COMPRESS(1)                          BSD                           COMPRESS(1)



NAME
     compress, uncompress, zcat - compress and expand data

SYNOPSIS
     compress [ -f ] [ -v ] [ -c ] [ -b bits ] [ file ... ]
     uncompress [ -f ] [ -v ] [ -c ] [ file ... ]
     zcat [ file ... ]

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 ownership modes, access, and
     modification times.  If you do not specify a file, compress compresses
     the standard input to the standard output.  Use uncompress or zcat to
     restore compressed files to their original form.

     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.  You can change the default 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 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
     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.

     Exit status is normally 0; if the last file is larger after (attempted)
     compression, the status is 2; if an error occurs, the exit status is 1.

OPTIONS
     -f        Forces compression of file.  This option is useful for
               compressing an entire directory, even if some of the files
               don't actually shrink. If you don't specify -f, and compress is
               run in the foreground, you are asked whether an existing file
               should be overwritten

     -v        Causes the printing of the percentage reduction of each file.

     -c        ("cat") Makes compress/uncompress write to the standard output;
               no files are changed.  The nondestructive behavior of zcat is
               identical to that of uncompress -c.

     -b bits   Determines the maximum number of bits to be used in the
               encoding scheme. bits must be between 9 and 16. The default is
               16.

DIAGNOSTICS
     Usage: compress [-fvc] [-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 xx bits, can handle only yy bits
             File was compressed by a program that could deal with more bits
             than the compress code on this machine.  Recompress the file with
             smaller bits.
     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 file cannot be compressed because its name is longer than 12
             characters.  Rename and try again.
     file 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, (for example, a
             directory), it is left unchanged.
     -- has xx other links: unchanged
             The input file has links; it is left unchanged.  See ln(1) for
             more information.
     -- file unchanged
             No savings is achieved by compression.  The input remains
             unchanged.

BUGS
     Files that were compressed using the older compact(1) utility must be
     expanded with uncompact(1).

     Although compressed files are compatible between machines with large
     memory, you should use -b12 for file transfer to architectures with a
     small process data space (64KB or less).

     Ideally compress would be more flexible about the existence of the .Z
     suffix.