1 \input texinfo @c -*-texinfo-*-
5 @settitle Gzip User's Manual
10 This manual is for Gzip
11 (version @value{VERSION}, @value{UPDATED}),
12 and documents commands for compressing and decompressing data.
14 Copyright @copyright{} 1998, 1999, 2001, 2002, 2006 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 1.2 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 @c Debian install-info (up through at least version 1.9.20) uses only the
30 @c first dircategory. But install-info 1.10.28 rejects any attempt to
31 @c put the more-useful individual utility first. So put the less-useful
32 @c general category first.
33 @dircategory Utilities
35 * Gzip: (gzip). The gzip command for compressing files.
38 @dircategory Individual utilities
40 * gzip: (gzip)Invoking gzip. Compress files.
45 @subtitle The data compression program
46 @subtitle for Gzip Version @value{VERSION}
47 @subtitle @value{UPDATED}
48 @author by Jean-loup Gailly
51 @vskip 0pt plus 1filll
59 @top Compressing Files
65 * Overview:: Preliminary information.
66 * Sample:: Sample output from @code{gzip}.
67 * Invoking gzip:: How to run @code{gzip}.
68 * Advanced usage:: Concatenated files.
69 * Environment:: The @code{GZIP} environment variable
70 * Tapes:: Using @code{gzip} on tapes.
71 * Problems:: Reporting bugs.
72 * Copying This Manual:: How to make copies of this manual.
73 * Concept Index:: Index of concepts.
76 @node Overview, Sample, , Top
80 @code{gzip} reduces the size of the named files using Lempel-Ziv coding
81 (LZ77). Whenever possible, each file is replaced by one with the
82 extension @samp{.gz}, while keeping the same ownership modes, access and
83 modification times. (The default extension is @samp{-gz} for VMS,
84 @samp{z} for MSDOS, OS/2 FAT and Atari.) If no files are specified or
85 if a file name is "-", the standard input is compressed to the standard
86 output. @code{gzip} will only attempt to compress regular files. In
87 particular, it will ignore symbolic links.
89 If the new file name is too long for its file system, @code{gzip}
90 truncates it. @code{gzip} attempts to truncate only the parts of the
91 file name longer than 3 characters. (A part is delimited by dots.) If
92 the name consists of small parts only, the longest parts are truncated.
93 For example, if file names are limited to 14 characters, gzip.msdos.exe
94 is compressed to gzi.msd.exe.gz. Names are not truncated on systems
95 which do not have a limit on file name length.
97 By default, @code{gzip} keeps the original file name and timestamp in
98 the compressed file. These are used when decompressing the file with the
99 @samp{-N} option. This is useful when the compressed file name was
100 truncated or when the time stamp was not preserved after a file
101 transfer. However, due to limitations in the current @code{gzip} file
102 format, fractional seconds are discarded. Also, time stamps must fall
103 within the range 1970-01-01 00:00:00 through 2106-02-07 06:28:15
104 @acronym{UTC}, and hosts whose operating systems use 32-bit time
105 stamps are further restricted to time stamps no later than 2038-01-19
106 03:14:07 @acronym{UTC}. The upper bounds assume the typical case
107 where leap seconds are ignored.
109 Compressed files can be restored to their original form using @samp{gzip -d}
110 or @code{gunzip} or @code{zcat}. If the original name saved in the
111 compressed file is not suitable for its file system, a new name is
112 constructed from the original one to make it legal.
114 @code{gunzip} takes a list of files on its command line and replaces
115 each file whose name ends with @samp{.gz}, @samp{.z}, @samp{.Z},
116 @samp{-gz}, @samp{-z} or @samp{_z} and which begins with the correct
117 magic number with an uncompressed file without the original extension.
118 @code{gunzip} also recognizes the special extensions @samp{.tgz} and
119 @samp{.taz} as shorthands for @samp{.tar.gz} and @samp{.tar.Z}
120 respectively. When compressing, @code{gzip} uses the @samp{.tgz}
121 extension if necessary instead of truncating a file with a @samp{.tar}
124 @code{gunzip} can currently decompress files created by @code{gzip},
125 @code{zip}, @code{compress} or @code{pack}. The detection of the input
126 format is automatic. When using the first two formats, @code{gunzip}
127 checks a 32 bit CRC (cyclic redundancy check). For @code{pack},
128 @code{gunzip} checks the uncompressed length. The @code{compress} format
129 was not designed to allow consistency checks. However @code{gunzip} is
130 sometimes able to detect a bad @samp{.Z} file. If you get an error when
131 uncompressing a @samp{.Z} file, do not assume that the @samp{.Z} file is
132 correct simply because the standard @code{uncompress} does not complain.
133 This generally means that the standard @code{uncompress} does not check
134 its input, and happily generates garbage output. The SCO @samp{compress
135 -H} format (@code{lzh} compression method) does not include a CRC but
136 also allows some consistency checks.
138 Files created by @code{zip} can be uncompressed by @code{gzip} only if
139 they have a single member compressed with the 'deflation' method. This
140 feature is only intended to help conversion of @code{tar.zip} files to
141 the @code{tar.gz} format. To extract a @code{zip} file with a single
142 member, use a command like @samp{gunzip <foo.zip} or @samp{gunzip -S
143 .zip foo.zip}. To extract @code{zip} files with several
144 members, use @code{unzip} instead of @code{gunzip}.
146 @code{zcat} is identical to @samp{gunzip -c}. @code{zcat}
147 uncompresses either a list of files on the command line or its standard
148 input and writes the uncompressed data on standard output. @code{zcat}
149 will uncompress files that have the correct magic number whether they
150 have a @samp{.gz} suffix or not.
152 @code{gzip} uses the Lempel-Ziv algorithm used in @code{zip} and PKZIP.
153 The amount of compression obtained depends on the size of the input and
154 the distribution of common substrings. Typically, text such as source
155 code or English is reduced by 60-70%. Compression is generally much
156 better than that achieved by LZW (as used in @code{compress}), Huffman
157 coding (as used in @code{pack}), or adaptive Huffman coding
160 Compression is always performed, even if the compressed file is slightly
161 larger than the original. The worst case expansion is a few bytes for
162 the @code{gzip} file header, plus 5 bytes every 32K block, or an expansion
163 ratio of 0.015% for large files. Note that the actual number of used
164 disk blocks almost never increases. @code{gzip} normally preserves the mode,
165 ownership and time stamps of files when compressing or decompressing.
167 The @code{gzip} file format is specified in P. Deutsch, @sc{gzip} file
168 format specification version 4.3,
169 @uref{ftp://ftp.isi.edu/in-notes/rfc1952.txt, Internet RFC 1952} (May
170 1996). The @code{zip} deflation format is specified in P. Deutsch,
171 @sc{deflate} Compressed Data Format Specification version 1.3,
172 @uref{ftp://ftp.isi.edu/in-notes/rfc1951.txt, Internet RFC 1951} (May
175 @node Sample, Invoking gzip, Overview, Top
176 @chapter Sample Output
179 Here are some realistic examples of running @code{gzip}.
181 This is the output of the command @samp{gzip -h}:
184 gzip @var{version-number}
185 usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
186 -c --stdout write on standard output, keep original files unchanged
187 -d --decompress decompress
188 -f --force force overwrite of output file and compress links
189 -h --help give this help
190 -l --list list compressed file contents
191 -L --license display software license
192 -n --no-name do not save or restore the original name and time stamp
193 -N --name save or restore the original name and time stamp
194 -q --quiet suppress all warnings
195 -r --recursive operate recursively on directories
196 -S .suf --suffix .suf use suffix .suf on compressed files
197 -t --test test compressed file integrity
198 -v --verbose verbose mode
199 -V --version display version number
200 -1 --fast compress faster
201 -9 --best compress better
202 file... files to (de)compress. If none given, use standard input.
203 Report bugs to <bug-gzip@@gnu.org>.
206 This is the output of the command @samp{gzip -v texinfo.tex}:
209 texinfo.tex: 69.7% -- replaced with texinfo.tex.gz
212 The following command will find all @code{gzip} files in the current
213 directory and subdirectories, and extract them in place without
214 destroying the original:
217 find . -name '*.gz' -print | sed 's/^\(.*\)[.]gz$/gunzip < "&" > "\1"/' | sh
220 @node Invoking gzip, Advanced usage, Sample, Top
221 @chapter Invoking @code{gzip}
225 The format for running the @code{gzip} program is:
228 gzip @var{option} @dots{}
231 @code{gzip} supports the following options:
237 Write output on standard output; keep original files unchanged.
238 If there are several input files, the output consists of a sequence of
239 independently compressed members. To obtain better compression,
240 concatenate all input files before compressing them.
249 Force compression or decompression even if the file has multiple links
250 or the corresponding file already exists, or if the compressed data
251 is read from or written to a terminal. If the input data is not in
252 a format recognized by @code{gzip}, and if the option @samp{--stdout} is also
253 given, copy the input data without change to the standard output: let
254 @code{zcat} behave as @code{cat}. If @samp{-f} is not given, and
255 when not running in the background, @code{gzip} prompts to verify
256 whether an existing file should be overwritten.
260 Print an informative help message describing the options then quit.
264 For each compressed file, list the following fields:
267 compressed size: size of the compressed file
268 uncompressed size: size of the uncompressed file
269 ratio: compression ratio (0.0% if unknown)
270 uncompressed_name: name of the uncompressed file
273 The uncompressed size is given as @samp{-1} for files not in @code{gzip}
274 format, such as compressed @samp{.Z} files. To get the uncompressed size for
275 such a file, you can use:
281 In combination with the @samp{--verbose} option, the following fields are also
285 method: compression method (deflate,compress,lzh,pack)
286 crc: the 32-bit CRC of the uncompressed data
287 date & time: time stamp for the uncompressed file
290 The crc is given as ffffffff for a file not in gzip format.
292 With @samp{--verbose}, the size totals and compression ratio for all files
293 is also displayed, unless some sizes are unknown. With @samp{--quiet},
294 the title and totals lines are not displayed.
296 The @code{gzip} format represents the input size modulo
297 @math{2^32}, so the uncompressed size and compression ratio are listed
298 incorrectly for uncompressed files 4 GB and larger. To work around
299 this problem, you can use the following command to discover a large
300 uncompressed file's true size:
308 Display the @code{gzip} license then quit.
312 When compressing, do not save the original file name and time stamp by
313 default. (The original name is always saved if the name had to be
314 truncated.) When decompressing, do not restore the original file name
315 if present (remove only the @code{gzip}
316 suffix from the compressed file name) and do not restore the original
317 time stamp if present (copy it from the compressed file). This option
318 is the default when decompressing.
322 When compressing, always save the original file name and time stamp; this
323 is the default. When decompressing, restore the original file name and
324 time stamp if present. This option is useful on systems which have
325 a limit on file name length or when the time stamp has been lost after
330 Suppress all warning messages.
334 Travel the directory structure recursively. If any of the file names
335 specified on the command line are directories, @code{gzip} will descend
336 into the directory and compress all the files it finds there (or
337 decompress them in the case of @code{gunzip}).
339 @item --suffix @var{suf}
341 Use suffix @samp{@var{suf}} instead of @samp{.gz}. Any suffix can be
342 given, but suffixes other than @samp{.z} and @samp{.gz} should be
343 avoided to avoid confusion when files are transferred to other systems.
344 A null suffix forces gunzip to try decompression on all given files
345 regardless of suffix, as in:
348 gunzip -S "" * (*.* for MSDOS)
351 Previous versions of gzip used the @samp{.z} suffix. This was changed to
352 avoid a conflict with @code{pack}.
356 Test. Check the compressed file integrity.
360 Verbose. Display the name and percentage reduction for each file compressed.
364 Version. Display the version number and compilation options, then quit.
369 Regulate the speed of compression using the specified digit @var{n},
370 where @samp{-1} or @samp{--fast} indicates the fastest compression
371 method (less compression) and @samp{--best} or @samp{-9} indicates the
372 slowest compression method (optimal compression). The default
373 compression level is @samp{-6} (that is, biased towards high compression at
377 @node Advanced usage, Environment, Invoking gzip, Top
378 @chapter Advanced usage
379 @cindex concatenated files
381 Multiple compressed files can be concatenated. In this case,
382 @code{gunzip} will extract all members at once. If one member is
383 damaged, other members might still be recovered after removal of the
384 damaged member. Better compression can be usually obtained if all
385 members are decompressed and then recompressed in a single step.
387 This is an example of concatenating @code{gzip} files:
390 gzip -c file1 > foo.gz
391 gzip -c file2 >> foo.gz
406 In case of damage to one member of a @samp{.gz} file, other members can
407 still be recovered (if the damaged member is removed). However,
408 you can get better compression by compressing all members at once:
411 cat file1 file2 | gzip > foo.gz
414 compresses better than
417 gzip -c file1 file2 > foo.gz
420 If you want to recompress concatenated files to get better compression, do:
423 zcat old.gz | gzip > new.gz
426 If a compressed file consists of several members, the uncompressed
427 size and CRC reported by the @samp{--list} option applies to the last member
428 only. If you need the uncompressed size for all members, you can use:
434 If you wish to create a single archive file with multiple members so
435 that members can later be extracted independently, use an archiver such
436 as @code{tar} or @code{zip}. GNU @code{tar} supports the @samp{-z}
437 option to invoke @code{gzip} transparently. @code{gzip} is designed as a
438 complement to @code{tar}, not as a replacement.
440 @node Environment, Tapes, Advanced usage, Top
444 The environment variable @code{GZIP} can hold a set of default options for
445 @code{gzip}. These options are interpreted first and can be overwritten by
446 explicit command line parameters. For example:
449 for sh: GZIP="-8v --name"; export GZIP
450 for csh: setenv GZIP "-8v --name"
451 for MSDOS: set GZIP=-8v --name
454 On Vax/VMS, the name of the environment variable is @code{GZIP_OPT}, to
455 avoid a conflict with the symbol set for invocation of the program.
457 @node Tapes, Problems, Environment, Top
458 @chapter Using @code{gzip} on tapes
461 When writing compressed data to a tape, it is generally necessary to pad
462 the output with zeroes up to a block boundary. When the data is read and
463 the whole block is passed to @code{gunzip} for decompression,
464 @code{gunzip} detects that there is extra trailing garbage after the
465 compressed data and emits a warning by default if the garbage contains
466 nonzero bytes. You have to use the
467 @samp{--quiet} option to suppress the warning. This option can be set in the
468 @code{GZIP} environment variable, as in:
471 for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
472 for csh: (setenv GZIP "-q"; tar -xfz --block-compress /dev/rst0)
475 In the above example, @code{gzip} is invoked implicitly by the @samp{-z}
476 option of GNU @code{tar}. Make sure that the same block size (@samp{-b}
477 option of @code{tar}) is used for reading and writing compressed data on
478 tapes. (This example assumes you are using the GNU version of
481 @node Problems, Copying This Manual, Tapes, Top
482 @chapter Reporting Bugs
485 If you find a bug in @code{gzip}, please send electronic mail to
486 @email{bug-gzip@@gnu.org}. Include the version number,
487 which you can find by running @w{@samp{gzip -V}}. Also include in your
488 message the hardware and operating system, the compiler used to compile
490 a description of the bug behavior, and the input to @code{gzip} that triggered
493 @node Copying This Manual, Concept Index, Problems, Top
494 @appendix Copying This Manual
497 * GNU Free Documentation License:: License for copying this manual.
502 @node Concept Index, , Copying This Manual, Top
503 @appendix Concept Index