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NAME    [Toc]    [Back]

       gzip, gunzip, gzcat - Compresses or expands files.

SYNOPSIS    [Toc]    [Back]

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

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

       gzcat [-afhlLnNrtvV] [-S suffix] [name...]

OPTIONS    [Toc]    [Back]

       Specifies  ascii  text  mode;  converts  end-of-line using
       local conventions. This option is supported only  on  some
       nonUnix  systems. For MSDOS, CR LF is converted to LF when
       compressing, and LF is converted to CR LF when decompressing.
   Writes  output  on  standard output; keeps 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.

              The gzcat command is equivalent to the gunzip -c or
              gzip -cd command.  Specifies an  uncompress  operation.

              The  gunzip  command  is  equivalent to the gzip -d
              command.  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 the gzip command,
  and if the -c option is also specified, copy
              the input data without change to the standard  output;
  that  is, let the gzcat command behave as the
              cat command. If the -f option is not specified, and
              when  not  running in the background, the gzip command
 prompts to verify  whether  an  existing  file
              should  be overwritten.  Displays a help screen and
              quits.  Lists the following fields  for  each  compressed
  file:  Specifies  size  of  the compressed
              file.  Specifies size  of  the  uncompressed  file.
              Specifies  compression  ratio (or 0.0% if unknown).
              Specifies the name of the uncompressed file.

              The uncompressed size is given as -1 for files that
              are  not  in  the  gzip  format, such as compressed

              When used with the -v option, the following  fields
              are  also  displayed: Specifies compression method.
              Specifies the 32-bit CRC of the uncompressed  data.
              Specifies the time stamp for the uncompressed file.

              The compression  methods  currently  supported  are
              deflate,  compress, lzh (SCO compress -H) and pack.
              The crc value is given as ffffffff for a file  that
              is  not in the gzip format.   When used with the -N
              option, the uncompressed name and the date and time
              are  those  stored  within  the compressed file, if

              When used with the -v option, the size  totals  and
              compression  ratio for all files is also displayed,
              unless some sizes are unknown. When the  -q  option
              is  specified,  the  title and totals lines are not
              displayed.  Displays the gzip  license  and  quits.
              Specifies  that  the  original  file  name and time
              stamp are not saved when  compressing  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.  Specifies,
 when compressing, to always save the original
              file  name  and   time  stamp; this is the default.
              Specifies, when decompressing, to restore the original
  file  name  and time stamp, if present.  This
              option is useful on systems which have a  limit  on
              the  length  of  a file name or when the time stamp
              has been lost after a  file  transfer.   Suppresses
              all  warnings.   Travels  the  directory  structure
              recursively. If any of the file names specified  on
              the  command line are directories, the gzip command
              descends into the directory and compresses all  the
              files  it  finds there (or decompresses them in the
              case of  the  gunzip  command).   Uses  the  suffix
              instead  of  and should be avoided to remove confusion
 when files are transferred to  other  systems.
              A  null suffix forces the gunzip command to attempt
              decompressing all given  files  regardless  of  the
              suffix, as follows:

              gunzip -S  *       (*.* for MSDOS)

              Previous versions of the gzip command used the suffix.
 This was changed to avoid a conflict with  the
              pack command.  Specifies that the compressed file's
              integrity be tested.  Specifies verbose mode.  Displays
  the  name  and percentage reduction for each
              file compressed  or  decompressed.   Specifies  the
              version  number  and  compilation  options and then
              quits.  Regulates the speed of compression by using
              the specified digit #, for which -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 the expense of speed).

DESCRIPTION    [Toc]    [Back]

       The gzip command reduces the size of specified files using
       the  Lempel-Ziv  coding  (LZ77).   Whenever possible, each
       file is replaced by one with the extension  modes,  access
       and modification times.

       If  files are not specified, or if a file name is "-", the
       standard input is compressed to the standard  output.  The
       gzip  command only attempts to compress regular files.  In
       particular, it ignores symbolic links.

       By default, the gzip command keeps the original file  name
       and time stamp 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 the gzip command with the -d option, or by using the
       gunzip command.

       The gunzip command takes a list  of  specified  files  and
       replaces each file that begins with the correct magic number
 and whose name ends with -gz, -z, _z or with an uncompressed
  file  without  the original extension. The gunzip
       command also recognizes the  special   extensions  and  as
       shorthands  for  and  respectively.  When compressing, the
       gzip command uses the extension if  necessary  instead  of
       truncating a file with a extension.

       The gunzip command can currently decompress files that are
       created by the gzip, zip, compress, compress  -H  or  pack
       commands.  The detection of the input format is automatic.
       When using the  first  two  formats,  the  gunzip  command
       checks  a  32-bit  CRC.   For the pack command, the gunzip
       command checks  the  uncompressed  length.   Although  the
       standard compress format was not designed to allow consistency
 checks, the gunzip  command  is  sometimes  able  to
       detect  a  bad  file in cases where the uncompress command
       does not. Therefore, if you get an error when  uncompressing
  a file, do not assume that the file is correct if the
       same file can be decompressed without error by the  uncompress
 command.  In this case, the uncompress command probably
 did not process the input  file  correctly,  and  the
       generated output file is not useful.

       The gzip command uses the Lempel-Ziv algorithm used in the
       zip  and  PKZIP  commands.   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 the compress command), Huffman coding (as used in
       the pack command), or adaptive Huffman coding (in the 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.  The gzip command preserves
       the  mode,  ownership  and  time stamps of files when compressing
 or decompressing.

   Advanced Usage    [Toc]    [Back]
       Multiple compressed files can  be  concatenated.  In  this
       case, the gunzip command extracts all members at once. For
       example: gzip -c file1  > foo.gz gzip -c file2 >> foo.gz

       Using the previous example, gunzip -c foo is equivalent to
       cat file1 file2.

       In  case  a member of a file is damaged, other members can
       still be recovered (if the  damaged  member  is  removed).
       However,  you  gain  better compression by compressing all
       members at once as follows:  cat  file1  file2  |  gzip  >

       The preceding command line compresses better than the following
 one: gzip -c file1 file2 > foo.gz

       If you want to recompress concatenated files to gain  better
  compression, do the following: gzip -cd old.gz | gzip
       > new.gz

       If a compressed file  consists  of  several  members,  the
       uncompressed  size  and  CRC  reported  by  the  -l option
       applies to the last member only.  If you need  the  uncompressed
  size  for all members, use the following command:
       gzip -cd file.gz | wc -c

       To create a single archive file with multiple  members  so
       that  members can later be extracted independently, use an
       archiver such as the tar or zip commands. GNU tar supports
       the  -z  option  to invoke the gzip command transparently.
       The gzip command is designed as a complement  to  the  tar
       command, not as a replacement.

   Environment    [Toc]    [Back]
       The  environment  variable  GZIP can hold a set of default
       options for the gzip command.  These  options  are  interpreted
  first  and  can be overwritten by explicit command
       line options as follows.

       GZIP="-8v --name"; export GZIP (for sh) setenv  GZIP  "-8v
       --name" (for csh)

RESTRICTIONS    [Toc]    [Back]

       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 the gunzip command for decompression, the gunzip
       command detects that there is extra trailing garbage after
       the compressed data and emits a warning by  default.   You
       have  to  use  the  -q  option  to suppress warnings. This
       option can be set in the GZIP environment variable as follows:

       GZIP="-q"   tar  -xfz  --block-compress /dev/tape/tape0_d1
       (for  sh)  (setenv  GZIP  -q;   tar   -xfz   --block-compr
       /dev/tape/tape0_d1 (for csh)

       In  the  previous  example,  the  gzip  command is invoked
       implicitly by the -z option of the GNU tar  command.  Make
       sure  that the same block size (specified by the -b option
       of the tar command) is used for reading and  writing  compressed
 data on tapes.  (This example assumes that you are
       using the GNU version of the tar command.)

       The --list flag reports incorrect sizes if they  exceed  2
       gigabytes.  The --list flag reports sizes as -1 and crc as
       ffffffff if the compressed file is on a nonseekable media.

       In  rare  cases,  the  --best flag gives worse compression
       than the default compression level (-6).  On  some  highly
       redundant  files,  the  compress command compresses better
       than gzip command.

       [Tru64  UNIX]  The  gzip  command  may  not  preserve  the
       extended  file  attributes  (property  list)  of  a  file,
       including any access control lists (ACL). Verify that  any
       ACLs are not removed or modified by using gzip.

EXIT STATUS    [Toc]    [Back]

       Success.  An error occurred.  A warning is encountered.

SEE ALSO    [Toc]    [Back]

       Commands:  compress(1), pack(1)

       Files:  acl(4)

[ Back ]
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