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[gzip] / gzip.doc

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 modi-
       fication 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 particu-
       lar, it will ignore symbolic links.

       If the compressed file name is too long for its file system, gzip trun-
       cates  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 com-
       pressed  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, _z or .Z 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,
       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 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 dis-
       tribution 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
       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 spec-
       ification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Inter-
       net 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 con-
              verted 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 com-
              pressed 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 for-
              mat, 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,  com-
              press,  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 ).

       -S .suf --suffix .suf
              Use suffix .suf instead of .gz. Any suffix  can  be  given,  but
              suffixes other than .z and .gz should be avoided to avoid confu-
              sion when files are transferred to other systems.  A null suffix
              forces  gunzip  to  try decompression on all given files regard-
              less of suffix, as in:

                  gunzip -S "" *       (*.* for MSDOS)

              Previous versions of gzip used the .z suffix. This  was  changed
              to avoid a conflict with 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 com-
              pression 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  transpar-
       ently. 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), com-
       press(1), pack(1), compact(1)

       The gzip file format is specified in P. Deutsch, GZIP file format spec-
       ification version 4.3, <ftp://ftp.isi.edu/in-notes/rfc1952.txt>, Inter-
       net  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.  Recom-
              press 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 multi-
              ply-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 com-
       pressed 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, com-
       press 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  per-
       mission notice identical to this one.

       Permission  is granted to copy and distribute translations of this man-
       ual into another language, under the above conditions for modified ver-
       sions,  except  that this permission notice may be stated in a transla-
       tion approved by the Foundation.

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