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, 2009-2016 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 @samp{-gz} for @abbr{VMS},
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 and time stamp in
102 the compressed file. These are used when decompressing the file with the
103 @option{-N} option. This is useful when the compressed file name was
104 truncated or when the time stamp was not preserved after a file
105 transfer. However, due to limitations in the current @command{gzip} file
106 format, fractional seconds are discarded. Also, time stamps must fall
107 within the range 1970-01-01 00:00:00 through 2106-02-07 06:28:15
108 @abbr{UTC}, and hosts whose operating systems use 32-bit time
109 stamps are further restricted to time stamps no later than 2038-01-19
110 03:14:07 @abbr{UTC}. The upper bounds assume the typical case
111 where leap seconds are ignored.
113 Compressed files can be restored to their original form using @samp{gzip -d}
114 or @command{gunzip} or @command{zcat}. If the original name saved in the
115 compressed file is not suitable for its file system, a new name is
116 constructed from the original one to make it legal.
118 @command{gunzip} takes a list of files on its command line and replaces
119 each file whose name ends with @samp{.gz}, @samp{.z}
120 @samp{-gz}, @samp{-z}, or @samp{_z} (ignoring case)
121 and which begins with the correct
122 magic number with an uncompressed file without the original extension.
123 @command{gunzip} also recognizes the special extensions @samp{.tgz} and
124 @samp{.taz} as shorthands for @samp{.tar.gz} and @samp{.tar.Z}
125 respectively. When compressing, @command{gzip} uses the @samp{.tgz}
126 extension if necessary instead of truncating a file with a @samp{.tar}
129 @command{gunzip} can currently decompress files created by @command{gzip},
130 @command{zip}, @command{compress} or @command{pack}. The detection of the input
131 format is automatic. When using the first two formats, @command{gunzip}
132 checks a 32 bit @abbr{CRC} (cyclic redundancy check). For @command{pack},
133 @command{gunzip} checks the uncompressed length. The @command{compress} format
134 was not designed to allow consistency checks. However @command{gunzip} is
135 sometimes able to detect a bad @samp{.Z} file. If you get an error when
136 uncompressing a @samp{.Z} file, do not assume that the @samp{.Z} file is
137 correct simply because the standard @command{uncompress} does not complain.
138 This generally means that the standard @command{uncompress} does not check
139 its input, and happily generates garbage output. The @abbr{SCO} @samp{compress
140 -H} format (@abbr{LZH} compression method) does not include a @abbr{CRC} but
141 also allows some consistency checks.
143 Files created by @command{zip} can be uncompressed by @command{gzip} only if
144 they have a single member compressed with the ``deflation'' method. This
145 feature is only intended to help conversion of @file{tar.zip} files to
146 the @file{tar.gz} format. To extract a @command{zip} file with a single
147 member, use a command like @samp{gunzip <foo.zip} or @samp{gunzip -S
148 .zip foo.zip}. To extract @command{zip} files with several
149 members, use @command{unzip} instead of @command{gunzip}.
151 @command{zcat} is identical to @samp{gunzip -c}. @command{zcat}
152 uncompresses either a list of files on the command line or its standard
153 input and writes the uncompressed data on standard output. @command{zcat}
154 will uncompress files that have the correct magic number whether they
155 have a @samp{.gz} suffix or not.
157 @command{gzip} uses the Lempel--Ziv algorithm used in @command{zip} and
159 The amount of compression obtained depends on the size of the input and
160 the distribution of common substrings. Typically, text such as source
161 code or English is reduced by 60--70%. Compression is generally much
162 better than that achieved by @abbr{LZW} (as used in @command{compress}), Huffman
163 coding (as used in @command{pack}), or adaptive Huffman coding
166 Compression is always performed, even if the compressed file is slightly
167 larger than the original. The worst case expansion is a few bytes for
168 the @command{gzip} file header, plus 5 bytes every 32K block, or an expansion
169 ratio of 0.015% for large files. Note that the actual number of used
170 disk blocks almost never increases. @command{gzip} normally preserves the mode,
171 ownership and time stamps of files when compressing or decompressing.
173 The @command{gzip} file format is specified in P. Deutsch, GZIP file
174 format specification version 4.3,
175 @uref{http://www.ietf.org/rfc/rfc1952.txt, Internet @abbr{RFC} 1952} (May
176 1996). The @command{zip} deflation format is specified in P. Deutsch,
177 DEFLATE Compressed Data Format Specification version 1.3,
178 @uref{http://www.ietf.org/rfc/rfc1951.txt, Internet @abbr{RFC} 1951} (May
182 @chapter Sample output
185 Here are some realistic examples of running @command{gzip}.
187 This is the output of the command @samp{gzip -h}:
190 Usage: gzip [OPTION]... [FILE]...
191 Compress or uncompress FILEs (by default, compress FILES in-place).
193 Mandatory arguments to long options are mandatory for short options too.
195 -c, --stdout write on standard output, keep original files unchanged
196 -d, --decompress decompress
197 -f, --force force overwrite of output file and compress links
198 -h, --help give this help
199 -k, --keep keep (don't delete) input files
200 -l, --list list compressed file contents
201 -L, --license display software license
202 -n, --no-name do not save or restore the original name and time stamp
203 -N, --name save or restore the original name and time stamp
204 -q, --quiet suppress all warnings
205 -r, --recursive operate recursively on directories
206 --rsyncable make rsync-friendly archive
207 -S, --suffix=SUF use suffix SUF on compressed files
208 --synchronous synchronous output (safer if system crashes, but slower)
209 -t, --test test compressed file integrity
210 -v, --verbose verbose mode
211 -V, --version display version number
212 -1, --fast compress faster
213 -9, --best compress better
215 With no FILE, or when FILE is -, read standard input.
217 Report bugs to <bug-gzip@@gnu.org>.
220 This is the output of the command @samp{gzip -v texinfo.tex}:
223 texinfo.tex: 69.3% -- replaced with texinfo.tex.gz
226 The following command will find all regular @samp{.gz} files in the
227 current directory and subdirectories (skipping file names that contain
228 newlines), and extract them in place without destroying the original,
229 stopping on the first failure:
233 *' -prune -o -name '*.gz' -type f -print |
236 s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/
242 @chapter Invoking @command{gzip}
246 The format for running the @command{gzip} program is:
249 gzip @var{option} @dots{}
252 @command{gzip} supports the following options:
258 Write output on standard output; keep original files unchanged.
259 If there are several input files, the output consists of a sequence of
260 independently compressed members. To obtain better compression,
261 concatenate all input files before compressing them.
270 Force compression or decompression even if the file has multiple links
271 or the corresponding file already exists, or if the compressed data
272 is read from or written to a terminal. If the input data is not in
273 a format recognized by @command{gzip}, and if the option @option{--stdout} is also
274 given, copy the input data without change to the standard output: let
275 @command{zcat} behave as @command{cat}. If @option{-f} is not given, and
276 when not running in the background, @command{gzip} prompts to verify
277 whether an existing file should be overwritten.
281 Print an informative help message describing the options then quit.
285 Keep (don't delete) input files during compression or decompression.
289 For each compressed file, list the following fields:
292 compressed size: size of the compressed file
293 uncompressed size: size of the uncompressed file
294 ratio: compression ratio (0.0% if unknown)
295 uncompressed_name: name of the uncompressed file
298 The uncompressed size is given as @minus{}1 for files not in @command{gzip}
299 format, such as compressed @samp{.Z} files. To get the uncompressed size for
300 such a file, you can use:
306 In combination with the @option{--verbose} option, the following fields are also
310 method: compression method (deflate,compress,lzh,pack)
311 crc: the 32-bit CRC of the uncompressed data
312 date & time: time stamp for the uncompressed file
315 The @abbr{CRC} is given as ffffffff for a file not in gzip format.
317 With @option{--verbose}, the size totals and compression ratio for all files
318 is also displayed, unless some sizes are unknown. With @option{--quiet},
319 the title and totals lines are not displayed.
321 The @command{gzip} format represents the input size modulo
322 @math{2^32}, so the uncompressed size and compression ratio are listed
323 incorrectly for uncompressed files 4 GiB and larger. To work around
324 this problem, you can use the following command to discover a large
325 uncompressed file's true size:
333 Display the @command{gzip} license then quit.
337 When compressing, do not save the original file name and time stamp by
338 default. (The original name is always saved if the name had to be
339 truncated.) When decompressing, do not restore the original file name
340 if present (remove only the @command{gzip}
341 suffix from the compressed file name) and do not restore the original
342 time stamp if present (copy it from the compressed file). This option
343 is the default when decompressing.
347 When compressing, always save the original file name and time stamp; this
348 is the default. When decompressing, restore the original file name and
349 time stamp if present. This option is useful on systems which have
350 a limit on file name length or when the time stamp has been lost after
355 Suppress all warning messages.
359 Travel the directory structure recursively. If any of the file names
360 specified on the command line are directories, @command{gzip} will descend
361 into the directory and compress all the files it finds there (or
362 decompress them in the case of @command{gunzip}).
365 Cater better to the @command{rsync} program by periodically resetting
366 the internal structure of the compressed data stream. This lets the
367 @code{rsync} program take advantage of similarities in the uncompressed
368 input when synchronizing two files compressed with this flag. The cost:
369 the compressed output is usually about one percent larger.
371 @item --suffix @var{suf}
373 Use suffix @var{suf} instead of @samp{.gz}. Any suffix can be
374 given, but suffixes other than @samp{.z} and @samp{.gz} should be
375 avoided to avoid confusion when files are transferred to other systems.
376 A null suffix forces gunzip to try decompression on all given files
377 regardless of suffix, as in:
380 gunzip -S "" * (*.* for MSDOS)
383 Previous versions of gzip used the @samp{.z} suffix. This was changed to
384 avoid a conflict with @command{pack}.
387 Use synchronous output, by transferring output data to the output
388 file's storage device when the file system supports this. Because
389 file system data can be cached, without this option if the system
390 crashes around the time a command like @samp{gzip FOO} is run the user
391 might lose both @file{FOO} and @file{FOO.gz}; this is the default with
392 @command{gzip}, just as it is the default with most applications that
393 move data. When this option is used, @command{gzip} is safer but can
394 be considerably slower.
398 Test. Check the compressed file integrity.
402 Verbose. Display the name and percentage reduction for each file compressed.
406 Version. Display the version number and compilation options, then quit.
411 Regulate the speed of compression using the specified digit @var{n},
412 where @option{-1} or @option{--fast} indicates the fastest compression
413 method (less compression) and @option{--best} or @option{-9} indicates the
414 slowest compression method (optimal compression). The default
415 compression level is @option{-6} (that is, biased towards high compression at
420 @chapter Advanced usage
421 @cindex concatenated files
423 Multiple compressed files can be concatenated. In this case,
424 @command{gunzip} will extract all members at once. If one member is
425 damaged, other members might still be recovered after removal of the
426 damaged member. Better compression can be usually obtained if all
427 members are decompressed and then recompressed in a single step.
429 This is an example of concatenating @command{gzip} files:
432 gzip -c file1 > foo.gz
433 gzip -c file2 >> foo.gz
450 In case of damage to one member of a @samp{.gz} file, other members can
451 still be recovered (if the damaged member is removed). However,
452 you can get better compression by compressing all members at once:
455 cat file1 file2 | gzip > foo.gz
459 compresses better than
462 gzip -c file1 file2 > foo.gz
465 If you want to recompress concatenated files to get better compression, do:
468 zcat old.gz | gzip > new.gz
471 If a compressed file consists of several members, the uncompressed
472 size and @abbr{CRC} reported by the @option{--list} option applies to
474 only. If you need the uncompressed size for all members, you can use:
480 If you wish to create a single archive file with multiple members so
481 that members can later be extracted independently, use an archiver such
482 as @command{tar} or @command{zip}. @acronym{GNU} @command{tar}
483 supports the @option{-z}
484 option to invoke @command{gzip} transparently. @command{gzip} is designed as a
485 complement to @command{tar}, not as a replacement.
491 The obsolescent environment variable @env{GZIP} can hold a set of
492 default options for @command{gzip}. These options are interpreted
493 first and can be overwritten by explicit command line parameters. As
494 this can cause problems when using scripts, this feature is supported
495 only for options that are reasonably likely to not cause too much
496 harm, and @command{gzip} warns if it is used. This feature will be
497 removed in a future release of @command{gzip}.
499 You can use an alias or script instead. For example, if
500 @command{gzip} is in the directory @samp{/usr/bin} you can prepend
501 @file{$HOME/bin} to your @env{PATH} and create an executable script
502 @file{$HOME/bin/gzip} containing the following:
510 On @abbr{VMS}, the name of the obsolescent environment variable is
511 @env{GZIP_OPT}, to avoid a conflict with the symbol set for invocation
515 @chapter Using @command{gzip} on tapes
518 When writing compressed data to a tape, it is generally necessary to pad
519 the output with zeroes up to a block boundary. When the data is read and
520 the whole block is passed to @command{gunzip} for decompression,
521 @command{gunzip} detects that there is extra trailing garbage after the
522 compressed data and emits a warning by default if the garbage contains
523 nonzero bytes. You can use the @option{--quiet} option to suppress
527 @chapter Reporting Bugs
530 If you find a bug in @command{gzip}, please send electronic mail to
531 @email{bug-gzip@@gnu.org}. Include the version number,
532 which you can find by running @w{@samp{gzip -V}}. Also include in your
533 message the hardware and operating system, the compiler used to compile
535 a description of the bug behavior, and the input to @command{gzip}
539 @node GNU Free Documentation License
540 @appendix GNU Free Documentation License
545 @appendix Concept index