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GZIP(1)                                                                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 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 -gz for VMS, z for MSDOS, OS/2 FAT, Windows NT FAT and Atari.)  If no files are spec-
       ified, 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 decom-
       pressing 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,
       _z or .Z 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 respec-
       tively.  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 compress format was not designed to allow consistency checks. How-
       ever 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 'defla-
       tion' 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 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), Huff-
       man 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  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, con-
              catenate 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 back-
              ground, 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 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  decom-
              pressing,  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 decom-
              press 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 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 exam-
       ple:

             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  inde-
       pendently,  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 invo-
       cation 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 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 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 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 (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 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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