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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 possi‐
       ble, each file is replaced by one with the extension .gz, while keeping the same ownership
       modes, access and modification 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 com‐
       press 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  trun‐
       cated.  For  example,  if  file names are limited to 14 characters, gzip.msdos.exe is com‐
       pressed 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  com‐
       pressed  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 num‐
       ber 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 com‐
       press 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  com‐
       plain.  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 com‐
       pressed 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.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  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,  , Internet RFC 1952 (May 1996).  The zip defla‐
       tion format is specified in P. Deutsch, DEFLATE Compressed Data Format Specification  ver‐
       sion 1.3, , 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  com‐
              presses 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 gun‐
       zip for decompression, gunzip detects that there is extra trailing garbage after the  com‐
       pressed  data  and  emits a warning by default. You have to use the --quiet option to sup‐
       press the warning. This option can be set in the GZIP environment 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 incor‐
       rect 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  uncom‐
       pressed 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 dis‐
       tributed under the terms of a permission notice identical to this one.

       Permission is granted to copy and distribute translations of this manual into another lan‐
       guage,  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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