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compress(1)							  compress(1)



NAME

  compress, uncompress,	zcat - Compresses and expands data

SYNOPSIS

  compress [-cCdfFnqvV]	[-b bits] [file...]

  uncompress [-cCfFnqvV] [file...]

  zcat [-n] [file...]

STANDARDS

  Interfaces documented	on this	reference page conform to industry standards
  as follows:

  compress:  XCU5.0

  uncompress:  XCU5.0

  zcat:	 XCU5.0

  Refer	to the standards(5) reference page for more information	about indus-
  try standards	and associated tags.

OPTIONS

  -b bits
      Specifies	the maximum number of bits to use to replace common sub-
      strings in the file.  The	default	for bits is 16,	with values of 9
      through 16 acceptable.  First, the algorithm uses	9-bit codes 257
      through 512.  Then it uses 10-bit	codes, continuing until	the bits
      limit is reached.	 (This option applies to the compress command only.)

      After the	bits limit is attained,	the compress command periodically
      checks the compression ratio.  If	it is increasing, compress continues
      to use the existing code dictionary.  However, if	the compression	ratio
      decreases, compress discards the table of	substrings and rebuilds	it
      from the beginning.  This	lets the algorithm adapt to the	next block of
      the file.

      [Tru64 UNIX]  The	-b option must be the last option on the command
      line.

  -c  Makes compress and uncompress write to the standard output; no files
      are changed.  The	nondestructive behavior	of zcat	is identical to	that
      of uncompress -c.

  -C  [Tru64 UNIX]  Produces output compatible with compress 2.0.

  -d  [Tru64 UNIX]  Uncompresses a file.

  -f or	-F
      When used	with the compress command, forces the compression of file
      even if no reduction in the size of the file will	occur.

      With both	the compress and uncompress commands, if you run the command
      in the background	and -f is not specified, you are prompted as to
      whether an existing file should be overwritten.

  -n  [Tru64 UNIX]  Specifies that no header has been added.

  -q  [Tru64 UNIX]  Specifies a	quiet operation. This is the default.  Diag-
      nostics messages,	which display if you specify the -v option, do not
      print. If	the -q and -v options are both specified, the last one on the
      command line is honored.

  -v  Prints the percentage reduction of each file when	compressing the	file.
      Prints messages to standard error	concerning the expansion of each file
      when uncompressing the file. If the -q and -v options are	both speci-
      fied, the	last one on the	command	line is	honored.

  -V  [Tru64 UNIX]  Specifies a	version.

OPERANDS

  file
      The path name of a file to be processed.	If - (hyphen) is used, stan-
      dard input will be read.

      The compress command will	create an output file name by appending	a
      suffix of	.Z to the input	file name.

      With the uncompress and zcat commands, if	the file name has a .Z suf-
      fix, the command will look for a file of that name.  If the suffix is
      not specified, the command will append the .Z suffix before searching
      for the file.

      The uncompress command will store	output in a file named with the	.Z
      suffix removed if	it was specified.  If no .Z suffix is specified, the
      uncompress command will accept file as the name of the output file and
      append a .Z suffix before	searching for the file.

DESCRIPTION

  The compress command reduces the size	of the named files using adaptive
  Lempel-Ziv coding.

  Whenever possible, each file is replaced by one with the extension .Z	while
  keeping the same ownership modes, access, and	modification times.  If	no
  files	are specified, standard	input is compressed to the standard output.

  Compressed files can be restored to their original form by using the
  uncompress or	zcat command.

  The uncompress command replaces the compressed .Z file with an uncompressed
  version of the file, identical to the	file that was originally compressed
  with compress; the .Z	suffix is removed.  When issuing an uncompress com-
  mand,	you can	refer to the compressed	target file with or without the	.Z
  suffix. If you do not	specify	the suffix, uncompress assumes it.

  The zcat command writes the uncompressed version of a	compressed file	to
  standard output.  The	compressed (.Z)	files remain intact. The zcat command
  is identical to uncompress -c.  When issuing a zcat command, you can refer
  to the compressed target file	with or	without	the .Z suffix; if you do not
  specify the suffix, zcat assumes it.


  [Tru64 UNIX]	The compress command uses the modified Lempel-Ziv algorithm
  popularized in "A Technique for High Performance Data	Compression," Terry
  A. Welch, IEEE Computer, vol.	17, no.	6 (June	1984), pp. 8-19.

  The amount of	compression obtained depends on	the size of the	input, the
  number of bits per code, and the distribution	of common substrings.
  Typically, files containing source code or plain text	are reduced by 50 to
  60 percent.  Compression is generally	much better than that achieved by
  Huffman coding (as used in the pack command) or adaptive Huffman coding,
  and takes less time to compute.

NOTES

  [Tru64 UNIX]	The compress command may not preserve the extended file
  attributes (property list) of	a file,	including the access control list
  (ACL)	if any.	Verify that any	ACLs have not been removed or modified after
  using	compress or uncompress.

EXIT STATUS

  The following	exit values are	returned:

  compress command


  0   Successful completion.

  1   An error occurred.

  2   One or more files	were not compressed because they became	larger.

  >>2  An error occurred.

  uncompress and zcat commands


  0   Successful completion.

  >>0  An error occurred.

DIAGNOSTICS

  Usage: compress [-fvc] [-b maxbits] [file ...]
      Invalid arguments	were specified on the command line.

  Missing maxbits
      maxbits must follow -b.

  file not in compressed format
      The file cannot be uncompressed because it was never compressed.

  file compressed with xx bits,	can only handle	yy bits.
      The file was compressed by a program that	could deal with	more bits
      than the compress	code on	this machine. Recompress the file with
      smaller bits.

  file already has .Z suffix --	no change
      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, or the	locale's equivalent of a y, if you want	the output
      file to be replaced; n, or the locale's equivalent of a n, if not.
      (The LC_MESSAGES variable	determines the locale's	equivalent of y	or
      n.)

  uncompress: corrupt input
      A	SIGSEGV	violation was detected,	which usually means that the input
      file is corrupted.

  Compression: xx.xx%
      Percentage of the	input saved by compression (relevant only for -v).

  -- not a regular file: unchanged
      When the input file is not a regular file, (for example, a directory),
      it is left unaltered.

  -- has xx other links: unchanged
      The input	file has links;	it is left unchanged.  (See the	ln command
      for more information.)

  -- file unchanged
      No saving	is achieved by compression.  The input remains unchanged.

EXAMPLES

   1.  To compress folder and print the	savings, enter:
	    compress -v	folder

       The system responds with	a message like:
	    folder: Compression: 43.94%	-- replaced with folder.Z

   2.  The following command displays the uncompressed version of the
       testlog.Z file:
	    zcat testlog.Z



ENVIRONMENT VARIABLES

  The following	environment variables affect the execution of compress,
  uncompress, and zcat:

  LANG
      Provides a default value for the internationalization variables that
      are unset	or null. If LANG is unset or null, the corresponding value
      from the default locale is used.	If any of the internationalization
      variables	contain	an invalid setting, the	utility	behaves	as if none of
      the variables had	been defined.

  LC_ALL
      If set to	a non-empty string value, overrides the	values of all the
      other internationalization variables.

  LC_CTYPE
      Determines the locale for	the interpretation of sequences	of bytes of
      text data	as characters (for example, single-byte	as opposed to multi-
      byte characters in arguments).

  LC_MESSAGES
      Determines the locale for	the format and contents	of diagnostic mes-
      sages written to standard	error.

  NLSPATH
      Determines the location of message catalogues for	the processing of
      LC_MESSAGES.







SEE ALSO

  Commands:  pack(1), pcat(1), unpack(1)


  Files:  acl(4)

  Standards:  standards(5)