\input texinfo @c -*-texinfo-*- @c %**start of header @setfilename gzip.info @documentencoding UTF-8 @include version.texi @settitle GNU Gzip @finalout @setchapternewpage odd @c %**end of header @copying This manual is for GNU Gzip (version @value{VERSION}, @value{UPDATED}), and documents commands for compressing and decompressing data. Copyright @copyright{} 1998-1999, 2001-2002, 2006-2007, 2009-2016 Free Software Foundation, Inc. Copyright @copyright{} 1992, 1993 Jean-loup Gailly @quotation Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. @end quotation @end copying @dircategory Compression @direntry * Gzip: (gzip). General (de)compression of files (lzw). @end direntry @dircategory Individual utilities @direntry * gunzip: (gzip)Overview. Decompression. * gzexe: (gzip)Overview. Compress executables. * zcat: (gzip)Overview. Decompression to stdout. * zdiff: (gzip)Overview. Compare compressed files. * zforce: (gzip)Overview. Force .gz extension on files. * zgrep: (gzip)Overview. Search compressed files. * zmore: (gzip)Overview. Decompression output by pages. @end direntry @titlepage @title GNU gzip @subtitle The data compression program @subtitle for Gzip version @value{VERSION} @subtitle @value{UPDATED} @author by Jean-loup Gailly @page @vskip 0pt plus 1filll @insertcopying @end titlepage @contents @ifnottex @node Top @top GNU Gzip: General file (de)compression @insertcopying @end ifnottex @menu * Overview:: Preliminary information. * Sample:: Sample output from @command{gzip}. * Invoking gzip:: How to run @command{gzip}. * Advanced usage:: Concatenated files. * Environment:: The @env{GZIP} environment variable * Tapes:: Using @command{gzip} on tapes. * Problems:: Reporting bugs. * GNU Free Documentation License:: Copying and sharing this manual. * Concept index:: Index of concepts. @end menu @node Overview @chapter Overview @cindex overview @command{gzip} reduces the size of the named files using Lempel--Ziv coding (LZ77). Whenever possible, each file is replaced by one with the extension @samp{.gz}, while keeping the same ownership modes, access and modification times. (The default extension is @samp{-gz} for @abbr{VMS}, @samp{z} for @abbr{MSDOS}, @abbr{OS/2} @abbr{FAT} and Atari.) If no files are specified or if a file name is @file{-}, the standard input is compressed to the standard output. @command{gzip} will only attempt to compress regular files. In particular, it will ignore symbolic links. If the new file name is too long for its file system, @command{gzip} truncates it. @command{gzip} attempts to truncate only the parts of the file name longer than 3 characters. (A part is delimited by dots.) If the name consists of small parts only, the longest parts are truncated. For example, if file names are limited to 14 characters, gzip.msdos.exe is compressed to gzi.msd.exe.gz. Names are not truncated on systems which do not have a limit on file name length. By default, @command{gzip} keeps the original file name and time stamp in the compressed file. These are used when decompressing the file with the @option{-N} option. This is useful when the compressed file name was truncated or when the time stamp was not preserved after a file transfer. However, due to limitations in the current @command{gzip} file format, fractional seconds are discarded. Also, time stamps must fall within the range 1970-01-01 00:00:00 through 2106-02-07 06:28:15 @abbr{UTC}, and hosts whose operating systems use 32-bit time stamps are further restricted to time stamps no later than 2038-01-19 03:14:07 @abbr{UTC}. The upper bounds assume the typical case where leap seconds are ignored. Compressed files can be restored to their original form using @samp{gzip -d} or @command{gunzip} or @command{zcat}. If the original name saved in the compressed file is not suitable for its file system, a new name is constructed from the original one to make it legal. @command{gunzip} takes a list of files on its command line and replaces each file whose name ends with @samp{.gz}, @samp{.z} @samp{-gz}, @samp{-z}, or @samp{_z} (ignoring case) and which begins with the correct magic number with an uncompressed file without the original extension. @command{gunzip} also recognizes the special extensions @samp{.tgz} and @samp{.taz} as shorthands for @samp{.tar.gz} and @samp{.tar.Z} respectively. When compressing, @command{gzip} uses the @samp{.tgz} extension if necessary instead of truncating a file with a @samp{.tar} extension. @command{gunzip} can currently decompress files created by @command{gzip}, @command{zip}, @command{compress} or @command{pack}. The detection of the input format is automatic. When using the first two formats, @command{gunzip} checks a 32 bit @abbr{CRC} (cyclic redundancy check). For @command{pack}, @command{gunzip} checks the uncompressed length. The @command{compress} format was not designed to allow consistency checks. However @command{gunzip} is sometimes able to detect a bad @samp{.Z} file. If you get an error when uncompressing a @samp{.Z} file, do not assume that the @samp{.Z} file is correct simply because the standard @command{uncompress} does not complain. This generally means that the standard @command{uncompress} does not check its input, and happily generates garbage output. The @abbr{SCO} @samp{compress -H} format (@abbr{LZH} compression method) does not include a @abbr{CRC} but also allows some consistency checks. Files created by @command{zip} can be uncompressed by @command{gzip} only if they have a single member compressed with the ``deflation'' method. This feature is only intended to help conversion of @file{tar.zip} files to the @file{tar.gz} format. To extract a @command{zip} file with a single member, use a command like @samp{gunzip . @end example This is the output of the command @samp{gzip -v texinfo.tex}: @example texinfo.tex: 69.3% -- replaced with texinfo.tex.gz @end example The following command will find all regular @samp{.gz} files in the current directory and subdirectories (skipping file names that contain newlines), and extract them in place without destroying the original, stopping on the first failure: @example find . -name '* *' -prune -o -name '*.gz' -type f -print | sed " s/'/'\\\\''/g s/^\\(.*\\)\\.gz$/gunzip <'\\1.gz' >'\\1'/ " | sh -e @end example @node Invoking gzip @chapter Invoking @command{gzip} @cindex invoking @cindex options The format for running the @command{gzip} program is: @example gzip @var{option} @dots{} @end example @command{gzip} supports the following options: @table @option @item --stdout @itemx --to-stdout @itemx -c Write output on standard output; keep original files unchanged. If there are several input files, the output consists of a sequence of independently compressed members. To obtain better compression, concatenate all input files before compressing them. @item --decompress @itemx --uncompress @itemx -d Decompress. @item --force @itemx -f Force compression or decompression even if the file has multiple links or the corresponding file already exists, or if the compressed data is read from or written to a terminal. If the input data is not in a format recognized by @command{gzip}, and if the option @option{--stdout} is also given, copy the input data without change to the standard output: let @command{zcat} behave as @command{cat}. If @option{-f} is not given, and when not running in the background, @command{gzip} prompts to verify whether an existing file should be overwritten. @item --help @itemx -h Print an informative help message describing the options then quit. @item --keep @itemx -k Keep (don't delete) input files during compression or decompression. @item --list @itemx -l For each compressed file, list the following fields: @example compressed size: size of the compressed file uncompressed size: size of the uncompressed file ratio: compression ratio (0.0% if unknown) uncompressed_name: name of the uncompressed file @end example The uncompressed size is given as @minus{}1 for files not in @command{gzip} format, such as compressed @samp{.Z} files. To get the uncompressed size for such a file, you can use: @example zcat file.Z | wc -c @end example In combination with the @option{--verbose} option, the following fields are also displayed: @example method: compression method (deflate,compress,lzh,pack) crc: the 32-bit CRC of the uncompressed data date & time: time stamp for the uncompressed file @end example The @abbr{CRC} is given as ffffffff for a file not in gzip format. With @option{--verbose}, the size totals and compression ratio for all files is also displayed, unless some sizes are unknown. With @option{--quiet}, the title and totals lines are not displayed. The @command{gzip} format represents the input size modulo @math{2^32}, so the uncompressed size and compression ratio are listed incorrectly for uncompressed files 4 GiB and larger. To work around this problem, you can use the following command to discover a large uncompressed file's true size: @example zcat file.gz | wc -c @end example @item --license @itemx -L Display the @command{gzip} license then quit. @item --no-name @itemx -n When compressing, do not save the original file name and time stamp by default. (The original name is always saved if the name had to be truncated.) When decompressing, do not restore the original file name if present (remove only the @command{gzip} suffix from the compressed file name) and do not restore the original time stamp if present (copy it from the compressed file). This option is the default when decompressing. @item --name @itemx -N When compressing, always save the original file name and time stamp; this is the default. When decompressing, restore the original file name and time stamp if present. This option is useful on systems which have a limit on file name length or when the time stamp has been lost after a file transfer. @item --quiet @itemx -q Suppress all warning messages. @item --recursive @itemx -r Travel the directory structure recursively. If any of the file names specified on the command line are directories, @command{gzip} will descend into the directory and compress all the files it finds there (or decompress them in the case of @command{gunzip}). @item --rsyncable Cater better to the @command{rsync} program by periodically resetting the internal structure of the compressed data stream. This lets the @code{rsync} program take advantage of similarities in the uncompressed input when synchronizing two files compressed with this flag. The cost: the compressed output is usually about one percent larger. @item --suffix @var{suf} @itemx -S @var{suf} Use suffix @var{suf} instead of @samp{.gz}. Any suffix can be given, but suffixes other than @samp{.z} and @samp{.gz} should be avoided to avoid confusion when files are transferred to other systems. A null suffix forces gunzip to try decompression on all given files regardless of suffix, as in: @example gunzip -S "" * (*.* for MSDOS) @end example Previous versions of gzip used the @samp{.z} suffix. This was changed to avoid a conflict with @command{pack}. @item --synchronous Use synchronous output, by transferring output data to the output file's storage device when the file system supports this. Because file system data can be cached, without this option if the system crashes around the time a command like @samp{gzip FOO} is run the user might lose both @file{FOO} and @file{FOO.gz}; this is the default with @command{gzip}, just as it is the default with most applications that move data. When this option is used, @command{gzip} is safer but can be considerably slower. @item --test @itemx -t Test. Check the compressed file integrity. @item --verbose @itemx -v Verbose. Display the name and percentage reduction for each file compressed. @item --version @itemx -V Version. Display the version number and compilation options, then quit. @item --fast @itemx --best @itemx -@var{n} Regulate the speed of compression using the specified digit @var{n}, where @option{-1} or @option{--fast} indicates the fastest compression method (less compression) and @option{--best} or @option{-9} indicates the slowest compression method (optimal compression). The default compression level is @option{-6} (that is, biased towards high compression at expense of speed). @end table @node Advanced usage @chapter Advanced usage @cindex concatenated files Multiple compressed files can be concatenated. In this case, @command{gunzip} will extract all members at once. If one member is damaged, other members might still be recovered after removal of the damaged member. Better compression can be usually obtained if all members are decompressed and then recompressed in a single step. This is an example of concatenating @command{gzip} files: @example gzip -c file1 > foo.gz gzip -c file2 >> foo.gz @end example @noindent Then @example gunzip -c foo @end example @noindent is equivalent to @example cat file1 file2 @end example In case of damage to one member of a @samp{.gz} file, other members can still be recovered (if the damaged member is removed). However, you can get better compression by compressing all members at once: @example cat file1 file2 | gzip > foo.gz @end example @noindent compresses better than @example gzip -c file1 file2 > foo.gz @end example If you want to recompress concatenated files to get better compression, do: @example zcat old.gz | gzip > new.gz @end example If a compressed file consists of several members, the uncompressed size and @abbr{CRC} reported by the @option{--list} option applies to the last member only. If you need the uncompressed size for all members, you can use: @example zcat file.gz | wc -c @end example If you wish to create a single archive file with multiple members so that members can later be extracted independently, use an archiver such as @command{tar} or @command{zip}. @acronym{GNU} @command{tar} supports the @option{-z} option to invoke @command{gzip} transparently. @command{gzip} is designed as a complement to @command{tar}, not as a replacement. @node Environment @chapter Environment @cindex Environment The obsolescent environment variable @env{GZIP} can hold a set of default options for @command{gzip}. These options are interpreted first and can be overwritten by explicit command line parameters. As this can cause problems when using scripts, this feature is supported only for options that are reasonably likely to not cause too much harm, and @command{gzip} warns if it is used. This feature will be removed in a future release of @command{gzip}. You can use an alias or script instead. For example, if @command{gzip} is in the directory @samp{/usr/bin} you can prepend @file{$HOME/bin} to your @env{PATH} and create an executable script @file{$HOME/bin/gzip} containing the following: @example #! /bin/sh export PATH=/usr/bin exec gzip -9 "$@@" @end example On @abbr{VMS}, the name of the obsolescent environment variable is @env{GZIP_OPT}, to avoid a conflict with the symbol set for invocation of the program. @node Tapes @chapter Using @command{gzip} on tapes @cindex tapes When writing compressed data to a tape, it is generally necessary to pad the output with zeroes up to a block boundary. When the data is read and the whole block is passed to @command{gunzip} for decompression, @command{gunzip} detects that there is extra trailing garbage after the compressed data and emits a warning by default if the garbage contains nonzero bytes. You can use the @option{--quiet} option to suppress the warning. @node Problems @chapter Reporting Bugs @cindex bugs If you find a bug in @command{gzip}, please send electronic mail to @email{bug-gzip@@gnu.org}. Include the version number, which you can find by running @w{@samp{gzip -V}}. Also include in your message the hardware and operating system, the compiler used to compile @command{gzip}, a description of the bug behavior, and the input to @command{gzip} that triggered the bug.@refill @node GNU Free Documentation License @appendix GNU Free Documentation License @include fdl.texi @node Concept index @appendix Concept index @printindex cp @bye