compress(1) CLIX compress(1)
NAME
compress, uncompress, zcat - Compresses or expands data
SYNOPSIS
compress [-f] [-v] [-c] [-V] [-d] [-b maxbits] [file ... ]
uncompress [-f] [-v] [-c] [-V] [file ... ]
zcat [-V] [file ... ]
DESCRIPTION
The compress command can be used to compress and expand data. The command
reduces the size of the named files using adaptive Lempel-Ziv coding.
Whenever possible, each file file is replaced by one with the form file.Z,
while keeping the same ownership modes, access and modification times. If
no files are specified, stdin is compressed to stdout. Compressed files
can be restored to their original form using uncompress, zcat, or compress
-d.
The -f flag forces compression of file. 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 flag makes compress and uncompress write to stdout; no files are
changed. The nondestructive behavior of zcat is identical to that of
uncompress -c. The compress command 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). The value of maxbits 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 maxbits 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 maxbits
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
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compress(1) CLIX compress(1)
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
or adaptive Huffman coding, and takes less time to compute.
Under the -v flag, a message is displayed yielding the percentage of
reduction for each file compressed.
If the -V flag is specified, the current version and compile options are
displayed on stderr.
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 (64K bytes or less, as exhibited by the DECTM
PDPTM series, the Intel 80286, and so on.)
DIAGNOSTICS
Usage: compress [-dfvcV] [-b bits] [file ...]
Invalid flags were specified on the command line.
Missing maxbits
Maxbits must follow -b. Maxbits than the compress code on this
machine. Recompress the file with smaller maxbits.
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
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 the output file should be replaced; ``n'' if not.
uncompress: corrupt input
A SIGSEGV violation was detected which usually means that the input
file has been 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 unaltered.
-- has xx other links: unchanged
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compress(1) CLIX compress(1)
The input file has links; it is left unchanged. See ln for more
information.
-- file unchanged
No savings are achieved by compression. The input remains virgin.
EXIT VALUES
Exit status is normally 0; if the last file is larger after (attempted)
compression, the status is 2; if an error occurs, exit status is 1.
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