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     COMPRESS(1)                                           COMPRESS(1)

     NAME
          compress - compress and expand data

     SYNOPSIS
          compress [ -dfvcV ] [ -b bits ] [ name ... ]

     DESCRIPTION
          Compress reduces the size of the named files using adaptive
          Lempel-Ziv coding.  Whenever possible, each file is replaced
          by one with the extension .Z, while keeping the same owner-
          ship modes, access and modification times.  If no files are
          specified, the standard input is compressed to the standard
          output.  Compressed files can be restored to their original
          form using compress -d.

          The -f option will force compression of name. 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.

          The -c option makes compress write to the standard output;
          no files are changed.

          The -v option prints compression statistics and the -V
          option prints the version of the program.

          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),
          pp. 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).  Bits must be between 9 and 16.  The default
          can be changed in the source to allow compress to be run on
          a smaller machine.

          After the bits 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 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.

          Note that the -b flag is omitted for compress -d, 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.

     COMPRESS(1)                                           COMPRESS(1)

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

          Under the -v option, a message is printed yielding the per-
          centage of reduction for each file compressed.

          If the -V option is specified, the current version and com-
          pile options are printed on stderr.

     BUGS
          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 (64KB or less,
          as exhibited by the DEC PDP series, the Intel 80286, etc.)