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compress(1)							   compress(1)

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

   Compress Files
       maxbits] [file ...]

       [file ...]

       [file ...]

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

       [options] [directory ...]

       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
				  removed.  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
				  directory.  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

				  options   may	  include  any	valid  command
				  options (they are passed through to To force
				  compression  of  all	files,	even  when the
				  result 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.

       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
				  entire 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
				  behavior of is identical to that of

	      Print  the current version and compile options onto the standard

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

       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

   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

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

       These commands return the following values upon completion:

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

       Invalid options were specified on the command line.

	      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.

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

       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.

       Compress the file named and print compression information to the termi‐

       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

       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:

       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).

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

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

       Compressed file created by
	      and removed by

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


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