GZIP(1)                                                                                         General Commands Manual                                                                                         GZIP(1)

NAME
       gzip, gunzip, zcat - compress or expand files

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

DESCRIPTION
       Gzip  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 modifica‐
       tion times.  (The default extension is -gz for VMS, 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.  Gzip 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.  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, 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 time stamp 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  for‐
       mat.  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.

       zcat  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.

       Gzip 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 expan‐
       sion 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 run‐
              ning 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: time stamp 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 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 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.

       -N --name
              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.

       -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 ).

       --rsyncable
              While compressing, synchronize the output occasionally based on the input.  This increases size by less than 1 percent most cases, but means that the rsync(1) program can take advantage of similarities
              in the uncompressed input when synchronizing two files compressed with this flag.  gunzip cannot tell the difference between a compressed file created with this option, and one created without it.

       -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 sys‐
              tems.

              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.

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

       -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).

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 trans‐
       parently. gzip is designed as a complement to tar, not as a replacement.

ENVIRONMENT
       The 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. For example:
             for sh:    GZIP="-8v --name"; export GZIP
             for csh:   setenv GZIP "-8v --name"
             for MSDOS: set GZIP=-8v --name

       On Vax/VMS, the name of the environment variable is GZIP_OPT, to avoid a conflict with the symbol set for invocation of the program.

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, <http://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, <http://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 -- no change
              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 have to use the --quiet option to suppress the warning. This option can be set in the  GZIP  environ‐
       ment variable as in:
         for sh:  GZIP="-q"  tar -xfz --block-compress /dev/rst0
         for csh: (setenv GZIP -q; tar -xfz --block-compr /dev/rst0

       In  the  above  example,  gzip is invoked implicitly by the -z option of GNU tar. Make sure that the same block size (-b option of tar) is used for reading and writing compressed data on tapes.  (This example
       assumes you are using the GNU version of tar.)

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.

COPYRIGHT NOTICE
       Copyright © 1998, 1999, 2001, 2002, 2012 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 transla‐
       tion approved by the Foundation.

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