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GZIP(1)                              General Commands Manual                              GZIP(1)



NAME
       gzip, gunzip, zcat - compress or expand files

SYNOPSIS
       gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ...  ]
       gunzip [ -acfhlLnNrtvV ] [-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, 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.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 pre-
       serve 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 suf-
       fix 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.  Typ-
       ically,  text  such as source code or English is reduced by 60-70%.  Compression is gener-
       ally 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.

       The  gzip  file  format is specified in P. Deutsch, GZIP file format specification version
       4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Internet RFC  1952  (May  1996).   The  zip
       deflation  format is specified in P. Deutsch, DEFLATE Compressed Data Format Specification
       version 1.3, <ftp://ftp.isi.edu/in-notes/rfc1951.txt>, Internet RFC 1951 (May 1996).


OPTIONS
       -a --ascii
              Ascii text mode: convert end-of-lines using local conventions. This option is  sup-
              ported only on some non-Unix systems. For MSDOS, CR LF is converted to LF when com-
              pressing, 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  sev-
              eral  input  files,  the  output consists of a sequence of independently compressed
              members. To obtain better compression, concatenate all input files before compress-
              ing them.

       -d --decompress --uncompress
              Decompress.

       -f --force
              Force  compression or decompression even if the file has multiple links or the cor-
              responding 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.

       -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  com-
              pressed .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 com-
              press -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 dis-
              played, 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 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 decom-
              pressing.

       -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 ).

       -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.

              pack(1).

       -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  indi-
              cates  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  mem-
       bers 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 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), pack(1),
       compact(1)

       The gzip file format is specified in P. Deutsch, GZIP file  format  specification  version
       4.3,  <ftp://ftp.isi.edu/in-notes/rfc1952.txt>,  Internet  RFC  1952  (May 1996).  The zip
       deflation format is specified in P. Deutsch, DEFLATE Compressed Data Format  Specification
       version 1.3, <ftp://ftp.isi.edu/in-notes/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 [-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 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 (C) 1998, 1999, 2001, 2002 Free Software Foundation, 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 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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