


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 own-
       ership 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 regu-
       lar  files.  In particular, it will ignore symbolic links.

       If the compressed file name is too long for its file  sys-
       tem,  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  times-
       tamp  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 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  neces-
       sary 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 for-
       mats, gunzip checks a 32 bit CRC. For pack, gunzip  checks
       the  uncompressed length. The standard compress format was



                                                                1





GZIP(1)                                                   GZIP(1)


       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  compres-
       sion  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 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  uncom-
       press  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.  Typ-
       ically,  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.


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



                                                                2





GZIP(1)                                                   GZIP(1)


              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  ouput:  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 fol-
              lowing 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



                                                                3





GZIP(1)                                                   GZIP(1)


              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 trans-
              ferred to other systems.  A null suffix forces gun-
              zip  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).





                                                                4





GZIP(1)                                                   GZIP(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 speci-
              fied 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  compres-
              sion 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 mem-
       bers can still be recovered  (if  the  damaged  member  is
       removed).  However, you can get better compression by com-
       pressing 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



                                                                5





GZIP(1)                                                   GZIP(1)


       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  sup-
       ports  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)

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  com-
               pressed.
       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



                                                                6





GZIP(1)                                                   GZIP(1)


               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 --list option reports incorrect sizes if they exceed 2
       gigabytes.  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.















                                                                7


