gzip Documentation

This is the 'gzip.doc' included with gzip. Italic paragraphs are unimplemented or meaningless in MacGzip.

GZIP(1)                  USER COMMANDS                    GZIP(1)

     gzip, gunzip, zcat - compress or expand files

     gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]

     gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]
     zcat [ -fhLV ] [ name ... ]

     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 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  out-
     put.   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  del-
     imited  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

     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 out-
     put.   The  SCO  compress -H format (lzh compression method)
     does not include a CRC  but  also  allows  some  consistency

     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.

     -a --ascii
          Ascii text mode: convert end-of-lines using local  con-
          ventions.  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

     -c --stdout --to-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

     -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 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 stan-
          dard 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 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 decompress-
          ing, 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 direc-
          tories,  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 compres-
          sion).  The default compression level is -6  (that  is,
          biased towards high compression at expense of speed).

     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

           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 indepen-
     dently, 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.

     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
           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.

     znew(1), zcmp(1),  zmore(1),  zforce(1),  gzexe(1),  zip(1),
     unzip(1), compress(1), pack(1), compact(1)

     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
     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.
             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  direc-
             tory,  (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.

     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:
       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.)

     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.