GZIP(1) 						      General Commands Manual							   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 exten-
       sion .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 out-
       put.  Gzip will only attempt to compress 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 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 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 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 trun-
       cating 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  uncom-
       pressing  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  com-
       pression 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 uncom-
       presses 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 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 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 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 --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  com-
	      pressed  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 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 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 (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 transparently. gzip is designed as a complement to tar, not as a replace-
       ment.

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), 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 [-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 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  envi-
       ronment 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
       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, com-
       press compresses better than gzip.

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