compress(1)                    DG/UX R4.11MU05                   compress(1)


NAME
       compress, uncompress, zcat - compress, expand or display expanded
       files

SYNOPSIS
       compress [ -cfv ] [ -b bits ] [ filename...  ]
       uncompress [ -cv ] [ filename...  ]
       zcat [ filename...  ]

DESCRIPTION
       compress reduces the size of the named files using adaptive
       Lempel-Ziv coding.  Whenever possible, each file is replaced by one
       with a .Z, extension.  The ownership modes, access time  and
       modification time will stay the same.  If no files are specified, the
       standard input is compressed to the standard output.

       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(1)], and takes less time
       to compute.  The bits parameter specified during compression is
       encoded within the compressed file, along with a magic number to
       ensure that neither decompression of random data nor recompression of
       compressed data is subsequently allowed.

       Compressed files can be restored to their original form using
       uncompress.

       zcat produces uncompressed output on the standard output, but leaves
       the compressed .Z file intact.

   Options
       -c     Write to the standard output; no files are changed.  The
              nondestructive behavior of zcat is identical to that of
              `uncompress -c'.

       -f     Force compression, even if the file does not actually shrink,
              or the corresponding .Z file already exists.  Except when
              running in the background (under /usr/bin/sh), if -f is not
              given, prompt to verify whether an existing .Z file should be
              overwritten.

       -v     Verbose.  Display the percentage reduction for each file
              compressed.

       -b bits
              Set the upper limit (in bits) for common substring codes.
              bits must be between 9 and 16 (16 is the default).  Lowering
              the number of bits will result in larger, less compressed
              files.

FILES
       /usr/bin/sh

DIAGNOSTICS
       Exit status is normally 0.  If the last file was not compressed
       because it became larger, the status is 2.  If an error occurs, exit
       status is 1.

       Usage: compress [-fvc] [-b maxbits] [filename ...]
                 Invalid options were specified on the command line.

       Missing maxbits
                 Maxbits must follow -b.

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

       filename: compressed with xxbits, can only handle yybits
                 filename was compressed by a program that could deal with
                 more bits than the compress code on this machine.
                 Recompress the file with smaller bits.

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

       filename: already exists; do you wish to overwrite (y or n)?
                 Respond y if you want the output file to be replaced; n if
                 not.

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

       -- has xx other links: unchanged
                 The input file has links; it is left unchanged.  See ln(1)
                 for more information.

       -- file unchanged
                 No savings are achieved by compression.  The input remains
                 uncompressed.

SEE ALSO
       pack(1)

       A Technique for High Performance Data Compression, Terry A. Welch,
       IEEE Computer, vol. 17, no. 6 (June 1984), pp. 8-19.

NOTES
       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).

       compress should be more flexible about the existence of the .Z
       suffix.


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