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 compress(1)							 compress(1)




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

 SYNOPSIS
    Compress Files
      compress [-d] [-f|-z] [-z] [-v] [-c] [-V] [-b maxbits] [file ...]

      uncompress [-f] [-v] [-c] [-V] [file ...]

      zcat [-V] [file ...]

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

      uncompressdir [options] [directory ...]

 DESCRIPTION
      The following commands compress and uncompress files and directory
      subtrees as indicated:

	   compress	       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 suffix .Z added to
			       indicate that it is a compressed file.
			       Original ownership, modes, access, and
			       modification times are preserved.  If no file
			       is specified, or if - is specified, standard
			       input is compressed to the standard output.

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

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

	   compressdir	       Front-end processor.  Recursively descend
			       each specified directory subtree and use
			       compress to compress each file in directory.
			       Existing files are replaced by a compressed
			       file having the same name plus the suffix .Z,
			       provided the resulting file is smaller than
			       the original.  If no directories are



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 compress(1)							 compress(1)




			       specified, compression is applied to all
			       files starting with the current directory.

			       options may include any valid compress
			       command options (they are passed through to
			       compress).  To force compression of all
			       files, even when the result is larger than
			       the original file, use the -f option.

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

      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 pack), or adaptive Huffman
      coding (compact), and takes less time to compute.

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

	   -d		       Decompress file.	 compress -d is equivalent
			       to uncompress.

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

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

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

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

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




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 compress(1)							 compress(1)




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

      compress 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 -b flag is reached
      (default 16).

      After the maxbits limit is attained, compress periodically checks the
      compression ratio.  If it is increasing, compress continues to use the
      existing code dictionary.	 However, if the compression ratio is
      decreasing, compress 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 -b flag is omitted for uncompress 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 recompression of compressed data is attempted.

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

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

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

      If any internationalization variable contains an invalid setting,
      compress, uncompress, and zcat 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:

	   0	Completed successfully.




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 compress(1)							 compress(1)




	   2	Last file is larger after (attempted) compression.
	   1	An error occurred.

 DIAGNOSTICS
      Usage: compress [-f|-z] [-dvcV] [-b maxbits] [file ...]
	   Invalid options were specified on the command line.

      Missing maxbits
	   maxbits must follow -b.

      file: not in compressed format
	   The file specified to uncompress has not been compressed.

      file: compressed with xxbits, can only handle yybits
	   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.

      file: already has .Z suffix -- no change
	   The file is assumed to be already compressed.  Rename the file
	   and try again.

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

      file already exists; do you wish to overwrite (y or n)?
	   Respond y if you want the output to replace the existing file;
	   otherwise, respond n.

      uncompress: corrupt input
	   A SIGSEGV violation was detected which usually means that the
	   input file has been 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 (a directory for
	   example), it is left unaltered.

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

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




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 compress(1)							 compress(1)




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

 EXAMPLES
      Compress the file named zenith and print compression information to
      the terminal:

	   compress -v zenith

      The terminal display shows either a line resembling

	   zenith: Compression: 23.55% -- replaced with zenith.Z

      indicating that the compressed file is 23.55% smaller than the
      original, or a line resembling

	   zenith: Compression: -12.04% -- file unchanged

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

      Undo the compression by typing either of the following commands:

	   uncompress zenith.Z
	   compress -d zenith.Z

      This restores file zenith.Z to its original uncompressed form and
      name.

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

	   uncompress -c arch.tar.Z | tar -tvf -

 WARNINGS
      Although compressed files are compatible between machines with large
      memory, -b12 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
      st_mode by stat(), but not copied to the new file (see stat(2)).

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

 FILES
      *.Z  Compressed file created by compress and removed by uncompress.




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 compress(1)							 compress(1)




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

 STANDARDS CONFORMANCE
      compress: XPG4

















































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