compress(1) (Directory and File Management Utilities) 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.
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compress(1) (Directory and File Management Utilities) compress(1)
FILES
/usr/bin/sh
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.
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.
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compress(1) (Directory and File Management Utilities) compress(1)
-- file unchanged
No savings are achieved by compression. The input remains
uncompressed.
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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