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gzip(1)								      gzip(1)



NAME

  gzip,	gunzip,	gzcat -	Compresses or expands files.

SYNOPSIS

  gzip [-acdfhlLnNrtvV19] [-S suffix] [name...]

  gunzip [-acfhlLnNrtvV] [-S suffix] [name...]

  gzcat	[-afhlLnNrtvV] [-S suffix] [name...]

OPTIONS

  -a (--ascii)
      Specifies	ascii text mode; converts end-of-line using local conven-
      tions. This option is supported only on some nonUnix systems. For
      MSDOS, CR	LF is converted	to LF when compressing,	and LF is converted
      to CR LF when decompressing.

  -c (--stdout --to--stdout)
      Writes output on standard	output;	keeps original files unchanged.	 If
      there are	several	input files, the output	consists of a sequence of
      independently compressed members.	 To obtain better compression, con-
      catenate all input files before compressing them.

      The gzcat	command	is equivalent to the gunzip -c or gzip -cd command.

  -d (--decompress --uncompress)
      Specifies	an uncompress operation.

      The gunzip command is equivalent to the gzip -d command.

  -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	for-
      mat recognized by	the gzip command, and if the -c	option is also speci-
      fied, copy the input data	without	change to the standard output; that
      is, let the gzcat	command	behave as the cat command. If the -f option
      is not specified,	and when not running in	the background,	the gzip com-
      mand prompts to verify whether an	existing file should be	overwritten.

  -h (--help)
      Displays a help screen and quits.

  -l (--list)
      Lists the	following fields for each compressed file:

      compressed size
	  Specifies size of the	compressed file.

      uncompressed size
	  Specifies size of the	uncompressed file.

      ratio
	  Specifies compression	ratio (or 0.0% if unknown).

      uncompressed_name
	  Specifies the	name of	the uncompressed file.

      The uncompressed size is given as	-1 for files that are not in the gzip
      format, such as compressed .Z files.

      When used	with the -v option, the	following fields are also displayed:

      method
	  Specifies compression	method.

      crc Specifies the	32-bit CRC of the uncompressed data.

      date && time
	  Specifies the	time stamp for the uncompressed	file.

      The compression methods currently	supported are deflate, compress, lzh
      (SCO compress -H)	and pack.  The crc value is given as ffffffff for a
      file that	is not in the gzip format.   When used with the	-N option,
      the uncompressed name and	the date and time are those stored within the
      compressed file, if present.

      When used	with the -v option, the	size totals and	compression ratio for
      all files	is also	displayed, unless some sizes are unknown. When the -q
      option is	specified, the title and totals	lines are not displayed.

  -L (--License)
      Displays the gzip	license	and quits.

  -n (--no-name)
      Specifies	that the original file name and	time stamp are not saved when
      compressing 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)
      Specifies, when compressing, to always save the original file name and
      time stamp; this is the default. Specifies, when decompressing, to
      restore the original file	name and time stamp, if	present.  This option
      is useful	on systems which have a	limit on the length of a file name or
      when the time stamp has been lost	after a	file transfer.

  -q (--quiet)
      Suppresses all warnings.

  -r (--recursive)
      Travels the directory structure recursively. If any of the file names
      specified	on the command line are	directories, the gzip command des-
      cends into the directory and compresses all the files it finds there
      (or decompresses them in the case	of the gunzip command).

  -S .suf (--suffix .suf)
      Uses the suffix .suf instead of .gz.  Any	suffix can be given, but suf-
      fixes other than .z and .gz should be avoided to remove confusion	when
      files are	transferred to other systems.  A null suffix forces the	gun-
      zip command to attempt decompressing all given files regardless of the
      suffix, as follows:


	   gunzip -S  *	      (*.* for MSDOS)

      Previous versions	of the gzip command used the .z	suffix.	This was
      changed to avoid a conflict with the pack	command.

  -t (--test)
      Specifies	that the compressed file's integrity be	tested.

  -v (--verbose)
      Specifies	verbose	mode. Displays the name	and percentage reduction for
      each file	compressed or decompressed.

  -V (--Version)
      Specifies	the version number and compilation options and then quits.

  -# (--fast --best)
      Regulates	the speed of compression by using the specified	digit #, for
      which -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 the expense of	speed).

DESCRIPTION

  The gzip command reduces the size of specified files using the Lempel-Ziv
  coding (LZ77).  Whenever possible, each file is replaced by one with the
  extension .gz, while keeping the same	ownership modes, access	and modifica-
  tion times.

  If files are not specified, or if a file name	is "-",	the standard input is
  compressed to	the standard  output.  The gzip	command	only attempts to
  compress regular files.  In particular, it ignores symbolic links.

  By default, the gzip command keeps the original file name and	time stamp 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	the gzip com-
  mand with the	-d option, or by using the gunzip command.

  The gunzip command takes a list of specified files and replaces each file
  that begins with the correct magic number and	whose name ends	with .gz,
  -gz, .z, -z, _z or .Z	with an	uncompressed file without the original exten-
  sion.	The gunzip command also	recognizes the special	extensions .tgz	and
  .taz as shorthands for .tar.gz and .tar.Z respectively.  When	compressing,
  the gzip command uses	the .tgz extension if necessary	instead	of truncating
  a file with a	.tar extension.

  The gunzip command can currently decompress files that are created by	the
  gzip,	zip, compress, compress	-H or pack commands. The detection of the
  input	format is automatic. When using	the first two formats, the gunzip
  command checks a 32-bit CRC.	For the	pack command, the gunzip command
  checks the uncompressed length.  Although the	standard compress format was
  not designed to allow	consistency checks, the	gunzip command is sometimes
  able to detect a bad .Z file in cases	where the uncompress command does
  not. Therefore, if you get an	error when uncompressing a .Z file, do not
  assume that the .Z file is correct if	the same file can be decompressed
  without error	by the uncompress command.  In this case, the uncompress com-
  mand probably	did not	process	the input file correctly, and the generated
  output file is not useful.

  The gzip command uses	the Lempel-Ziv algorithm used in the zip and PKZIP
  commands.  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 the compress command), Huffman
  coding (as used in the pack command),	or adaptive Huffman coding (in the
  compact command).


  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.  The gzip command preserves the mode,	ownership and
  time stamps of files when compressing	or decompressing.

  Advanced Usage


  Multiple compressed files can	be concatenated. In this case, the gunzip
  command extracts all members at once.	For example:

       gzip -c file1  >>	foo.gz
       gzip -c file2 >>>>	foo.gz

  Using	the previous example, gunzip -c	foo is equivalent to cat file1 file2.

  In case a member of a	.gz file is damaged, other members can still be
  recovered (if	the damaged member is removed).	 However, you gain better
  compression by compressing all members at once as follows:

       cat file1 file2 | gzip >>	foo.gz

  The preceding	command	line compresses	better than the	following one:

       gzip -c file1 file2 >> foo.gz

  If you want to recompress concatenated files to gain better compression, do
  the following:

       gzip -cd	old.gz | gzip >>	new.gz

  If a compressed file consists	of several members, the	uncompressed size and
  CRC reported by the -l option	applies	to the last member only.  If you need
  the uncompressed size	for all	members, use the following command:

       gzip -cd	file.gz	| wc -c

  To create a single archive file with multiple	members	so that	members	can
  later	be extracted independently, use	an archiver such as the	tar or zip
  commands. GNU	tar supports the -z option to invoke the gzip command tran-
  sparently.  The gzip command is designed as a	complement to the tar com-
  mand,	not as a replacement.

  Environment


  The environment variable GZIP	can hold a set of default options for the
  gzip command.	 These options are interpreted first and can be	overwritten
  by explicit command line options as follows.

       GZIP="-8v --name"; export GZIP (for sh)
       setenv GZIP "-8v	--name"	(for csh)







RESTRICTIONS

  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 the gunzip command for decompression, the
  gunzip command detects that there is extra trailing garbage after the
  compressed data and emits a warning by default.  You have to use the -q
  option to suppress warnings. This option can be set in the GZIP environment
  variable as follows:

       GZIP="-q"  tar -xfz --block-compress /dev/tape/tape0_d1	(for sh)
       (setenv GZIP -q;	tar -xfz --block-compr /dev/tape/tape0_d1 (for csh)

  In the previous example, the gzip command is invoked implicitly by the -z
  option of the	GNU tar	command. Make sure that	the same block size (speci-
  fied by the -b option	of the tar command) is used for	reading	and writing
  compressed data on tapes.  (This example assumes that	you are	using the GNU
  version of the tar command.)

  The --list flag reports incorrect sizes if they exceed 2 gigabytes. The --
  list flag reports sizes as -1	and crc	as ffffffff if the compressed file is
  on a nonseekable media.

  In rare cases, the --best flag gives worse compression than the default
  compression level (-6).  On some highly redundant files, the compress	com-
  mand compresses better than gzip command.

  [Tru64 UNIX]	The gzip command may not preserve the extended file attri-
  butes	(property list)	of a file, including any access	control	lists (ACL).
  Verify that any ACLs are not removed or modified by using gzip.

EXIT STATUS

  0 (Zero)
      Success.

  1   An error occurred.

  2   A	warning	is encountered.

SEE ALSO

  Commands:  compress(1), pack(1)

  Files:  acl(4)