gunzip

GZIP(1)                      General Commands Manual                     GZIP(1)



NAME
       gzip, gunzip, zcat - compress or expand files

SYNOPSIS
       gzip [ -acdfhklLnNrtvV19 ] [-S suffix] [ name ...  ]
       gunzip [ -acfhklLnNrtvV ] [-S suffix] [ name ...  ]
       zcat [ -fhLV ] [ name ...  ]

DESCRIPTION
       The gzip command reduces the size of the named files using Lempel-Ziv
       coding (LZ77).  Whenever possible, each file is replaced by one with the
       extension .gz, while keeping the same ownership modes, access and
       modification times.  (The default extension is z for MSDOS, OS/2 FAT,
       Windows NT FAT and Atari.)  If no files are specified, or if a file name
       is "-", the standard input is compressed to the standard output.  The
       gzip command will only attempt to compress regular files.  In particular,
       it will ignore symbolic links.

       If the compressed file name is too long for its file system, gzip
       truncates it.  The gzip command 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, gzip keeps the original file name and timestamp in the
       compressed file. These are used when decompressing the file with the -N
       option. This is useful when the compressed file name was truncated or
       when the timestamp was not preserved after a file transfer.

       Compressed files can be restored to their original form using gzip -d or
       gunzip or 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.

       gunzip takes a list of files on its command line and replaces each file
       whose name ends with .gz, -gz, .z, -z, or _z (ignoring case) and which
       begins with the correct magic number with an uncompressed file without
       the original extension.  gunzip also recognizes the special extensions
       .tgz and .taz as shorthands for .tar.gz and .tar.Z respectively.  When
       compressing, gzip uses the .tgz extension if necessary instead of
       truncating a file with a .tar extension.

       gunzip can currently decompress files created by gzip, zip, compress,
       compress -H or pack.  The detection of the input format is automatic.
       When using the first two formats, gunzip checks a 32 bit CRC. For pack
       and gunzip checks the uncompressed length. The standard compress format
       was not designed to allow consistency checks. However gunzip is sometimes
       able to detect a bad .Z file. If you get an error when uncompressing a .Z
       file, do not assume that the .Z file is correct simply because the
       standard uncompress does not complain. This generally means that the
       standard uncompress does not check its input, and happily generates
       garbage output.  The SCO compress -H format (lzh compression method) does
       not include a CRC but also allows some consistency checks.

       Files created by zip can be uncompressed by gzip only if they have a
       single member compressed with the 'deflation' method. This feature is
       only intended to help conversion of tar.zip files to the tar.gz format.
       To extract a zip file with a single member, use a command like 'gunzip
       <foo.zip' or 'gunzip -S .zip foo.zip'.  To extract zip files with several
       members, use unzip instead of gunzip.

       The zcat command is identical to gunzip -c.  (On some systems, zcat may
       be installed as gzcat to preserve the original link to compress.)  zcat
       uncompresses either a list of files on the command line or its standard
       input and writes the uncompressed data on standard output.  zcat will
       uncompress files that have the correct magic number whether they have a
       .gz suffix or not.

       The gzip command uses the Lempel-Ziv algorithm used in zip and PKZIP.
       The amount of compression obtained depends on the size of the input and
       the distribution of common substrings.  Typically, text such as source
       code or English is reduced by 60-70%.  Compression is generally much
       better than that achieved by LZW (as used in compress), Huffman coding
       (as used in pack), or adaptive Huffman coding (compact).

       Compression is always performed, even if the compressed file is slightly
       larger than the original. The worst case expansion is a few bytes for the
       gzip file header, plus 5 bytes every 32K block, or an expansion ratio of
       0.015% for large files. Note that the actual number of used disk blocks
       almost never increases.  gzip preserves the mode, ownership and
       timestamps of files when compressing or decompressing.

OPTIONS
       -a --ascii
              Ascii text mode: convert end-of-lines using local conventions.
              This option is supported only on some non-Unix systems. For MSDOS,
              CR LF is converted to LF when compressing, and LF is converted to
              CR LF when decompressing.

       -c --stdout --to-stdout
              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.

       -d --decompress --uncompress
              Decompress.

       -f --force
              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 gzip, and if the
              option --stdout is also given, copy the input data without change
              to the standard output: let zcat behave as cat.  If -f is not
              given, and when not running in the background, gzip prompts to
              verify whether an existing file should be overwritten.

       -h --help
              Display a help screen and quit.

       -k --keep
              Keep (don't delete) input files during compression or
              decompression.

       -l --list
              For each compressed file, list the following fields:

                  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

              The uncompressed size is given as -1 for files not in gzip format,
              such as compressed .Z files. To get the uncompressed size for such
              a file, you can use:

                  zcat file.Z | wc -c

              In combination with the --verbose option, the following fields are
              also displayed:

                  method: compression method
                  crc: the 32-bit CRC of the uncompressed data
                  date & time: timestamp for the uncompressed file

              The compression methods currently supported are deflate, compress,
              lzh (SCO compress -H) and pack.  The crc is given as ffffffff for
              a file not in gzip format.

              With --name, the uncompressed name,  date and time  are those
              stored within the compress file if present.

              With --verbose, the size totals and compression ratio for all
              files is also displayed, unless some sizes are unknown. With
              --quiet, the title and totals lines are not displayed.

       -L --license
              Display the gzip license and quit.

       -n --no-name
              When compressing, do not save the original file name and timestamp
              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 gzip suffix from the
              compressed file name) and do not restore the original timestamp if
              present (copy it from the compressed file). This option is the
              default when decompressing.

       -N --name
              When compressing, always save the original file name and
              timestamp; this is the default. When decompressing, restore the
              original file name and timestamp if present. This option is useful
              on systems which have a limit on file name length or when the
              timestamp has been lost after a file transfer.

       -q --quiet
              Suppress all warnings.

       -r --recursive
              Travel the directory structure recursively. If any of the file
              names specified on the command line are directories, gzip will
              descend into the directory and compress all the files it finds
              there (or decompress them in the case of gunzip ).

       -S .suf --suffix .suf
              When compressing, use suffix .suf instead of .gz.  Any non-empty
              suffix can be given, but suffixes other than .z and .gz should be
              avoided to avoid confusion when files are transferred to other
              systems.

              When decompressing, add .suf to the beginning of the list of
              suffixes to try, when deriving an output file name from an input
              file name.

       --synchronous
              Use synchronous output.  With this option, gzip is less likely to
              lose data during a system crash, but it can be considerably
              slower.

       -t --test
              Test. Check the compressed file integrity then quit.

       -v --verbose
              Verbose. Display the name and percentage reduction for each file
              compressed or decompressed.

       -V --version
              Version. Display the version number and compilation options then
              quit.

       -# --fast --best
              Regulate the speed of compression using the specified digit #,
              where -1 or --fast indicates the fastest compression method (less
              compression) and -9 or --best indicates the slowest compression
              method (best compression).  The default compression level is -6
              (that is, biased towards high compression at expense of speed).

       --rsyncable
              When you synchronize a compressed file between two computers, this
              option allows rsync to transfer only files that were changed in
              the archive instead of the entire archive.  Normally, after a
              change is made to any file in the archive, the compression
              algorithm can generate a new version of the archive that does not
              match the previous version of the archive. In this case, rsync
              transfers the entire new version of the archive to the remote
              computer.  With this option, rsync can transfer only the changed
              files as well as a small amount of metadata that is required to
              update the archive structure in the area that was changed.

ADVANCED USAGE
       Multiple compressed files can be concatenated. In this case, gunzip will
       extract all members at once. For example:

             gzip -c file1  > foo.gz
             gzip -c file2 >> foo.gz

       Then

             gunzip -c foo

       is equivalent to

             cat file1 file2

       In case of damage to one member of a .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:

             cat file1 file2 | gzip > foo.gz

       compresses better than

             gzip -c file1 file2 > foo.gz

       If you want to recompress concatenated files to get better compression,
       do:

             gzip -cd old.gz | gzip > new.gz

       If a compressed file consists of several members, the uncompressed size
       and CRC reported by the --list option applies to the last member only. If
       you need the uncompressed size for all members, you can use:

             gzip -cd file.gz | wc -c

       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 tar
       or zip. GNU tar supports the -z option to invoke gzip transparently. gzip
       is designed as a complement to tar, not as a replacement.

ENVIRONMENT
       The obsolescent environment variable GZIP can hold a set of default
       options for 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 gzip warns if
       it is used.  This feature will be removed in a future release of gzip.

       You can use an alias or script instead.  For example, if gzip is in the
       directory /usr/bin you can prepend $HOME/bin to your PATH and create an
       executable script $HOME/bin/gzip containing the following:

             #! /bin/sh
             export PATH=/usr/bin
             exec gzip -9 "$@"

SEE ALSO
       znew(1), zcmp(1), zmore(1), zforce(1), gzexe(1), zip(1), unzip(1),
       compress(1)

       The gzip file format is specified in P. Deutsch, GZIP file format
       specification version 4.3, <https://www.ietf.org/rfc/rfc1952.txt>,
       Internet RFC 1952 (May 1996).  The zip deflation format is specified in
       P. Deutsch, DEFLATE Compressed Data Format Specification version 1.3,
       <https://www.ietf.org/rfc/rfc1951.txt>, Internet RFC 1951 (May 1996).

DIAGNOSTICS
       Exit status is normally 0; if an error occurs, exit status is 1. If a
       warning occurs, exit status is 2.

       Usage: gzip [-cdfhklLnNrtvV19] [-S suffix] [file ...]
              Invalid options were specified on the command line.

       file: not in gzip format
              The file specified to gunzip has not been compressed.

       file: Corrupt input. Use zcat to recover some data.
              The compressed file has been damaged. The data up to the point of
              failure can be recovered using

                    zcat file > recover

       file: compressed with xx bits, can only handle yy bits
              File was compressed (using LZW) by a program that could deal with
              more bits than the decompress code on this machine.  Recompress
              the file with gzip, which compresses better and uses less memory.

       file: already has .gz suffix -- unchanged
              The file is assumed to be already compressed.  Rename the file and
              try again.

       file already exists; do you wish to overwrite (y or n)?
              Respond "y" if you want the output file to be replaced; "n" if
              not.

       gunzip: corrupt input
              A SIGSEGV violation was detected which usually means that the
              input file has been corrupted.

       xx.x% Percentage of the input saved by compression.
              (Relevant only for -v and -l.)

       -- not a regular file or directory: ignored
              When the input file is not a regular file or directory, (e.g. a
              symbolic link, socket, FIFO, device file), it is left unaltered.

       -- has xx other links: unchanged
              The input file has links; it is left unchanged.  See ln(1) for
              more information. Use the -f flag to force compression of
              multiply-linked files.

CAVEATS
       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 gunzip for decompression, gunzip detects
       that there is extra trailing garbage after the compressed data and emits
       a warning by default.  You can use the --quiet option to suppress the
       warning.

BUGS
       The gzip format represents the input size modulo 2^32, so the --list
       option reports incorrect uncompressed sizes and compression ratios for
       uncompressed files 4 GB and larger.  To work around this problem, you can
       use the following command to discover a large uncompressed file's true
       size:

             zcat file.gz | wc -c

       The --list option reports sizes as -1 and crc as ffffffff if the
       compressed file is on a non seekable media.

       In some rare cases, the --best option gives worse compression than the
       default compression level (-6). On some highly redundant files, compress
       compresses better than gzip.

REPORTING BUGS
       Report bugs to: bug-gzip@gnu.org
       GNU gzip home page: <https://www.gnu.org/software/gzip/>
       General help using GNU software: <https://www.gnu.org/gethelp/>

COPYRIGHT NOTICE
       Copyright © 1998-1999, 2001-2002, 2012, 2015-2021 Free Software
       Foundation, Inc.
       Copyright © 1992, 1993 Jean-loup Gailly

       Permission is granted to make and distribute verbatim copies of this
       manual provided the copyright notice and this permission notice are
       preserved on all copies.

       Permission is granted to copy and distribute modified versions of this
       manual under the conditions for verbatim copying, provided that the
       entire resulting derived work is distributed under the terms of a
       permission notice identical to this one.

       Permission is granted to copy and distribute translations of this manual
       into another language, under the above conditions for modified versions,
       except that this permission notice may be stated in a translation
       approved by the Foundation.




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