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. Put this one first, as it is more useful in practice.
31 @dircategory Individual utilities
33 * gzip: (gzip)Invoking gzip. Compress files.
36 @dircategory Utilities
38 * Gzip: (gzip). The gzip command for compressing files.
43 @subtitle The data compression program
44 @subtitle for Gzip Version @value{VERSION}
45 @subtitle @value{UPDATED}
46 @author by Jean-loup Gailly
49 @vskip 0pt plus 1filll
57 @top Compressing Files
63 * Overview:: Preliminary information.
64 * Sample:: Sample output from @code{gzip}.
65 * Invoking gzip:: How to run @code{gzip}.
66 * Advanced usage:: Concatenated files.
67 * Environment:: The @code{GZIP} environment variable
68 * Tapes:: Using @code{gzip} on tapes.
69 * Problems:: Reporting bugs.
70 * Copying This Manual:: How to make copies of this manual.
71 * Concept Index:: Index of concepts.
74 @node Overview, Sample, , Top
78 @code{gzip} reduces the size of the named files using Lempel-Ziv coding
79 (LZ77). Whenever possible, each file is replaced by one with the
80 extension @samp{.gz}, while keeping the same ownership modes, access and
81 modification times. (The default extension is @samp{-gz} for VMS,
82 @samp{z} for MSDOS, OS/2 FAT and Atari.) If no files are specified or
83 if a file name is "-", the standard input is compressed to the standard
84 output. @code{gzip} will only attempt to compress regular files. In
85 particular, it will ignore symbolic links.
87 If the new file name is too long for its file system, @code{gzip}
88 truncates it. @code{gzip} attempts to truncate only the parts of the
89 file name longer than 3 characters. (A part is delimited by dots.) If
90 the name consists of small parts only, the longest parts are truncated.
91 For example, if file names are limited to 14 characters, gzip.msdos.exe
92 is compressed to gzi.msd.exe.gz. Names are not truncated on systems
93 which do not have a limit on file name length.
95 By default, @code{gzip} keeps the original file name and timestamp in
96 the compressed file. These are used when decompressing the file with the
97 @samp{-N} option. This is useful when the compressed file name was
98 truncated or when the time stamp was not preserved after a file
101 Compressed files can be restored to their original form using @samp{gzip -d}
102 or @code{gunzip} or @code{zcat}. If the original name saved in the
103 compressed file is not suitable for its file system, a new name is
104 constructed from the original one to make it legal.
106 @code{gunzip} takes a list of files on its command line and replaces
107 each file whose name ends with @samp{.gz}, @samp{.z}, @samp{.Z},
108 @samp{-gz}, @samp{-z} or @samp{_z} and which begins with the correct
109 magic number with an uncompressed file without the original extension.
110 @code{gunzip} also recognizes the special extensions @samp{.tgz} and
111 @samp{.taz} as shorthands for @samp{.tar.gz} and @samp{.tar.Z}
112 respectively. When compressing, @code{gzip} uses the @samp{.tgz}
113 extension if necessary instead of truncating a file with a @samp{.tar}
116 @code{gunzip} can currently decompress files created by @code{gzip},
117 @code{zip}, @code{compress} or @code{pack}. The detection of the input
118 format is automatic. When using the first two formats, @code{gunzip}
119 checks a 32 bit CRC (cyclic redundancy check). For @code{pack},
120 @code{gunzip} checks the uncompressed length. The @code{compress} format
121 was not designed to allow consistency checks. However @code{gunzip} is
122 sometimes able to detect a bad @samp{.Z} file. If you get an error when
123 uncompressing a @samp{.Z} file, do not assume that the @samp{.Z} file is
124 correct simply because the standard @code{uncompress} does not complain.
125 This generally means that the standard @code{uncompress} does not check
126 its input, and happily generates garbage output. The SCO @samp{compress
127 -H} format (@code{lzh} compression method) does not include a CRC but
128 also allows some consistency checks.
130 Files created by @code{zip} can be uncompressed by @code{gzip} only if
131 they have a single member compressed with the 'deflation' method. This
132 feature is only intended to help conversion of @code{tar.zip} files to
133 the @code{tar.gz} format. To extract a @code{zip} file with a single
134 member, use a command like @samp{gunzip <foo.zip} or @samp{gunzip -S
135 .zip foo.zip}. To extract @code{zip} files with several
136 members, use @code{unzip} instead of @code{gunzip}.
138 @code{zcat} is identical to @samp{gunzip -c}. @code{zcat}
139 uncompresses either a list of files on the command line or its standard
140 input and writes the uncompressed data on standard output. @code{zcat}
141 will uncompress files that have the correct magic number whether they
142 have a @samp{.gz} suffix or not.
144 @code{gzip} uses the Lempel-Ziv algorithm used in @code{zip} and PKZIP.
145 The amount of compression obtained depends on the size of the input and
146 the distribution of common substrings. Typically, text such as source
147 code or English is reduced by 60-70%. Compression is generally much
148 better than that achieved by LZW (as used in @code{compress}), Huffman
149 coding (as used in @code{pack}), or adaptive Huffman coding
152 Compression is always performed, even if the compressed file is slightly
153 larger than the original. The worst case expansion is a few bytes for
154 the @code{gzip} file header, plus 5 bytes every 32K block, or an expansion
155 ratio of 0.015% for large files. Note that the actual number of used
156 disk blocks almost never increases. @code{gzip} preserves the mode,
157 ownership and timestamps of files when compressing or decompressing.
159 The @code{gzip} file format is specified in P. Deutsch, @sc{gzip} file
160 format specification version 4.3,
161 @uref{ftp://ftp.isi.edu/in-notes/rfc1952.txt, Internet RFC 1952} (May
162 1996). The @code{zip} deflation format is specified in P. Deutsch,
163 @sc{deflate} Compressed Data Format Specification version 1.3,
164 @uref{ftp://ftp.isi.edu/in-notes/rfc1951.txt, Internet RFC 1951} (May
167 @node Sample, Invoking gzip, Overview, Top
168 @chapter Sample Output
171 Here are some realistic examples of running @code{gzip}.
173 This is the output of the command @samp{gzip -h}:
176 gzip @var{version-number}
177 usage: gzip [-cdfhlLnNrtvV19] [-S suffix] [file ...]
178 -c --stdout write on standard output, keep original files unchanged
179 -d --decompress decompress
180 -f --force force overwrite of output file and compress links
181 -h --help give this help
182 -l --list list compressed file contents
183 -L --license display software license
184 -n --no-name do not save or restore the original name and time stamp
185 -N --name save or restore the original name and time stamp
186 -q --quiet suppress all warnings
187 -r --recursive operate recursively on directories
188 -S .suf --suffix .suf use suffix .suf on compressed files
189 -t --test test compressed file integrity
190 -v --verbose verbose mode
191 -V --version display version number
192 -1 --fast compress faster
193 -9 --best compress better
194 file... files to (de)compress. If none given, use standard input.
195 Report bugs to <bug-gzip@@gnu.org>.
198 This is the output of the command @samp{gzip -v texinfo.tex}:
201 texinfo.tex: 69.7% -- replaced with texinfo.tex.gz
204 The following command will find all @code{gzip} files in the current
205 directory and subdirectories, and extract them in place without
206 destroying the original:
209 find . -name '*.gz' -print | sed 's/^\(.*\)[.]gz$/gunzip < "&" > "\1"/' | sh
212 @node Invoking gzip, Advanced usage, Sample, Top
213 @chapter Invoking @code{gzip}
217 The format for running the @code{gzip} program is:
220 gzip @var{option} @dots{}
223 @code{gzip} supports the following options:
229 Write output on standard output; keep original files unchanged.
230 If there are several input files, the output consists of a sequence of
231 independently compressed members. To obtain better compression,
232 concatenate all input files before compressing them.
241 Force compression or decompression even if the file has multiple links
242 or the corresponding file already exists, or if the compressed data
243 is read from or written to a terminal. If the input data is not in
244 a format recognized by @code{gzip}, and if the option @samp{--stdout} is also
245 given, copy the input data without change to the standard output: let
246 @code{zcat} behave as @code{cat}. If @samp{-f} is not given, and
247 when not running in the background, @code{gzip} prompts to verify
248 whether an existing file should be overwritten.
252 Print an informative help message describing the options then quit.
256 For each compressed file, list the following fields:
259 compressed size: size of the compressed file
260 uncompressed size: size of the uncompressed file
261 ratio: compression ratio (0.0% if unknown)
262 uncompressed_name: name of the uncompressed file
265 The uncompressed size is given as @samp{-1} for files not in @code{gzip}
266 format, such as compressed @samp{.Z} files. To get the uncompressed size for
267 such a file, you can use:
273 In combination with the @samp{--verbose} option, the following fields are also
277 method: compression method (deflate,compress,lzh,pack)
278 crc: the 32-bit CRC of the uncompressed data
279 date & time: time stamp for the uncompressed file
282 The crc is given as ffffffff for a file not in gzip format.
284 With @samp{--verbose}, the size totals and compression ratio for all files
285 is also displayed, unless some sizes are unknown. With @samp{--quiet},
286 the title and totals lines are not displayed.
288 The @code{gzip} format represents the the input size modulo
289 @math{2^32}, so the uncompressed size and compression ratio are listed
290 incorrectly for uncompressed files 4 GB and larger. To work around
291 this problem, you can use the following command to discover a large
292 uncompressed file's true size:
300 Display the @code{gzip} license then quit.
304 When compressing, do not save the original file name and time stamp by
305 default. (The original name is always saved if the name had to be
306 truncated.) When decompressing, do not restore the original file name
307 if present (remove only the @code{gzip}
308 suffix from the compressed file name) and do not restore the original
309 time stamp if present (copy it from the compressed file). This option
310 is the default when decompressing.
314 When compressing, always save the original file name and time stamp; this
315 is the default. When decompressing, restore the original file name and
316 time stamp if present. This option is useful on systems which have
317 a limit on file name length or when the time stamp has been lost after
322 Suppress all warning messages.
326 Travel the directory structure recursively. If any of the file names
327 specified on the command line are directories, @code{gzip} will descend
328 into the directory and compress all the files it finds there (or
329 decompress them in the case of @code{gunzip}).
331 @item --suffix @var{suf}
333 Use suffix @samp{@var{suf}} instead of @samp{.gz}. Any suffix can be
334 given, but suffixes other than @samp{.z} and @samp{.gz} should be
335 avoided to avoid confusion when files are transferred to other systems.
336 A null suffix forces gunzip to try decompression on all given files
337 regardless of suffix, as in:
340 gunzip -S "" * (*.* for MSDOS)
343 Previous versions of gzip used the @samp{.z} suffix. This was changed to
344 avoid a conflict with @code{pack}.
348 Test. Check the compressed file integrity.
352 Verbose. Display the name and percentage reduction for each file compressed.
356 Version. Display the version number and compilation options, then quit.
361 Regulate the speed of compression using the specified digit @var{n},
362 where @samp{-1} or @samp{--fast} indicates the fastest compression
363 method (less compression) and @samp{--best} or @samp{-9} indicates the
364 slowest compression method (optimal compression). The default
365 compression level is @samp{-6} (that is, biased towards high compression at
369 @node Advanced usage, Environment, Invoking gzip, Top
370 @chapter Advanced usage
371 @cindex concatenated files
373 Multiple compressed files can be concatenated. In this case,
374 @code{gunzip} will extract all members at once. If one member is
375 damaged, other members might still be recovered after removal of the
376 damaged member. Better compression can be usually obtained if all
377 members are decompressed and then recompressed in a single step.
379 This is an example of concatenating @code{gzip} files:
382 gzip -c file1 > foo.gz
383 gzip -c file2 >> foo.gz
398 In case of damage to one member of a @samp{.gz} file, other members can
399 still be recovered (if the damaged member is removed). However,
400 you can get better compression by compressing all members at once:
403 cat file1 file2 | gzip > foo.gz
406 compresses better than
409 gzip -c file1 file2 > foo.gz
412 If you want to recompress concatenated files to get better compression, do:
415 zcat old.gz | gzip > new.gz
418 If a compressed file consists of several members, the uncompressed
419 size and CRC reported by the @samp{--list} option applies to the last member
420 only. If you need the uncompressed size for all members, you can use:
426 If you wish to create a single archive file with multiple members so
427 that members can later be extracted independently, use an archiver such
428 as @code{tar} or @code{zip}. GNU @code{tar} supports the @samp{-z}
429 option to invoke @code{gzip} transparently. @code{gzip} is designed as a
430 complement to @code{tar}, not as a replacement.
432 @node Environment, Tapes, Advanced usage, Top
436 The environment variable @code{GZIP} can hold a set of default options for
437 @code{gzip}. These options are interpreted first and can be overwritten by
438 explicit command line parameters. For example:
441 for sh: GZIP="-8v --name"; export GZIP
442 for csh: setenv GZIP "-8v --name"
443 for MSDOS: set GZIP=-8v --name
446 On Vax/VMS, the name of the environment variable is @code{GZIP_OPT}, to
447 avoid a conflict with the symbol set for invocation of the program.
449 @node Tapes, Problems, Environment, Top
450 @chapter Using @code{gzip} on tapes
453 When writing compressed data to a tape, it is generally necessary to pad
454 the output with zeroes up to a block boundary. When the data is read and
455 the whole block is passed to @code{gunzip} for decompression,
456 @code{gunzip} detects that there is extra trailing garbage after the
457 compressed data and emits a warning by default if the garbage contains
458 nonzero bytes. You have to use the
459 @samp{--quiet} option to suppress the warning. This option can be set in the
460 @code{GZIP} environment variable, as in:
463 for sh: GZIP="-q" tar -xfz --block-compress /dev/rst0
464 for csh: (setenv GZIP "-q"; tar -xfz --block-compress /dev/rst0)
467 In the above example, @code{gzip} is invoked implicitly by the @samp{-z}
468 option of GNU @code{tar}. Make sure that the same block size (@samp{-b}
469 option of @code{tar}) is used for reading and writing compressed data on
470 tapes. (This example assumes you are using the GNU version of
473 @node Problems, Copying This Manual, Tapes, Top
474 @chapter Reporting Bugs
477 If you find a bug in @code{gzip}, please send electronic mail to
478 @email{bug-gzip@@gnu.org}. Include the version number,
479 which you can find by running @w{@samp{gzip -V}}. Also include in your
480 message the hardware and operating system, the compiler used to compile
482 a description of the bug behavior, and the input to @code{gzip} that triggered
485 @node Copying This Manual, Concept Index, Problems, Top
486 @appendix Copying This Manual
489 * GNU Free Documentation License:: License for copying this manual.
494 @node Concept Index, , Copying This Manual, Top
495 @appendix Concept Index