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.
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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.)
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