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 -S, --suffix=SUF use suffix SUF on compressed files
207 -t, --test test compressed file integrity
208 -v, --verbose verbose mode
209 -V, --version display version number
210 -1, --fast compress faster
211 -9, --best compress better
213 With no FILE, or when FILE is -, read standard input.
215 Report bugs to <bug-gzip@@gnu.org>.
218 This is the output of the command @samp{gzip -v texinfo.tex}:
221 texinfo.tex: 69.3% -- replaced with texinfo.tex.gz
224 The following command will find all regular @samp{.gz} files in the
225 current directory and subdirectories (skipping file names that contain
226 newlines), and extract them in place without destroying the original,
227 stopping on the first failure:
231 *' -prune -o -name '*.gz' -type f -print |
234 s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/
240 @chapter Invoking @command{gzip}
244 The format for running the @command{gzip} program is:
247 gzip @var{option} @dots{}
250 @command{gzip} supports the following options:
256 Write output on standard output; keep original files unchanged.
257 If there are several input files, the output consists of a sequence of
258 independently compressed members. To obtain better compression,
259 concatenate all input files before compressing them.
268 Force compression or decompression even if the file has multiple links
269 or the corresponding file already exists, or if the compressed data
270 is read from or written to a terminal. If the input data is not in
271 a format recognized by @command{gzip}, and if the option @option{--stdout} is also
272 given, copy the input data without change to the standard output: let
273 @command{zcat} behave as @command{cat}. If @option{-f} is not given, and
274 when not running in the background, @command{gzip} prompts to verify
275 whether an existing file should be overwritten.
279 Print an informative help message describing the options then quit.
283 Keep (don't delete) input files during compression or decompression.
287 For each compressed file, list the following fields:
290 compressed size: size of the compressed file
291 uncompressed size: size of the uncompressed file
292 ratio: compression ratio (0.0% if unknown)
293 uncompressed_name: name of the uncompressed file
296 The uncompressed size is given as @minus{}1 for files not in @command{gzip}
297 format, such as compressed @samp{.Z} files. To get the uncompressed size for
298 such a file, you can use:
304 In combination with the @option{--verbose} option, the following fields are also
308 method: compression method (deflate,compress,lzh,pack)
309 crc: the 32-bit CRC of the uncompressed data
310 date & time: time stamp for the uncompressed file
313 The @abbr{CRC} is given as ffffffff for a file not in gzip format.
315 With @option{--verbose}, the size totals and compression ratio for all files
316 is also displayed, unless some sizes are unknown. With @option{--quiet},
317 the title and totals lines are not displayed.
319 The @command{gzip} format represents the input size modulo
320 @math{2^32}, so the uncompressed size and compression ratio are listed
321 incorrectly for uncompressed files 4 GiB and larger. To work around
322 this problem, you can use the following command to discover a large
323 uncompressed file's true size:
331 Display the @command{gzip} license then quit.
335 When compressing, do not save the original file name and time stamp by
336 default. (The original name is always saved if the name had to be
337 truncated.) When decompressing, do not restore the original file name
338 if present (remove only the @command{gzip}
339 suffix from the compressed file name) and do not restore the original
340 time stamp if present (copy it from the compressed file). This option
341 is the default when decompressing.
345 When compressing, always save the original file name and time stamp; this
346 is the default. When decompressing, restore the original file name and
347 time stamp if present. This option is useful on systems which have
348 a limit on file name length or when the time stamp has been lost after
353 Suppress all warning messages.
357 Travel the directory structure recursively. If any of the file names
358 specified on the command line are directories, @command{gzip} will descend
359 into the directory and compress all the files it finds there (or
360 decompress them in the case of @command{gunzip}).
362 @item --suffix @var{suf}
364 Use suffix @var{suf} instead of @samp{.gz}. Any suffix can be
365 given, but suffixes other than @samp{.z} and @samp{.gz} should be
366 avoided to avoid confusion when files are transferred to other systems.
367 A null suffix forces gunzip to try decompression on all given files
368 regardless of suffix, as in:
371 gunzip -S "" * (*.* for MSDOS)
374 Previous versions of gzip used the @samp{.z} suffix. This was changed to
375 avoid a conflict with @command{pack}.
379 Test. Check the compressed file integrity.
383 Verbose. Display the name and percentage reduction for each file compressed.
387 Version. Display the version number and compilation options, then quit.
392 Regulate the speed of compression using the specified digit @var{n},
393 where @option{-1} or @option{--fast} indicates the fastest compression
394 method (less compression) and @option{--best} or @option{-9} indicates the
395 slowest compression method (optimal compression). The default
396 compression level is @option{-6} (that is, biased towards high compression at
401 @chapter Advanced usage
402 @cindex concatenated files
404 Multiple compressed files can be concatenated. In this case,
405 @command{gunzip} will extract all members at once. If one member is
406 damaged, other members might still be recovered after removal of the
407 damaged member. Better compression can be usually obtained if all
408 members are decompressed and then recompressed in a single step.
410 This is an example of concatenating @command{gzip} files:
413 gzip -c file1 > foo.gz
414 gzip -c file2 >> foo.gz
431 In case of damage to one member of a @samp{.gz} file, other members can
432 still be recovered (if the damaged member is removed). However,
433 you can get better compression by compressing all members at once:
436 cat file1 file2 | gzip > foo.gz
440 compresses better than
443 gzip -c file1 file2 > foo.gz
446 If you want to recompress concatenated files to get better compression, do:
449 zcat old.gz | gzip > new.gz
452 If a compressed file consists of several members, the uncompressed
453 size and @abbr{CRC} reported by the @option{--list} option applies to
455 only. If you need the uncompressed size for all members, you can use:
461 If you wish to create a single archive file with multiple members so
462 that members can later be extracted independently, use an archiver such
463 as @command{tar} or @command{zip}. @acronym{GNU} @command{tar}
464 supports the @option{-z}
465 option to invoke @command{gzip} transparently. @command{gzip} is designed as a
466 complement to @command{tar}, not as a replacement.
472 The obsolescent environment variable @env{GZIP} can hold a set of
473 default options for @command{gzip}. These options are interpreted
474 first and can be overwritten by explicit command line parameters. As
475 this can cause problems when using scripts, this feature is supported
476 only for options that are reasonably likely to not cause too much
477 harm, and @command{gzip} warns if it is used. This feature will be
478 removed in a future release of @command{gzip}.
480 You can use an alias or script instead. For example, if
481 @command{gzip} is in the directory @samp{/usr/bin} you can prepend
482 @file{$HOME/bin} to your @env{PATH} and create an executable script
483 @file{$HOME/bin/gzip} containing the following:
491 On @abbr{VMS}, the name of the obsolescent environment variable is
492 @env{GZIP_OPT}, to avoid a conflict with the symbol set for invocation
496 @chapter Using @command{gzip} on tapes
499 When writing compressed data to a tape, it is generally necessary to pad
500 the output with zeroes up to a block boundary. When the data is read and
501 the whole block is passed to @command{gunzip} for decompression,
502 @command{gunzip} detects that there is extra trailing garbage after the
503 compressed data and emits a warning by default if the garbage contains
504 nonzero bytes. You can use the @option{--quiet} option to suppress
508 @chapter Reporting Bugs
511 If you find a bug in @command{gzip}, please send electronic mail to
512 @email{bug-gzip@@gnu.org}. Include the version number,
513 which you can find by running @w{@samp{gzip -V}}. Also include in your
514 message the hardware and operating system, the compiler used to compile
516 a description of the bug behavior, and the input to @command{gzip}
520 @node GNU Free Documentation License
521 @appendix GNU Free Documentation License
526 @appendix Concept index