Size of deflate blocks produced by DEFLATE COMPRESSION CALL in bytes in decimal or hexadecimal form. Default value is ‘1048576’ (1 megabyte). When using DEFLATE COMPRESSION CALL to compress a file containing heterogeneous data (e.g. a ‘.tar’ archive containing text and binary files), setting this to a smaller value may improve compression ratio. ‘DFLTCC_FIRST_FHT_BLOCK_SIZE’ Size of the first fixed deflate block produced by DEFLATE COMPRESSION CALL in bytes in decimal or hexadecimal form. Default value is ‘4096’ (4 kilobytes). When using DEFLATE COMPRESSION CALL to compress a small file, setting this to a larger value may improve compression ratio. ‘DFLTCC_RIBM’ Value of "Reserved for IBM" field of DEFLATE COMPRESSION CALL parameter block. Default value is ‘0’. ‘SOURCE_DATE_EPOCH’ If set to any value, disables compression with DEFLATE COMPRESSION CALL. This variable is normally set during reproducible builds, where DEFLATE COMPRESSION CALL must be disabled, because its output may not be reproducible. 6 Using ‘gzip’ on tapes *********************** When writing compressed data to a tape, it is generally necessary to pad the output with zeroes up to a block boundary. When the data is read and the whole block is passed to ‘gunzip’ for decompression, ‘gunzip’ detects that there is extra trailing garbage after the compressed data and emits a warning by default if the garbage contains nonzero bytes. You can use the ‘--quiet’ option to suppress the warning. 7 Reporting Bugs **************** If you find a bug in ‘gzip’, please send electronic mail to . Include the version number, which you can find by running ‘gzip -V’. Also include in your message the hardware and operating system, the compiler used to compile ‘gzip’, a description of the bug behavior, and the input to ‘gzip’ that triggered the bug. Appendix A GNU Free Documentation License ***************************************** Version 1.3, 3 November 2008 Copyright © 2000–2002, 2007–2008, 2023 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”. 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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. Appendix B Concept index ************************ * Menu: * bugs: Problems. (line 459) * concatenated files: Advanced usage. (line 339) * Environment: Environment. (line 388) * invoking: Invoking gzip. (line 181) * options: Invoking gzip. (line 181) * overview: Overview. (line 31) * sample: Sample. (line 128) * tapes: Tapes. (line 449)