1 \input texinfo @c -*-texinfo-*-
4 @settitle Gzip User's Manual
10 @dircategory Utilities
12 * Gzip: (gzip). The gzip command for compressing files.
15 This file documents the the GNU @code{gzip} command for compressing files.
17 Copyright (C) 1998, 1999, 2001 Free Software Foundation, Inc.
18 Copyright (C) 1992-1993 Jean-loup Gailly
20 Permission is granted to make and distribute verbatim copies of
21 this manual provided the copyright notice and this permission notice
22 are preserved on all copies.
25 Permission is granted to process this file through TeX and print the
26 results, provided the printed document carries copying permission
27 notice identical to this one except for the removal of this paragraph
28 (this paragraph not being relevant to the printed manual).
31 Permission is granted to copy and distribute modified versions of this
32 manual under the conditions for verbatim copying, provided that the entire
33 resulting derived work is distributed under the terms of a permission
34 notice identical to this one.
36 Permission is granted to copy and distribute translations of this manual
37 into another language, under the above conditions for modified versions,
38 except that this permission notice may be stated in a translation approved
44 @subtitle The data compression program
45 @subtitle Edition 1.3.2, for Gzip Version 1.3.2
46 @subtitle November 2001
47 @author by Jean-loup Gailly
50 @vskip 0pt plus 1filll
51 Copyright @copyright{} 1998, 1999, 2001 Free Software Foundation, Inc.
53 Copyright @copyright{} 1992-1993 Jean-loup Gailly
55 Permission is granted to make and distribute verbatim copies of
56 this manual provided the copyright notice and this permission notice
57 are preserved on all copies.
59 Permission is granted to copy and distribute modified versions of this
60 manual under the conditions for verbatim copying, provided that the entire
61 resulting derived work is distributed under the terms of a permission
62 notice identical to this one.
64 Permission is granted to copy and distribute translations of this manual
65 into another language, under the above conditions for modified versions,
66 except that this permission notice may be stated in a translation approved
73 This file documents the @code{gzip} command to compress files.
77 * Overview:: Preliminary information.
78 * Sample:: Sample output from @code{gzip}.
79 * Invoking gzip:: How to run @code{gzip}.
80 * Advanced usage:: Concatenated files.
81 * Environment:: The @code{GZIP} environment variable
82 * Tapes:: Using @code{gzip} on tapes.
83 * Problems:: Reporting bugs.
84 * Concept Index:: Index of concepts.
87 @node Overview, Sample, , Top
91 @code{gzip} reduces the size of the named files using Lempel-Ziv coding
92 (LZ77). Whenever possible, each file is replaced by one with the
93 extension @samp{.gz}, while keeping the same ownership modes, access and
94 modification times. (The default extension is @samp{-gz} for VMS,
95 @samp{z} for MSDOS, OS/2 FAT and Atari.) If no files are specified or
96 if a file name is "-", the standard input is compressed to the standard
97 output. @code{gzip} will only attempt to compress regular files. In
98 particular, it will ignore symbolic links.
100 If the new file name is too long for its file system, @code{gzip}
101 truncates it. @code{gzip} attempts to truncate only the parts of the
102 file name longer than 3 characters. (A part is delimited by dots.) If
103 the name consists of small parts only, the longest parts are truncated.
104 For example, if file names are limited to 14 characters, gzip.msdos.exe
105 is compressed to gzi.msd.exe.gz. Names are not truncated on systems
106 which do not have a limit on file name length.
108 By default, @code{gzip} keeps the original file name and timestamp in
109 the compressed file. These are used when decompressing the file with the
110 @samp{-N} option. This is useful when the compressed file name was
111 truncated or when the time stamp was not preserved after a file
114 Compressed files can be restored to their original form using @samp{gzip -d}
115 or @code{gunzip} or @code{zcat}. If the original name saved in the
116 compressed file is not suitable for its file system, a new name is
117 constructed from the original one to make it legal.
119 @code{gunzip} takes a list of files on its command line and replaces
120 each file whose name ends with @samp{.gz}, @samp{.z}, @samp{.Z},
121 @samp{-gz}, @samp{-z} or @samp{_z} and which begins with the correct
122 magic number with an uncompressed file without the original extension.
123 @code{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, @code{gzip} uses the @samp{.tgz}
126 extension if necessary instead of truncating a file with a @samp{.tar}
129 @code{gunzip} can currently decompress files created by @code{gzip},
130 @code{zip}, @code{compress} or @code{pack}. The detection of the input
131 format is automatic. When using the first two formats, @code{gunzip}
132 checks a 32 bit CRC (cyclic redundancy check). For @code{pack},
133 @code{gunzip} checks the uncompressed length. The @code{compress} format
134 was not designed to allow consistency checks. However @code{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 @code{uncompress} does not complain.
138 This generally means that the standard @code{uncompress} does not check
139 its input, and happily generates garbage output. The SCO @samp{compress
140 -H} format (@code{lzh} compression method) does not include a CRC but
141 also allows some consistency checks.
143 Files created by @code{zip} can be uncompressed by @code{gzip} only if
144 they have a single member compressed with the 'deflation' method. This
145 feature is only intended to help conversion of @code{tar.zip} files to
146 the @code{tar.gz} format. To extract @code{zip} files with several
147 members, use @code{unzip} instead of @code{gunzip}.
149 @code{zcat} is identical to @samp{gunzip -c}. @code{zcat}
150 uncompresses either a list of files on the command line or its standard
151 input and writes the uncompressed data on standard output. @code{zcat}
152 will uncompress files that have the correct magic number whether they
153 have a @samp{.gz} suffix or not.
155 @code{gzip} uses the Lempel-Ziv algorithm used in @code{zip} and PKZIP.
156 The amount of compression obtained depends on the size of the input and
157 the distribution of common substrings. Typically, text such as source
158 code or English is reduced by 60-70%. Compression is generally much
159 better than that achieved by LZW (as used in @code{compress}), Huffman
160 coding (as used in @code{pack}), or adaptive Huffman coding
163 Compression is always performed, even if the compressed file is slightly
164 larger than the original. The worst case expansion is a few bytes for
165 the @code{gzip} file header, plus 5 bytes every 32K block, or an expansion
166 ratio of 0.015% for large files. Note that the actual number of used
167 disk blocks almost never increases. @code{gzip} preserves the mode,
168 ownership and timestamps of files when compressing or decompressing.
170 The @code{gzip} file format is specified in P. Deutsch, @sc{gzip} file
171 format specification version 4.3,
172 @uref{ftp://ftp.isi.edu/in-notes/rfc1952.txt, Internet RFC 1952} (May
173 1996). The @code{zip} deflation format is specified in P. Deutsch,
174 @sc{deflate} Compressed Data Format Specification version 1.3,
175 @uref{ftp://ftp.isi.edu/in-notes/rfc1951.txt, Internet RFC 1951} (May
178 @node Sample, Invoking gzip, Overview, Top
179 @chapter Sample Output
182 Here are some realistic examples of running @code{gzip}.
184 This is the output of the command @samp{gzip -h}:
189 usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
190 -c --stdout write on standard output, keep original files unchanged
191 -d --decompress decompress
192 -f --force force overwrite of output file and compress links
193 -h --help give this help
194 -l --list list compressed file contents
195 -L --license display software license
196 -n --no-name do not save or restore the original name and time stamp
197 -N --name save or restore the original name and time stamp
198 -q --quiet suppress all warnings
199 -r --recursive operate recursively on directories
200 -S .suf --suffix .suf use suffix .suf on compressed files
201 -t --test test compressed file integrity
202 -v --verbose verbose mode
203 -V --version display version number
204 -1 --fast compress faster
205 -9 --best compress better
206 file... files to (de)compress. If none given, use standard input.
207 Report bugs to <bug-gzip@@gnu.org>.
210 This is the output of the command @samp{gzip -v texinfo.tex}:
213 texinfo.tex: 69.7% -- replaced with texinfo.tex.gz
216 The following command will find all @code{gzip} files in the current
217 directory and subdirectories, and extract them in place without
218 destroying the original:
221 find . -name '*.gz' -print | sed 's/^\(.*\)[.]gz$/gunzip < "&" > "\1"/' | sh
224 @node Invoking gzip, Advanced usage, Sample, Top
225 @chapter Invoking @code{gzip}
229 The format for running the @code{gzip} program is:
232 gzip @var{option} @dots{}
235 @code{gzip} supports the following options:
241 Write output on standard output; keep original files unchanged.
242 If there are several input files, the output consists of a sequence of
243 independently compressed members. To obtain better compression,
244 concatenate all input files before compressing them.
253 Force compression or decompression even if the file has multiple links
254 or the corresponding file already exists, or if the compressed data
255 is read from or written to a terminal. If the input data is not in
256 a format recognized by @code{gzip}, and if the option @samp{--stdout} is also
257 given, copy the input data without change to the standard output: let
258 @code{zcat} behave as @code{cat}. If @samp{-f} is not given, and
259 when not running in the background, @code{gzip} prompts to verify
260 whether an existing file should be overwritten.
264 Print an informative help message describing the options then quit.
268 For each compressed file, list the following fields:
271 compressed size: size of the compressed file
272 uncompressed size: size of the uncompressed file
273 ratio: compression ratio (0.0% if unknown)
274 uncompressed_name: name of the uncompressed file
277 The uncompressed size is given as @samp{-1} for files not in @code{gzip}
278 format, such as compressed @samp{.Z} files. To get the uncompressed size for
279 such a file, you can use:
285 In combination with the @samp{--verbose} option, the following fields are also
289 method: compression method (deflate,compress,lzh,pack)
290 crc: the 32-bit CRC of the uncompressed data
291 date & time: time stamp for the uncompressed file
294 The crc is given as ffffffff for a file not in gzip format.
296 With @samp{--verbose}, the size totals and compression ratio for all files
297 is also displayed, unless some sizes are unknown. With @samp{--quiet},
298 the title and totals lines are not displayed.
300 The @code{gzip} format represents the the input size modulo
301 @math{2^32}, so the uncompressed size and compression ratio are listed
302 incorrectly for uncompressed files 4 GB and larger. To work around
303 this problem, you can use the following command to discover a large
304 uncompressed file's true size:
312 Display the @code{gzip} license then quit.
316 When compressing, do not save the original file name and time stamp by
317 default. (The original name is always saved if the name had to be
318 truncated.) When decompressing, do not restore the original file name
319 if present (remove only the @code{gzip}
320 suffix from the compressed file name) and do not restore the original
321 time stamp if present (copy it from the compressed file). This option
322 is the default when decompressing.
326 When compressing, always save the original file name and time stamp; this
327 is the default. When decompressing, restore the original file name and
328 time stamp if present. This option is useful on systems which have
329 a limit on file name length or when the time stamp has been lost after
334 Suppress all warning messages.
338 Travel the directory structure recursively. If any of the file names
339 specified on the command line are directories, @code{gzip} will descend
340 into the directory and compress all the files it finds there (or
341 decompress them in the case of @code{gunzip}).
343 @item --suffix @var{suf}
345 Use suffix @samp{@var{suf}} instead of @samp{.gz}. Any suffix can be
346 given, but suffixes other than @samp{.z} and @samp{.gz} should be
347 avoided to avoid confusion when files are transferred to other systems.
348 A null suffix forces gunzip to try decompression on all given files
349 regardless of suffix, as in:
352 gunzip -S "" * (*.* for MSDOS)
355 Previous versions of gzip used the @samp{.z} suffix. This was changed to
356 avoid a conflict with @code{pack}.
360 Test. Check the compressed file integrity.
364 Verbose. Display the name and percentage reduction for each file compressed.
368 Version. Display the version number and compilation options, then quit.
373 Regulate the speed of compression using the specified digit @var{n},
374 where @samp{-1} or @samp{--fast} indicates the fastest compression
375 method (less compression) and @samp{--best} or @samp{-9} indicates the
376 slowest compression method (optimal compression). The default
377 compression level is @samp{-6} (that is, biased towards high compression at
381 @node Advanced usage, Environment, Invoking gzip, Top
382 @chapter Advanced usage
383 @cindex concatenated files
385 Multiple compressed files can be concatenated. In this case,
386 @code{gunzip} will extract all members at once. If one member is
387 damaged, other members might still be recovered after removal of the
388 damaged member. Better compression can be usually obtained if all
389 members are decompressed and then recompressed in a single step.
391 This is an example of concatenating @code{gzip} files:
394 gzip -c file1 > foo.gz
395 gzip -c file2 >> foo.gz
410 In case of damage to one member of a @samp{.gz} file, other members can
411 still be recovered (if the damaged member is removed). However,
412 you can get better compression by compressing all members at once:
415 cat file1 file2 | gzip > foo.gz
418 compresses better than
421 gzip -c file1 file2 > foo.gz
424 If you want to recompress concatenated files to get better compression, do:
427 zcat old.gz | gzip > new.gz
430 If a compressed file consists of several members, the uncompressed
431 size and CRC reported by the @samp{--list} option applies to the last member
432 only. If you need the uncompressed size for all members, you can use:
438 If you wish to create a single archive file with multiple members so
439 that members can later be extracted independently, use an archiver such
440 as @code{tar} or @code{zip}. GNU @code{tar} supports the @samp{-z}
441 option to invoke @code{gzip} transparently. @code{gzip} is designed as a
442 complement to @code{tar}, not as a replacement.
444 @node Environment, Tapes, Advanced usage, Top
448 The environment variable @code{GZIP} can hold a set of default options for
449 @code{gzip}. These options are interpreted first and can be overwritten by
450 explicit command line parameters. For example:
453 for sh: GZIP="-8v --name"; export GZIP
454 for csh: setenv GZIP "-8v --name"
455 for MSDOS: set GZIP=-8v --name
458 On Vax/VMS, the name of the environment variable is @code{GZIP_OPT}, to
459 avoid a conflict with the symbol set for invocation of the program.
461 @node Tapes, Problems, Environment, Top
462 @chapter Using @code{gzip} on tapes
465 When writing compressed data to a tape, it is generally necessary to pad
466 the output with zeroes up to a block boundary. When the data is read and
467 the whole block is passed to @code{gunzip} for decompression,
468 @code{gunzip} detects that there is extra trailing garbage after the
469 compressed data and emits a warning by default if the garbage contains
470 nonzero bytes. You have to use the
471 @samp{--quiet} option to suppress the warning. This option can be set in the
472 @code{GZIP} environment variable, as in:
475 for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
476 for csh: (setenv GZIP "-q"; tar -xfz --block-compress /dev/rst0)
479 In the above example, @code{gzip} is invoked implicitly by the @samp{-z}
480 option of GNU @code{tar}. Make sure that the same block size (@samp{-b}
481 option of @code{tar}) is used for reading and writing compressed data on
482 tapes. (This example assumes you are using the GNU version of
485 @node Problems, Concept Index, Tapes, Top
486 @chapter Reporting Bugs
489 If you find a bug in @code{gzip}, please send electronic mail to
490 @email{bug-gzip@@gnu.org}. Include the version number,
491 which you can find by running @w{@samp{gzip -V}}. Also include in your
492 message the hardware and operating system, the compiler used to compile
494 a description of the bug behavior, and the input to @code{gzip} that triggered
497 @node Concept Index, , Problems, Top
498 @unnumbered Concept Index