man pages section 1: User Commands

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Updated: July 2014

gzcat (1)


gzcat - compress or expand files


gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ...  ]
gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ...  ]
gzcat [ -fhLV ] [ name ...  ]


User Commands                                             GZIP(1)

     gzip, gunzip, gzcat - compress or expand files

     gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ...  ]
     gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ...  ]
     gzcat [ -fhLV ] [ name ...  ]

     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 modification times.  (The  default  exten-
     sion  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 out-
     put.  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  com-
     pressed  to  Names are not truncated on sys-
     tems 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 gzcat.  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 exten-
     sion.  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,

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User Commands                                             GZIP(1)

     gunzip checks a 32 bit CRC.  For  pack,  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 out-
     put.  The SCO compress -H format  (lzh  compression  method)
     does  not  include  a  CRC  but also allows some consistency

     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  files  to the tar.gz format.  To extract a zip file
     with a single member, use a command like gunzip <  or
     gunzip  -S  .zip  To extract zip files with several
     members, use unzip instead of gunzip.

     gzcat is identical to gunzip -c.  (On  some  systems,  gzcat
     may  be installed as ggzcat to preserve the original link to
     compress.)  gzcat uncompresses either a list of files on the
     command  line  or  its  standard input and writes the uncom-
     pressed data on  standard  output.   gzcat  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.   Typi-
     cally,  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 expan-
     sion 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.

     The gzip file format is specified in P. Deutsch,  GZIP  file
     format  specification  version  4.3,  <
     notes/rfc1952.txt>, Internet RFC 1952 (May 1996).   The  zip
     deflation  format  is  specified in P. Deutsch, DEFLATE Com-
     pressed   Data    Format    Specification    version    1.3,
     <>,  Internet RFC 1951
     (May 1996).

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User Commands                                             GZIP(1)

     -a --ascii
          Ascii text mode: convert end-of-lines using local  con-
          ventions.  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 decom-

     -c --stdout --to-stdout
          Write output on standard output;  keep  original  files
          unchanged.   If there are several input files, the out-
          put consists of a sequence of independently  compressed
          members.  To obtain better compression, concatenate all
          input files before compressing them.

     -d --decompress --uncompress

     -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 writ-
          ten 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 stan-
          dard output: let gzcat 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

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

     -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:

              gzcat 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

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User Commands                                             GZIP(1)

              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 decom-
          pressing, do not restore  the  original  file  name  if
          present  (remove  only  the  gzip  suffix from the com-
          pressed 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 decompress-
          ing, 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 direc-
          tories,  gzip  will descend into the directory and com-
          press 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

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User Commands                                             GZIP(1)

          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 com-
          pression method (less compression)  and  -9  or  --best
          indicates the slowest compression method (best compres-
          sion).  The default compression level is -6  (that  is,
          biased towards high compression at expense of speed).

     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


           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:

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User Commands                                             GZIP(1)

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

     If a compressed file consists of several members, the uncom-
     pressed  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  indepen-
     dently, 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.

     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
           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 invo-
     cation of the program.

     See  attributes(5)  for  descriptions   of   the   following

     |Availability   | compress/gzip    |
     |Stability      | Committed        |
     gznew(1), gzcmp(1), gzmore(1), gzforce(1), gzexe(1), zip(1),
     unzip(1), compress(1), pack(1), compact(1)

     The gzip file format is specified in P. Deutsch,  GZIP  file
     format  specification  version  4.3,  <
     notes/rfc1952.txt>, Internet RFC 1952 (May 1996).   The  zip
     deflation  format  is  specified in P. Deutsch, DEFLATE Com-
     pressed   Data    Format    Specification    version    1.3,
     <>,  Internet RFC 1951
     (May 1996).

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User Commands                                             GZIP(1)

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

     Usage: gzip [-cdfhlLnNrtvV19] [-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 gzcat to recover some data.
          The  compressed  file  has been damaged. The data up to
          the point of failure can be recovered using

                gzcat 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.

     When writing compressed data to a tape, it is generally nec-
     essary 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

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User Commands                                             GZIP(1)

     trailing garbage after the compressed data and emits a warn-
     ing  by  default. You have to use the --quiet option to sup-
     press the warning. This option can be set in the GZIP  envi-
     ronment 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.)

     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 com-
     mand to discover a large uncompressed file's true size:

           gzcat 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 compres-
     sion than the default compression level (-6). On some highly
     redundant files, compress compresses better than gzip.

     Copyright  (C)  1998, 1999, 2001, 2002 Free Software Founda-
     tion, Inc.
     Copyright (C) 1992, 1993 Jean-loup Gailly

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

     Permission is granted to copy and distribute  modified  ver-
     sions of this manual under the conditions for verbatim copy-
     ing, 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 language, under the above condi-
     tions for modified versions,  except  that  this  permission
     notice  may be stated in a translation approved by the Foun-

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User Commands                                             GZIP(1)

     This  software  was   built   from   source   available   at    The  original
     community       source       was       downloaded       from

     Further  information about this software can be found on the
     open source community  website  at

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