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COMPRESS(1)               BSD General Commands Manual              COMPRESS(1)

NAME
     compress, uncompress -- compress and expand data

SYNOPSIS
     compress [-fv] [-b bits] [file ...]
     compress -c [-b bits] [file]
     uncompress [-fv] [file ...]
     uncompress -c [file]

DESCRIPTION
     The compress utility reduces the size of files using adaptive Lempel-Ziv coding.  Each file is renamed
     to the same name plus the extension .Z.  A file argument with a .Z extension will be ignored except it
     will cause an error exit after other arguments are processed.  If compression would not reduce the size
     of a file, the file is ignored.

     The uncompress utility restores compressed files to their original form, renaming the files by deleting
     the .Z extensions.  A file specification need not include the file's .Z extension.  If a file's name in
     its file system does not have a .Z extension, it will not be uncompressed and it will cause an error
     exit after other arguments are processed.

     If renaming the files would cause files to be overwritten and the standard input device is a terminal,
     the user is prompted (on the standard error output) for confirmation.  If prompting is not possible or
     confirmation is not received, the files are not overwritten.

     As many of the modification time, access time, file flags, file mode, user ID, and group ID as allowed
     by permissions are retained in the new file.

     If no files are specified or a file argument is a single dash (`-'), the standard input is compressed
     or uncompressed to the standard output.  If either the input and output files are not regular files,
     the checks for reduction in size and file overwriting are not performed, the input file is not removed,
     and the attributes of the input file are not retained in the output file.

     The options are as follows:

     -b bits  The code size (see below) is limited to bits, which must be in the range 9..16.  The default
              is 16.

     -c       Compressed or uncompressed output is written to the standard output.  No files are modified.
              The -v option is ignored.  Compression is attempted even if the results will be larger than
              the original.

     -f       Files are overwritten without prompting for confirmation.  Also, for compress, files are com-pressed compressed
              pressed even if they are not actually reduced in size.

     -v       Print the percentage reduction of each file.  Ignored by uncompress or if the -c option is
              also used.

     The compress utility uses a modified Lempel-Ziv algorithm.  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 option or its default is reached.

     After the limit is reached, compress 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 scratch.  This allows the algorithm to
     adapt to the next "block" of the file.

     The -b option is unavailable for uncompress since the bits 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.

     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, text such as source code or English is reduced by
     50-60%.  Compression is generally much better than that achieved by Huffman coding (as used in the his-torical historical
     torical command pack), or adaptive Huffman coding (as used in the historical command compact), and
     takes less time to compute.

EXIT STATUS
     The compress and uncompress utilities exit 0 on success, and >0 if an error occurs.

     The compress utility exits 2 if attempting to compress a file would not reduce its size and the -f
     option was not specified and if no other error occurs.

SEE ALSO
     gunzip(1), gzexe(1), gzip(1), zcat(1), zmore(1), znew(1)

     Welch, Terry A., "A Technique for High Performance Data Compression", IEEE Computer, 17:6, pp. 8-19,
     June, 1984.

STANDARDS
     The compress and uncompress utilities conform to IEEE Std 1003.1-2001 (``POSIX.1'').

HISTORY
     The compress command appeared in 4.3BSD.

BUGS
     Some of these might be considered otherwise-undocumented features.

     compress: If the utility does not compress a file because doing so would not reduce its size, and a
     file of the same name except with an .Z extension exists, the named file is not really ignored as
     stated above; it causes a prompt to confirm the overwriting of the file with the extension.  If the
     operation is confirmed, that file is deleted.

     uncompress: If an empty file is compressed (using -f), the resulting .Z file is also empty.  That seems
     right, but if uncompress is then used on that file, an error will occur.

     Both utilities: If a `-' argument is used and the utility prompts the user, the standard input is taken
     as the user's reply to the prompt.

     Both utilities: If the specified file does not exist, but a similarly-named one with (for compress) or
     without (for uncompress) a .Z extension does exist, the utility will waste the user's time by not imme-diately immediately
     diately emitting an error message about the missing file and continuing.  Instead, it first asks for
     confirmation to overwrite the existing file and then does not overwrite it.

BSD                              May 17, 2002                              BSD

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