Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
compress(1)		    General Commands Manual		   compress(1)

NAME
       compress,  uncompress,  zcat, compressdir, uncompressdir	- compress and
       expand data

SYNOPSIS
   Compress Files
       maxbits]	[file ...]

       [file ...]

       [file ...]

   Compress Entire Directory Subtrees
       [options] [directory ...]

       [options] [directory ...]

DESCRIPTION
       The following commands compress and uncompress files and	directory sub-
       trees as	indicated:

	      Reduce the size of the named
				  files	 using adaptive	Lempel-Ziv coding.  If
				  reduction is possible, each file is replaced
				  by a new file	of the same name with the suf-
				  fix added to indicate	 that  it  is  a  com-
				  pressed  file.   Original  ownership,	modes,
				  access,  and	modification  times  are  pre-
				  served.   If	no file	is specified, or if is
				  specified, standard input is	compressed  to
				  the standard output.

	      Restore the compressed
				  files	 to  original  form.   Resulting files
				  have the original filename,  ownership,  and
				  permissions,	and the	filename suffix	is re-
				  moved.  If no	file is	specified,  or	if  is
				  specified, standard input is uncompressed to
				  the standard output.

	      Restore the compressed
				  files	to original form and send  the	result
				  to  standard	output.	  If no	file is	speci-
				  fied,	or if is specified, standard input  is
				  uncompressed to the standard output.

	      Front-end	processor.
				  Recursively descend each specified directory
				  subtree and use to compress each file	in di-
				  rectory.   Existing  files are replaced by a
				  compressed file having the  same  name  plus
				  the  suffix  provided	 the resulting file is
				  smaller than the original.  If  no  directo-
				  ries	are  specified,	compression is applied
				  to all files starting	with the  current  di-
				  rectory.

				  options  may	include	 any valid command op-
				  tions	(they are passed through to  To	 force
				  compression  of all files, even when the re-
				  sult is larger than the original  file,  use
				  the option.

	      Opposite of	  Restore  compressed  files to	their original
				  form.	 options may include any valid command
				  options (they	are passed through to

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

   Options
       These commands recognize	the  following	options	 in  the  combinations
       shown above in SYNOPSIS:

	      Decompress	  file.	 is equivalent to

	      Force compression	of
				  file.	 This is useful	for compressing	an en-
				  tire directory, even if some of the files do
				  not actually shrink.	If is not given	and is
				  run in the foreground, the user is  prompted
				  as  to  whether  an  existing	file should be
				  overwritten.

	      This is the same as the
				  option except	that it	does  not  force  com-
				  pression when	there is null compression.

	      Print a message describing the percentage	of
				  reduction for	each file compressed.

	      Force		  and  to  write  to  the  standard output; no
				  files	are changed.  The  nondestructive  be-
				  havior of is identical to that of

	      Print  the current version and compile options onto the standard
	      error.

	      Specify the maximum number of bits the
				  algorithm will use.  The default is  16  and
				  the  range  can be any integer between 9 and
				  16.

       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), pages 8-19.  Common substrings in	the  file  are
       first  replaced	by  9-bit codes	257 and	up.  When code 512 is reached,
       the algorithm switches to 10-bit	codes and continues to use  more  bits
       until the limit specified by the	flag is	reached	(default 16).

       After  the  maxbits limit is attained, periodically checks the compres-
       sion ratio.  If it is increasing, continues to use  the	existing  code
       dictionary.   However, if the compression ratio is decreasing, discards
       the table of substrings and rebuilds it from scratch.  This allows  the
       algorithm to adapt to the next "block" of the file.

       Note that the flag is omitted for since the maxbits parameter specified
       during compression is encoded within the	output,	 along	with  a	 magic
       number  to  ensure that neither decompression of	random data nor	recom-
       pression	of compressed data is attempted.

   Access Control Lists
       retains a file's	access control list  when  compressing	and  expanding
       data.

EXTERNAL INFLUENCES
   Environment Variables
       determines the language in which	messages are displayed.

       If  is  not specified in	the environment	or is set to the empty string,
       the value of is used as a default for each unspecified or  empty	 vari-
       able.   If is not specified or is set to	the empty string, a default of
       "C" (see	lang(5)) is used instead of

       If any internationalization variable contains an	invalid	 setting,  and
       behave  as  if  all internationalization	variables are set to "C".  See
       environ(5).

   International Code Set Support
       Single- and multi-byte character	code sets are supported.

RETURN VALUE
       These commands return the following values upon completion:

	      Completed	successfully.
	      Last file	is larger after	(attempted) compression.
	      An error occurred.

DIAGNOSTICS
       Invalid options were specified on the command line.

       maxbits
	      must follow

       The file	specified to
	      has not been compressed.

       file   was compressed by	a program that could deal with a higher	 value
	      of  maxbits  than	the compress code on this machine.  Recompress
	      the file with a lower value of maxbits.

       The file	is assumed to be already compressed.
	      Rename the file and try again.

       The output file name, which is the source file name with	a
	      extension, is too	long for the file system on which  the	source
	      file resides.  Make the source file name shorter and try again.

       Respond
	      if  you want the output to replace the existing file; otherwise,
	      respond

       A      violation	was detected which usually means that the  input  file
	      has been corrupted.

       Percentage of the input saved by	compression.
	      (Relevant	only for

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

       The input file has links	which are not symbolic links and has been
	      left unchanged. See ln(1)	for more information.

       The input file has symbolic links and has been left unchanged.
	      See ln(1)	for more information.

       No savings is achieved by compression.
	      The input	remains	unaltered.

EXAMPLES
       Compress	the file named and print compression information to the	termi-
       nal:

       The terminal display shows either a line	resembling

       indicating that the compressed file is 23.55% smaller than  the	origi-
       nal, or a line resembling

       indicating that an additional 12.04% space must be used to compress the
       file.

       Undo the	compression by typing either of	the following commands:

       This restores file to its original uncompressed form and	name.

       will perform on standard	input if no files are specified.  For example,
       to list a compressed tar	file:

WARNINGS
       Although	 compressed  files  are	compatible between machines with large
       memory, should be used for file transfer	to architectures with a	 small
       process data space (64K bytes or	less).

   NFS
       Access  control lists of	networked files	are summarized (as returned in
       by but not copied to the	new file (see stat(2)).

AUTHOR
       was developed by	Joseph M. Orost,  Kenneth  E.  Turkowski,  Spencer  W.
       Thomas, and James A. Woods.

FILES
       Compressed file created by
	      and removed by

SEE ALSO
       compact(1), pack(1), acl(5).

STANDARDS CONFORMANCE
								   compress(1)

NAME | SYNOPSIS | DESCRIPTION | EXTERNAL INFLUENCES | RETURN VALUE | DIAGNOSTICS | EXAMPLES | WARNINGS | AUTHOR | FILES | SEE ALSO | STANDARDS CONFORMANCE

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=compress&sektion=1&manpath=HP-UX+11.22>

home | help