Manual page for COMPRESS(1)
compress, uncompress, zcat - compress or expand files, display expanded contents
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 the extension
.Z,
while keeping the same ownership modes, as well as access and
modification times.
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
sys-unconfig.8
or adaptive Huffman coding
(old-compact(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
sh.1
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).
SEE ALSO
ln.1v
old-compact.1
sh.1
sys-unconfig.8
A Technique for High Performance Data Compression,
Terry A. Welch,
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.1v
for more information.
- -- file unchanged
-
No savings are achieved by compression.
The input remains uncompressed.
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).
compress
should be more flexible about the existence of the
.Z
suffix.
Created by unroff & hp-tools.
© somebody (See intro for details). All Rights Reserved.
Last modified 11/5/97