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 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 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 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, or _z (ignoring case)
       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
       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), 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 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, 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
              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 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 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 confusion
              when files are transferred to other systems.  A null suffix forces gunzip to  try decompression on all given files regardless 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 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 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 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 © 1998, 1999, 2001, 2002 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 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.

                                                                     local                                                             GZIP(1)
 

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