1 GZIP
NAME
     gzip, gunzip, zcat - compress or expand files

SYNOPSIS
     gzip [ -cdfhLrtvV19 ] [ name ... ]
     gunzip [ -cfhLrtvV ] [ name ... ]
     zcat [ -hLV ] [ name ... ]

2 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 exten-
     sion is -gz for VMS, z for MSDOS, OS/2  and  Atari.)  If  no
     files are specified, the standard input is compressed to the
     standard output. If the new file  name  is  too  long,  gzip
     truncates  it  and  keeps  the  original  file  name  in the
     compressed file.  Gzip will only attempt to compress regular
     files.  In particular, it will ignore symbolic links.

     Compressed files can be  restored  to  their  original  form
     using gzip -d or gunzip or zcat.

     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.

     gunzip can currently decompress files created by gzip,  zip,
     compress  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 compress format was not designed to  allow  con-
     sistency  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 gen-
     erally means that the standard uncompress does not check its
     input, and happily generates garbage output.

     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  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.   Typi-
     cally,  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 expan-
     sion 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.

2 OPTIONS
     -c --stdout
          Write output on standard output;  keep  original  files
          unchanged.   If there are several input files, the out-
          put 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 writ-
          ten  to  a  terminal.  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 --license
          Display the gzip license.

     -q --quiet
          Suppress all warnings.

     -r --recurse
          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 -z --suffix -z
          Use suffix -z 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.  Previous versions of gzip used the -z suffix.
          This was changed to avoid a conflict with pack.

     -t --test
          Test. Check the compressed file integrity.

     -v --verbose
          Verbose. Display the name and percentage reduction  for
          each file compressed.

     -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  (optimal
          compression).  The default compression level is -5.

2 ENVIRONMENT
     The environment variable GZIP_OPT 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:
           GZIP_OPT == "-8 -v"

2 SEE ALSO
     compress, zip, unzip

2 DIAGNOSTICS
     Exit status is normally 0; if an error occurs,  exit  status
     is 1. If a warning occurs, exit status is 2.

     Usage: gzip [-cdfhLrStvV19] [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
                   define /user sys$output file.recover
                   zcat file
     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.)
     -- not a regular file or directory: ignored
             When the input file is not a regular file or  direc-
             tory, it is left unaltered.

2 CAVEATS
     On VMS:
     - upper case options need quotes: gzip "-V".
     - restoration of timestamps and version numbers is not supported
     - If a compressed file already exists, gzip -f overwrites it, it
       does not create a new version.
     - multi-part gzip files are not supported.
     - gunzip does not preserve the input file format. You can use a
       separate utility to restore the original format.
     - gunzip and zcat can be used only if you have created the
       links to gzip as documented in makegzip.com.  Otherwise
       you must use explicit parameters ("gzip -c" or "gzip -dc").

     When writing compressed data to  a  tape,  it  is  generally
     necessary  to pad the output with zeroes up to a block boun-
     dary. 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 warn-
     ing  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:
         GZIP == "-q"
         tar xfz /dev/rmt/datn

     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.

2 BUGS
     On VMS, files in VFC record format are not correctly handled by
     the C runtime library (the linefeed character is suppressed).
