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NAME
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compress, uncompress, zcat – compress and expand data
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SYNOPSIS
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compress [ –f ] [ –v ] [ –c ] [ –V ] [ –b bits ] [ name ... ]
uncompress [ –f ] [ –v ] [ –c ] [ –V ] [ name ... ]
zcat [ –V ] [ name ... ]
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DESCRIPTION
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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 no files are specified, the standard input
is compressed to the standard output. Compressed files can be
restored to their
original form using uncompress or zcat.
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/uncompress write to the standard output;
no files are changed. The nondestructive behavior of zcat is identical
to that of uncompress –c.
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 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%. 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 percentage
of reduction for each file compressed.
If the –V option is specified, the current version and compile
options are printed on stderr.
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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SEE ALSO
DIAGNOSTICS
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Usage: compress [–dfvcV] [–b maxbits] [file ...]
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Invalid options were specified on the command line.
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Missing maxbits
file: not in compressed format
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The file specified to uncompress has not been compressed.
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file: compressed with xx bits, can only handle yy bits
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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.
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file: already has .Z suffix -- no change
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The file is assumed to be already compressed. Rename the file
and try again.
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file: filename too long to tack on .Z
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The file cannot be compressed because its name is longer than
12 characters. Rename and try again. This message does not occur
on BSD systems.
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file already exists; do you wish to overwrite (y or n)?
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Respond "y" if you want the output file to be replaced; "n" if not.
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uncompress: corrupt input
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A SIGSEGV violation was detected which usually means that the
input file has been corrupted.
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Compression: xx.xx%
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Percentage of the input saved by compression. (Relevant only for
–v.)
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-- not a regular file: unchanged
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When the input file is not a regular file, (e.g. a directory),
it is left unaltered.
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-- has xx other links: unchanged
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The input file has links; it is left unchanged. See ln(1) for
more information.
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-- file unchanged
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No savings is achieved by compression. The input remains virgin.
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SOURCE
BUGS
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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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