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GZIP(1)								       GZIP(1)

NAME
       gzip, gunzip, zcat - compress or expand files

SYNOPSIS
       gzip [ -acdfhlLnrtvV19 ] [-S suffix] [ name ...	]
       gunzip [ -acfhlLnrtvV ] [-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
       modification times.  (The default extension  is	-gz  for  VMS,	z  for
       MSDOS,  OS/2  FAT  and Atari.)  If no files are specified, the standard
       input is compressed to the standard output.  Gzip will only attempt  to
       compress regular files.	In particular, it will ignore symbolic links.

       If the new file name is too long for its file system, gzip truncates it
       and keeps the original file name in the compressed file.	 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.

       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
       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 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
	      converted 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
	      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 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 --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 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.

       -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 .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.	 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   (optimal	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:

	     zcat 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.

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="-8 -v"; export GZIP
	     for csh:	setenv GZIP "-8 -v"
	     for MSDOS: set GZIP=-8 -v

       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),  zip(1), unzip(1),
       compress(1), pack(1), compact(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 [-cdfhlLnrtvV19] [-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
	       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.

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 /dev/rmt/datn
	   for csh:   (setenv 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.

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
       compressed file is on a non seekable media.

Free Software Foundation	August 10, 1993			       GZIP(1)
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