1 \input texinfo @c -*-texinfo-*-
10 This manual is for GNU Gzip
11 (version @value{VERSION}, @value{UPDATED}),
12 and documents commands for compressing and decompressing data.
14 Copyright @copyright{} 1998-1999, 2001-2002, 2006-2007, 2009-2012 Free Software
17 Copyright @copyright{} 1992, 1993 Jean-loup Gailly
20 Permission is granted to copy, distribute and/or modify this document
21 under the terms of the GNU Free Documentation License, Version 1.3 or
22 any later version published by the Free Software Foundation; with no
23 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
24 Texts. A copy of the license is included in the section entitled ``GNU
25 Free Documentation License''.
29 @dircategory Compression
31 * Gzip: (gzip). General (de)compression of files (lzw).
34 @dircategory Individual utilities
36 * gunzip: (gzip)Overview. Decompression.
37 * gzexe: (gzip)Overview. Compress executables.
38 * zcat: (gzip)Overview. Decompression to stdout.
39 * zdiff: (gzip)Overview. Compare compressed files.
40 * zforce: (gzip)Overview. Force .gz extension on files.
41 * zgrep: (gzip)Overview. Search compressed files.
42 * zmore: (gzip)Overview. Decompression output by pages.
47 @subtitle The data compression program
48 @subtitle for Gzip version @value{VERSION}
49 @subtitle @value{UPDATED}
50 @author by Jean-loup Gailly
53 @vskip 0pt plus 1filll
61 @top GNU Gzip: General file (de)compression
67 * Overview:: Preliminary information.
68 * Sample:: Sample output from @command{gzip}.
69 * Invoking gzip:: How to run @command{gzip}.
70 * Advanced usage:: Concatenated files.
71 * Environment:: The @env{GZIP} environment variable
72 * Tapes:: Using @command{gzip} on tapes.
73 * Problems:: Reporting bugs.
74 * GNU Free Documentation License:: Copying and sharing this manual.
75 * Concept index:: Index of concepts.
82 @command{gzip} reduces the size of the named files using Lempel-Ziv coding
83 (LZ77). Whenever possible, each file is replaced by one with the
84 extension @samp{.gz}, while keeping the same ownership modes, access and
85 modification times. (The default extension is @samp{-gz} for @abbr{VMS},
86 @samp{z} for @abbr{MSDOS}, @abbr{OS/2} @abbr{FAT} and Atari.)
87 If no files are specified or
88 if a file name is "-", the standard input is compressed to the standard
89 output. @command{gzip} will only attempt to compress regular files. In
90 particular, it will ignore symbolic links.
92 If the new file name is too long for its file system, @command{gzip}
93 truncates it. @command{gzip} attempts to truncate only the parts of the
94 file name longer than 3 characters. (A part is delimited by dots.) If
95 the name consists of small parts only, the longest parts are truncated.
96 For example, if file names are limited to 14 characters, gzip.msdos.exe
97 is compressed to gzi.msd.exe.gz. Names are not truncated on systems
98 which do not have a limit on file name length.
100 By default, @command{gzip} keeps the original file name and time stamp in
101 the compressed file. These are used when decompressing the file with the
102 @option{-N} option. This is useful when the compressed file name was
103 truncated or when the time stamp was not preserved after a file
104 transfer. However, due to limitations in the current @command{gzip} file
105 format, fractional seconds are discarded. Also, time stamps must fall
106 within the range 1970-01-01 00:00:00 through 2106-02-07 06:28:15
107 @abbr{UTC}, and hosts whose operating systems use 32-bit time
108 stamps are further restricted to time stamps no later than 2038-01-19
109 03:14:07 @abbr{UTC}. The upper bounds assume the typical case
110 where leap seconds are ignored.
112 Compressed files can be restored to their original form using @samp{gzip -d}
113 or @command{gunzip} or @command{zcat}. If the original name saved in the
114 compressed file is not suitable for its file system, a new name is
115 constructed from the original one to make it legal.
117 @command{gunzip} takes a list of files on its command line and replaces
118 each file whose name ends with @samp{.gz}, @samp{.z}
119 @samp{-gz}, @samp{-z}, or @samp{_z} (ignoring case)
120 and which begins with the correct
121 magic number with an uncompressed file without the original extension.
122 @command{gunzip} also recognizes the special extensions @samp{.tgz} and
123 @samp{.taz} as shorthands for @samp{.tar.gz} and @samp{.tar.Z}
124 respectively. When compressing, @command{gzip} uses the @samp{.tgz}
125 extension if necessary instead of truncating a file with a @samp{.tar}
128 @command{gunzip} can currently decompress files created by @command{gzip},
129 @command{zip}, @command{compress} or @command{pack}. The detection of the input
130 format is automatic. When using the first two formats, @command{gunzip}
131 checks a 32 bit @abbr{CRC} (cyclic redundancy check). For @command{pack},
132 @command{gunzip} checks the uncompressed length. The @command{compress} format
133 was not designed to allow consistency checks. However @command{gunzip} is
134 sometimes able to detect a bad @samp{.Z} file. If you get an error when
135 uncompressing a @samp{.Z} file, do not assume that the @samp{.Z} file is
136 correct simply because the standard @command{uncompress} does not complain.
137 This generally means that the standard @command{uncompress} does not check
138 its input, and happily generates garbage output. The @abbr{SCO} @samp{compress
139 -H} format (@abbr{LZH} compression method) does not include a @abbr{CRC} but
140 also allows some consistency checks.
142 Files created by @command{zip} can be uncompressed by @command{gzip} only if
143 they have a single member compressed with the 'deflation' method. This
144 feature is only intended to help conversion of @file{tar.zip} files to
145 the @file{tar.gz} format. To extract a @command{zip} file with a single
146 member, use a command like @samp{gunzip <foo.zip} or @samp{gunzip -S
147 .zip foo.zip}. To extract @command{zip} files with several
148 members, use @command{unzip} instead of @command{gunzip}.
150 @command{zcat} is identical to @samp{gunzip -c}. @command{zcat}
151 uncompresses either a list of files on the command line or its standard
152 input and writes the uncompressed data on standard output. @command{zcat}
153 will uncompress files that have the correct magic number whether they
154 have a @samp{.gz} suffix or not.
156 @command{gzip} uses the Lempel-Ziv algorithm used in @command{zip} and
158 The amount of compression obtained depends on the size of the input and
159 the distribution of common substrings. Typically, text such as source
160 code or English is reduced by 60-70%. Compression is generally much
161 better than that achieved by @abbr{LZW} (as used in @command{compress}), Huffman
162 coding (as used in @command{pack}), or adaptive Huffman coding
165 Compression is always performed, even if the compressed file is slightly
166 larger than the original. The worst case expansion is a few bytes for
167 the @command{gzip} file header, plus 5 bytes every 32K block, or an expansion
168 ratio of 0.015% for large files. Note that the actual number of used
169 disk blocks almost never increases. @command{gzip} normally preserves the mode,
170 ownership and time stamps of files when compressing or decompressing.
172 The @command{gzip} file format is specified in P. Deutsch, @sc{gzip} file
173 format specification version 4.3,
174 @uref{ftp://ftp.isi.edu/in-notes/rfc1952.txt, Internet @abbr{RFC} 1952} (May
175 1996). The @command{zip} deflation format is specified in P. Deutsch,
176 @sc{deflate} Compressed Data Format Specification version 1.3,
177 @uref{ftp://ftp.isi.edu/in-notes/rfc1951.txt, Internet @abbr{RFC} 1951} (May
181 @chapter Sample output
184 Here are some realistic examples of running @command{gzip}.
186 This is the output of the command @samp{gzip -h}:
189 Usage: gzip [OPTION]... [FILE]...
190 Compress or uncompress FILEs (by default, compress FILES in-place).
192 Mandatory arguments to long options are mandatory for short options too.
194 -c, --stdout write on standard output, keep original files unchanged
195 -d, --decompress decompress
196 -f, --force force overwrite of output file and compress links
197 -h, --help give this help
198 -l, --list list compressed file contents
199 -L, --license display software license
200 -n, --no-name do not save or restore the original name and time stamp
201 -N, --name save or restore the original name and time stamp
202 -q, --quiet suppress all warnings
203 -r, --recursive operate recursively on directories
204 -S, --suffix=SUF use suffix SUF on compressed files
205 -t, --test test compressed file integrity
206 -v, --verbose verbose mode
207 -V, --version display version number
208 -1, --fast compress faster
209 -9, --best compress better
210 --rsyncable Make rsync-friendly archive
212 With no FILE, or when FILE is -, read standard input.
214 Report bugs to <bug-gzip@@gnu.org>.
217 This is the output of the command @samp{gzip -v texinfo.tex}:
220 texinfo.tex: 69.3% -- replaced with texinfo.tex.gz
223 The following command will find all regular @samp{.gz} files in the
224 current directory and subdirectories (skipping file names that contain
225 newlines), and extract them in place without destroying the original,
226 stopping on the first failure:
230 *' -prune -o -name '*.gz' -type f -print |
233 s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/
239 @chapter Invoking @command{gzip}
243 The format for running the @command{gzip} program is:
246 gzip @var{option} @dots{}
249 @command{gzip} supports the following options:
255 Write output on standard output; keep original files unchanged.
256 If there are several input files, the output consists of a sequence of
257 independently compressed members. To obtain better compression,
258 concatenate all input files before compressing them.
267 Force compression or decompression even if the file has multiple links
268 or the corresponding file already exists, or if the compressed data
269 is read from or written to a terminal. If the input data is not in
270 a format recognized by @command{gzip}, and if the option @option{--stdout} is also
271 given, copy the input data without change to the standard output: let
272 @command{zcat} behave as @command{cat}. If @option{-f} is not given, and
273 when not running in the background, @command{gzip} prompts to verify
274 whether an existing file should be overwritten.
278 Print an informative help message describing the options then quit.
282 For each compressed file, list the following fields:
285 compressed size: size of the compressed file
286 uncompressed size: size of the uncompressed file
287 ratio: compression ratio (0.0% if unknown)
288 uncompressed_name: name of the uncompressed file
291 The uncompressed size is given as @minus{}1 for files not in @command{gzip}
292 format, such as compressed @samp{.Z} files. To get the uncompressed size for
293 such a file, you can use:
299 In combination with the @option{--verbose} option, the following fields are also
303 method: compression method (deflate,compress,lzh,pack)
304 crc: the 32-bit CRC of the uncompressed data
305 date & time: time stamp for the uncompressed file
308 The @abbr{CRC} is given as ffffffff for a file not in gzip format.
310 With @option{--verbose}, the size totals and compression ratio for all files
311 is also displayed, unless some sizes are unknown. With @option{--quiet},
312 the title and totals lines are not displayed.
314 The @command{gzip} format represents the input size modulo
315 @math{2^32}, so the uncompressed size and compression ratio are listed
316 incorrectly for uncompressed files 4 GiB and larger. To work around
317 this problem, you can use the following command to discover a large
318 uncompressed file's true size:
326 Display the @command{gzip} license then quit.
330 When compressing, do not save the original file name and time stamp by
331 default. (The original name is always saved if the name had to be
332 truncated.) When decompressing, do not restore the original file name
333 if present (remove only the @command{gzip}
334 suffix from the compressed file name) and do not restore the original
335 time stamp if present (copy it from the compressed file). This option
336 is the default when decompressing.
340 When compressing, always save the original file name and time stamp; this
341 is the default. When decompressing, restore the original file name and
342 time stamp if present. This option is useful on systems which have
343 a limit on file name length or when the time stamp has been lost after
348 Suppress all warning messages.
352 Travel the directory structure recursively. If any of the file names
353 specified on the command line are directories, @command{gzip} will descend
354 into the directory and compress all the files it finds there (or
355 decompress them in the case of @command{gunzip}).
358 While compressing, synchronize the output occasionally based on the
359 input. This can reduce the compression slightly in some cases, but
360 means that the @code{rsync} program can take advantage of similarities
361 in the uncompressed input when syncronizing two files compressed with
362 this flag. @code{gunzip} cannot tell the difference between a
363 compressed file created with this option, and one created without it.
366 While compressing, synchronize the output occasionally based on
367 the input. This increases size by less than 1 percent most
368 cases, but means that the @command{rsync} program can much more efficiently
369 synchronize files compressed with this flag. @command{gunzip}
370 cannot tell the difference between a compressed file created
371 with this option, and one created without it.
374 While compressing, synchronize the output occasionally based on the
375 input. This can reduce the compression slightly in some cases, but
376 means that the @code{rsync} program can take advantage of similarities
377 in the uncompressed input when syncronizing two files compressed with
378 this flag. @code{gunzip} cannot tell the difference between a
379 compressed file created with this option, and one created without it.
381 @item --suffix @var{suf}
383 Use suffix @var{suf} instead of @samp{.gz}. Any suffix can be
384 given, but suffixes other than @samp{.z} and @samp{.gz} should be
385 avoided to avoid confusion when files are transferred to other systems.
386 A null suffix forces gunzip to try decompression on all given files
387 regardless of suffix, as in:
390 gunzip -S "" * (*.* for MSDOS)
393 Previous versions of gzip used the @samp{.z} suffix. This was changed to
394 avoid a conflict with @command{pack}.
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 environment variable @env{GZIP} can hold a set of default options for
492 @command{gzip}. These options are interpreted first and can be overwritten by
493 explicit command line parameters. For example:
496 for sh: GZIP="-8v --name"; export GZIP
497 for csh: setenv GZIP "-8v --name"
498 for MSDOS: set GZIP=-8v --name
501 On @abbr{VMS}, the name of the environment variable is @env{GZIP_OPT}, to
502 avoid a conflict with the symbol set for invocation of the program.
505 @chapter Using @command{gzip} on tapes
508 When writing compressed data to a tape, it is generally necessary to pad
509 the output with zeroes up to a block boundary. When the data is read and
510 the whole block is passed to @command{gunzip} for decompression,
511 @command{gunzip} detects that there is extra trailing garbage after the
512 compressed data and emits a warning by default if the garbage contains
513 nonzero bytes. You have to use the
514 @option{--quiet} option to suppress the warning. This option can be set in the
515 @env{GZIP} environment variable, as in:
518 for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
519 for csh: (setenv GZIP "-q"; tar -xfz --block-compress /dev/rst0)
522 In the above example, @command{gzip} is invoked implicitly by the @option{-z}
523 option of @acronym{GNU} @command{tar}. Make sure that the same block
525 option of @command{tar}) is used for reading and writing compressed data on
526 tapes. (This example assumes you are using the @acronym{GNU} version of
530 @chapter Reporting Bugs
533 If you find a bug in @command{gzip}, please send electronic mail to
534 @email{bug-gzip@@gnu.org}. Include the version number,
535 which you can find by running @w{@samp{gzip -V}}. Also include in your
536 message the hardware and operating system, the compiler used to compile
538 a description of the bug behavior, and the input to @command{gzip}
542 @node GNU Free Documentation License
543 @appendix GNU Free Documentation License
548 @appendix Concept index