1 \input texinfo @c -*-texinfo-*-
4 @documentencoding UTF-8
11 This manual is for GNU Gzip
12 (version @value{VERSION}, @value{UPDATED}),
13 and documents commands for compressing and decompressing data.
15 Copyright @copyright{} 1998-1999, 2001-2002, 2006-2007, 2009-2018 Free Software
18 Copyright @copyright{} 1992, 1993 Jean-loup Gailly
21 Permission is granted to copy, distribute and/or modify this document
22 under the terms of the GNU Free Documentation License, Version 1.3 or
23 any later version published by the Free Software Foundation; with no
24 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
25 Texts. A copy of the license is included in the section entitled ``GNU
26 Free Documentation License''.
30 @dircategory Compression
32 * Gzip: (gzip). General (de)compression of files (lzw).
35 @dircategory Individual utilities
37 * gunzip: (gzip)Overview. Decompression.
38 * gzexe: (gzip)Overview. Compress executables.
39 * zcat: (gzip)Overview. Decompression to stdout.
40 * zdiff: (gzip)Overview. Compare compressed files.
41 * zforce: (gzip)Overview. Force .gz extension on files.
42 * zgrep: (gzip)Overview. Search compressed files.
43 * zmore: (gzip)Overview. Decompression output by pages.
48 @subtitle The data compression program
49 @subtitle for Gzip version @value{VERSION}
50 @subtitle @value{UPDATED}
51 @author by Jean-loup Gailly
54 @vskip 0pt plus 1filll
62 @top GNU Gzip: General file (de)compression
68 * Overview:: Preliminary information.
69 * Sample:: Sample output from @command{gzip}.
70 * Invoking gzip:: How to run @command{gzip}.
71 * Advanced usage:: Concatenated files.
72 * Environment:: The @env{GZIP} environment variable
73 * Tapes:: Using @command{gzip} on tapes.
74 * Problems:: Reporting bugs.
75 * GNU Free Documentation License:: Copying and sharing this manual.
76 * Concept index:: Index of concepts.
83 @command{gzip} reduces the size of the named files using Lempel--Ziv coding
84 (LZ77). Whenever possible, each file is replaced by one with the
85 extension @samp{.gz}, while keeping the same ownership modes, access and
86 modification times. (The default extension is
87 @samp{z} for @abbr{MSDOS}, @abbr{OS/2} @abbr{FAT} and Atari.)
88 If no files are specified or
89 if a file name is @file{-}, the standard input is compressed to the standard
90 output. @command{gzip} will only attempt to compress regular files. In
91 particular, it will ignore symbolic links.
93 If the new file name is too long for its file system, @command{gzip}
94 truncates it. @command{gzip} attempts to truncate only the parts of the
95 file name longer than 3 characters. (A part is delimited by dots.) If
96 the name consists of small parts only, the longest parts are truncated.
97 For example, if file names are limited to 14 characters, gzip.msdos.exe
98 is compressed to gzi.msd.exe.gz. Names are not truncated on systems
99 which do not have a limit on file name length.
101 By default, @command{gzip} keeps the original file name in the
102 compressed file. This can be useful when decompressing the file with
103 @option{-N} if the compressed file name was truncated after a file
106 If the original is a regular file, @command{gzip} by default keeps its
107 timestamp in the compressed file. This can be useful when
108 decompressing the file with @option{-N} if the timestamp was not
109 preserved after a file transfer.
110 However, due to limitations in the current @command{gzip} file
111 format, fractional seconds are discarded. Also, timestamps must fall
112 within the range 1970-01-01 00:00:01 through 2106-02-07 06:28:15
113 @abbr{UTC}, and hosts whose operating systems use 32-bit timestamps
114 are further restricted to timestamps no later than 2038-01-19
115 03:14:07 @abbr{UTC}. The upper bounds assume the typical case
116 where leap seconds are ignored.
118 Compressed files can be restored to their original form using @samp{gzip -d}
119 or @command{gunzip} or @command{zcat}. If the original name saved in the
120 compressed file is not suitable for its file system, a new name is
121 constructed from the original one to make it legal.
123 @command{gunzip} takes a list of files on its command line and replaces
124 each file whose name ends with @samp{.gz}, @samp{.z}
125 @samp{-gz}, @samp{-z}, or @samp{_z} (ignoring case)
126 and which begins with the correct
127 magic number with an uncompressed file without the original extension.
128 @command{gunzip} also recognizes the special extensions @samp{.tgz} and
129 @samp{.taz} as shorthands for @samp{.tar.gz} and @samp{.tar.Z}
130 respectively. When compressing, @command{gzip} uses the @samp{.tgz}
131 extension if necessary instead of truncating a file with a @samp{.tar}
134 @command{gunzip} can currently decompress files created by @command{gzip},
135 @command{zip}, @command{compress} or @command{pack}. The detection of the input
136 format is automatic. When using the first two formats, @command{gunzip}
137 checks a 32 bit @abbr{CRC} (cyclic redundancy check). For @command{pack},
138 @command{gunzip} checks the uncompressed length. The @command{compress} format
139 was not designed to allow consistency checks. However @command{gunzip} is
140 sometimes able to detect a bad @samp{.Z} file. If you get an error when
141 uncompressing a @samp{.Z} file, do not assume that the @samp{.Z} file is
142 correct simply because the standard @command{uncompress} does not complain.
143 This generally means that the standard @command{uncompress} does not check
144 its input, and happily generates garbage output. The @abbr{SCO} @samp{compress
145 -H} format (@abbr{LZH} compression method) does not include a @abbr{CRC} but
146 also allows some consistency checks.
148 Files created by @command{zip} can be uncompressed by @command{gzip} only if
149 they have a single member compressed with the ``deflation'' method. This
150 feature is only intended to help conversion of @file{tar.zip} files to
151 the @file{tar.gz} format. To extract a @command{zip} file with a single
152 member, use a command like @samp{gunzip <foo.zip} or @samp{gunzip -S
153 .zip foo.zip}. To extract @command{zip} files with several
154 members, use @command{unzip} instead of @command{gunzip}.
156 @command{zcat} is identical to @samp{gunzip -c}. @command{zcat}
157 uncompresses either a list of files on the command line or its standard
158 input and writes the uncompressed data on standard output. @command{zcat}
159 will uncompress files that have the correct magic number whether they
160 have a @samp{.gz} suffix or not.
162 @command{gzip} uses the Lempel--Ziv algorithm used in @command{zip} and
164 The amount of compression obtained depends on the size of the input and
165 the distribution of common substrings. Typically, text such as source
166 code or English is reduced by 60--70%. Compression is generally much
167 better than that achieved by @abbr{LZW} (as used in @command{compress}), Huffman
168 coding (as used in @command{pack}), or adaptive Huffman coding
171 Compression is always performed, even if the compressed file is slightly
172 larger than the original. The worst case expansion is a few bytes for
173 the @command{gzip} file header, plus 5 bytes every 32K block, or an expansion
174 ratio of 0.015% for large files. Note that the actual number of used
175 disk blocks almost never increases. @command{gzip} normally preserves the mode,
176 ownership and timestamps of files when compressing or decompressing.
178 The @command{gzip} file format is specified in P. Deutsch, GZIP file
179 format specification version 4.3,
180 @uref{https://www.ietf.org/rfc/rfc1952.txt, Internet @abbr{RFC} 1952} (May
181 1996). The @command{zip} deflation format is specified in P. Deutsch,
182 DEFLATE Compressed Data Format Specification version 1.3,
183 @uref{https://www.ietf.org/rfc/rfc1951.txt, Internet @abbr{RFC} 1951} (May
187 @chapter Sample output
190 Here are some realistic examples of running @command{gzip}.
192 This is the output of the command @samp{gzip -h}:
195 Usage: gzip [OPTION]... [FILE]...
196 Compress or uncompress FILEs (by default, compress FILES in-place).
198 Mandatory arguments to long options are mandatory for short options too.
200 -c, --stdout write on standard output, keep original files unchanged
201 -d, --decompress decompress
202 -f, --force force overwrite of output file and compress links
203 -h, --help give this help
204 -k, --keep keep (don't delete) input files
205 -l, --list list compressed file contents
206 -L, --license display software license
207 -n, --no-name do not save or restore the original name and timestamp
208 -N, --name save or restore the original name and timestamp
209 -q, --quiet suppress all warnings
210 -r, --recursive operate recursively on directories
211 --rsyncable make rsync-friendly archive
212 -S, --suffix=SUF use suffix SUF on compressed files
213 --synchronous synchronous output (safer if system crashes, but slower)
214 -t, --test test compressed file integrity
215 -v, --verbose verbose mode
216 -V, --version display version number
217 -1, --fast compress faster
218 -9, --best compress better
220 With no FILE, or when FILE is -, read standard input.
222 Report bugs to <bug-gzip@@gnu.org>.
225 This is the output of the command @samp{gzip -v texinfo.tex}:
228 texinfo.tex: 69.3% -- replaced with texinfo.tex.gz
231 The following command will find all regular @samp{.gz} files in the
232 current directory and subdirectories (skipping file names that contain
233 newlines), and extract them in place without destroying the original,
234 stopping on the first failure:
238 *' -prune -o -name '*.gz' -type f -print |
241 s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/
247 @chapter Invoking @command{gzip}
251 The format for running the @command{gzip} program is:
254 gzip @var{option} @dots{}
257 @command{gzip} supports the following options:
263 Write output on standard output; keep original files unchanged.
264 If there are several input files, the output consists of a sequence of
265 independently compressed members. To obtain better compression,
266 concatenate all input files before compressing them.
275 Force compression or decompression even if the file has multiple links
276 or the corresponding file already exists, or if the compressed data
277 is read from or written to a terminal. If the input data is not in
278 a format recognized by @command{gzip}, and if the option @option{--stdout} is also
279 given, copy the input data without change to the standard output: let
280 @command{zcat} behave as @command{cat}. If @option{-f} is not given, and
281 when not running in the background, @command{gzip} prompts to verify
282 whether an existing file should be overwritten.
286 Print an informative help message describing the options then quit.
290 Keep (don't delete) input files during compression or decompression.
294 For each compressed file, list the following fields:
297 compressed size: size of the compressed file
298 uncompressed size: size of the uncompressed file
299 ratio: compression ratio (0.0% if unknown)
300 uncompressed_name: name of the uncompressed file
303 The uncompressed size is given as @minus{}1 for files not in @command{gzip}
304 format, such as compressed @samp{.Z} files. To get the uncompressed size for
305 such a file, you can use:
311 In combination with the @option{--verbose} option, the following fields are also
315 method: compression method (deflate,compress,lzh,pack)
316 crc: the 32-bit CRC of the uncompressed data
317 date & time: timestamp for the uncompressed file
320 The @abbr{CRC} is given as ffffffff for a file not in gzip format.
322 With @option{--verbose}, the size totals and compression ratio for all files
323 is also displayed, unless some sizes are unknown. With @option{--quiet},
324 the title and totals lines are not displayed.
326 The @command{gzip} format represents the input size modulo
327 @math{2^32}, so the uncompressed size and compression ratio are listed
328 incorrectly for uncompressed files 4 GiB and larger. To work around
329 this problem, you can use the following command to discover a large
330 uncompressed file's true size:
338 Display the @command{gzip} license then quit.
342 When compressing, do not save the original file name and timestamp by
343 default. (The original name is always saved if the name had to be
344 truncated.) When decompressing, do not restore the original file name
345 if present (remove only the @command{gzip}
346 suffix from the compressed file name) and do not restore the original
347 timestamp if present (copy it from the compressed file). This option
348 is the default when decompressing.
352 When compressing, always save the original file name, and save
353 the original timestamp if the original is a regular file; this
354 is the default. When decompressing, restore the original file name and
355 timestamp if present. This option is useful on systems which have
356 a limit on file name length or when the timestamp has been lost after
361 Suppress all warning messages.
365 Travel the directory structure recursively. If any of the file names
366 specified on the command line are directories, @command{gzip} will descend
367 into the directory and compress all the files it finds there (or
368 decompress them in the case of @command{gunzip}).
371 Cater better to the @command{rsync} program by periodically resetting
372 the internal structure of the compressed data stream. This lets the
373 @code{rsync} program take advantage of similarities in the uncompressed
374 input when synchronizing two files compressed with this flag. The cost:
375 the compressed output is usually about one percent larger.
377 @item --suffix @var{suf}
379 Use suffix @var{suf} instead of @samp{.gz}. Any suffix can be
380 given, but suffixes other than @samp{.z} and @samp{.gz} should be
381 avoided to avoid confusion when files are transferred to other systems.
382 A null suffix forces gunzip to try decompression on all given files
383 regardless of suffix, as in:
386 gunzip -S "" * (*.* for MSDOS)
389 Previous versions of gzip used the @samp{.z} suffix. This was changed to
390 avoid a conflict with @command{pack}.
393 Use synchronous output, by transferring output data to the output
394 file's storage device when the file system supports this. Because
395 file system data can be cached, without this option if the system
396 crashes around the time a command like @samp{gzip FOO} is run the user
397 might lose both @file{FOO} and @file{FOO.gz}; this is the default with
398 @command{gzip}, just as it is the default with most applications that
399 move data. When this option is used, @command{gzip} is safer but can
400 be considerably slower.
404 Test. Check the compressed file integrity.
408 Verbose. Display the name and percentage reduction for each file compressed.
412 Version. Display the version number and compilation options, then quit.
417 Regulate the speed of compression using the specified digit @var{n},
418 where @option{-1} or @option{--fast} indicates the fastest compression
419 method (less compression) and @option{--best} or @option{-9} indicates the
420 slowest compression method (optimal compression). The default
421 compression level is @option{-6} (that is, biased towards high compression at
426 @chapter Advanced usage
427 @cindex concatenated files
429 Multiple compressed files can be concatenated. In this case,
430 @command{gunzip} will extract all members at once. If one member is
431 damaged, other members might still be recovered after removal of the
432 damaged member. Better compression can be usually obtained if all
433 members are decompressed and then recompressed in a single step.
435 This is an example of concatenating @command{gzip} files:
438 gzip -c file1 > foo.gz
439 gzip -c file2 >> foo.gz
456 In case of damage to one member of a @samp{.gz} file, other members can
457 still be recovered (if the damaged member is removed). However,
458 you can get better compression by compressing all members at once:
461 cat file1 file2 | gzip > foo.gz
465 compresses better than
468 gzip -c file1 file2 > foo.gz
471 If you want to recompress concatenated files to get better compression, do:
474 zcat old.gz | gzip > new.gz
477 If a compressed file consists of several members, the uncompressed
478 size and @abbr{CRC} reported by the @option{--list} option applies to
480 only. If you need the uncompressed size for all members, you can use:
486 If you wish to create a single archive file with multiple members so
487 that members can later be extracted independently, use an archiver such
488 as @command{tar} or @command{zip}. @acronym{GNU} @command{tar}
489 supports the @option{-z}
490 option to invoke @command{gzip} transparently. @command{gzip} is designed as a
491 complement to @command{tar}, not as a replacement.
497 The obsolescent environment variable @env{GZIP} can hold a set of
498 default options for @command{gzip}. These options are interpreted
499 first and can be overwritten by explicit command line parameters. As
500 this can cause problems when using scripts, this feature is supported
501 only for options that are reasonably likely to not cause too much
502 harm, and @command{gzip} warns if it is used. This feature will be
503 removed in a future release of @command{gzip}.
505 You can use an alias or script instead. For example, if
506 @command{gzip} is in the directory @samp{/usr/bin} you can prepend
507 @file{$HOME/bin} to your @env{PATH} and create an executable script
508 @file{$HOME/bin/gzip} containing the following:
517 @chapter Using @command{gzip} on tapes
520 When writing compressed data to a tape, it is generally necessary to pad
521 the output with zeroes up to a block boundary. When the data is read and
522 the whole block is passed to @command{gunzip} for decompression,
523 @command{gunzip} detects that there is extra trailing garbage after the
524 compressed data and emits a warning by default if the garbage contains
525 nonzero bytes. You can use the @option{--quiet} option to suppress
529 @chapter Reporting Bugs
532 If you find a bug in @command{gzip}, please send electronic mail to
533 @email{bug-gzip@@gnu.org}. Include the version number,
534 which you can find by running @w{@samp{gzip -V}}. Also include in your
535 message the hardware and operating system, the compiler used to compile
537 a description of the bug behavior, and the input to @command{gzip}
541 @node GNU Free Documentation License
542 @appendix GNU Free Documentation License
547 @appendix Concept index