COMPRESS(1)                OpenBSD Reference Manual                COMPRESS(1)

NAME
     compress, uncompress - compress and expand data



SYNOPSIS
     compress [-cdftgOqv] [-b bits] [-o filename] [file ...]
     uncompress [-cftoqv] [-o filename] [file ...]

DESCRIPTION
     The compress utility reduces the size of the named files using adaptive
     Lempel-Ziv coding.  Each file is renamed to the same name plus the exten-
     sion ``.Z''. 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 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 removing the ``.Z'' extension.

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

     If no files are specified, the standard input is compressed or uncom-
     pressed to the standard output.  If either the input or 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.

     The options are as follows:

     -b bits
             Specify the bits code limit (see below).

     -c      Compressed or uncompressed output is written to the standard out-
             put.  No files are modified.

     -d      Decompress the source files instead of compressing them.

     -f      Force compression of file, even if it is not actually reduced in
             size.  Additionally, files are overwritten without prompting for
             confirmation.

     -g      Use deflate scheme which reportedly provides better compression
             rates.

     -O      Use old compression method (default is based on the program
             name).

     -o filename
             Set the output file name.

     -t      Test the integrity of each file leaving any files intact.

     -q      Be quiet, suppress any messages.

     -v      Print the percentage reduction of each file.

     compress 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 flag is reached.  bits must be
     between 9 and 16 (the default is 16).
     After the bits limit is reached, compress periodically checks the com-
     pression ratio.  If it is increasing, compress continues to use the ex-
     isting 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 flag is omitted for uncompress since the bits parameter specified
     during compression is encoded within the output, along with a magic num-
     ber 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.  Typ-
     ically, text such as source code or English is reduced by 50-60%.  Com-
     pression is generally much better than that achieved by Huffman coding
     (as used in the historical command pack), or adaptive Huffman coding (as
     used in the historical command compact), and takes less time to compute.

     The compress utility exits 0 on success, 1 if an error occurred, or 2 if
     one or more files were not compressed because they would have grown in
     size (and -f was not specified).

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

STANDARDS
     The compress utility is compliant with the IEEE Std1003.2-1992
     (``POSIX.2'') specification.

HISTORY
     The compress command appeared in 4.3BSD. The deflate compression support
     was added in OpenBSD 2.1.

OpenBSD 2.6                     April 18, 1994                               2