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gunzip(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.
       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


		  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.

       -# --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

       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-

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