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Linux 2.6 - man page for gzip (linux section 1)

GZIP(1) 			     General Commands Manual				  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 possi-
       ble, 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 com-
       press 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  trun-
       cated.  For  example,  if  file names are limited to 14 characters, gzip.msdos.exe is com-
       pressed 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  com-
       pressed	file  name  was  truncated  or when the time stamp was not preserved after a file

       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 com-
       pressing, 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 com-
       press 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  com-
       plain.  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 com-
       pressed 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 pre-
       serve 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 suf-
       fix 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 gener-
       ally 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).

       -a --ascii
	      Ascii text mode: convert end-of-lines using local conventions. This option is  sup-
	      ported only on some non-Unix systems. For MSDOS, CR LF is converted to LF when com-
	      pressing, 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  sev-
	      eral  input  files,  the	output consists of a sequence of independently compressed
	      members. To obtain better compression, concatenate all input files before compress-
	      ing them.

       -d --decompress --uncompress

       -f --force
	      Force  compression or decompression even if the file has multiple links or the cor-
	      responding 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  com-
	      pressed .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 com-
	      press -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 dis-
	      played, 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  decom-
	      pressing,  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 decom-

       -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 ).

	      While  compressing,  synchronize	the output occasionally based on the input.  This
	      increases size by less than 1 percent most cases, but means that the rsync(1)  pro-
	      gram can much more efficiently synchronize files compressed with this flag.  gunzip
	      cannot tell the difference between a compressed file created with this option,  and
	      one created without it.

       -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

       -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 indi-
	      cates the slowest compression method (best compression).	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 mem-
       bers 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 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

       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 invocation of the program.

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

       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 com-
	      presses 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

       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.

       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 gun-
       zip  for decompression, gunzip detects that there is extra trailing garbage after the com-
       pressed data and emits a warning by default. You have to use the --quiet  option  to  sup-
       press 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 gzip format represents the input size modulo 2^32, so the --list option reports incor-
       rect 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 uncom-
       pressed 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 (C) 1998, 1999, 2001, 2002 Free Software Foundation, Inc.
       Copyright (C) 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  dis-
       tributed under the terms of a permission notice identical to this one.

       Permission is granted to copy and distribute translations of this manual into another lan-
       guage, 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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