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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 ownership modes, access and modi-
       fication	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 regular files.  In particu-
       lar, it will ignore symbolic links.

       If the compressed file name is too long for its file system, gzip trun-
       cates  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 timestamp in the com-
       pressed	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 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	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	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  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  dis-
       tribution 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 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 con-
	      verted 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	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 com-
	      pressed 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 for-
	      mat, 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,  com-
	      press,  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  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	confu-
	      sion when	files are transferred to other systems.	 A null	suffix
	      forces  gunzip  to  try decompression on all given files regard-
	      less 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 com-
	      pression method (best  compression).   The  default  compression
	      level is -6 (that	is, biased towards high	compression at expense
	      of speed).

ADVANCED USAGE
       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

       Then
	     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  transpar-
       ently. 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),	compress(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 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 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 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	 unal-
	       tered.
       -- 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  mul-
	       tiply-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 com-
       pressed 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, com-
       press compresses	better than gzip.

								       GZIP(1)

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | ADVANCED USAGE | ENVIRONMENT | SEE ALSO | DIAGNOSTICS | CAVEATS | BUGS

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