compress(1) compress(1)
NAME
compress, uncompress, zcat - compress and expand data
SYNOPSIS
compress [-f] [-v] [-c] [-V] [-b maxbits] [files]
uncompress [-f] [-v] [-c] [-V] [files]
zcat [-V] [files]
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 uncompress or zcat.
FLAG OPTIONS
-f force compression of files. 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 running in the foreground, the user is
prompted as to whether an existing file should be
overwritten.
-c make compress or uncompress write to the standard out-
put; no files are changed. The nondestructive behavior
of zcat is identical to that of uncompress -c.
-b maxbits
use maxbits as the maximum number of bits to use in
codes when compressing file. compress uses a modified
Lempel-Ziv algorithm according to which common sub-
strings 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 maxbits specification 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 exist-
ing code dictionary. However, if the compression ratio
decreases, compress discards the table of substrings
and rebuilds it from scratch. This allows the algo-
rithm to adapt to the next ``block'' of the file.
-v print a message yielding the percentage of reduction
for each file compressed.
-V print the current version and compile options on the
April, 1990 1
compress(1) compress(1)
standard output.
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 maximum number of bits per code, and the dis-
tribution of common substrings. Typically, text such as
source code or English is reduced by 50 to 60 percent.
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.
Exit status is normally 0; if the last file is larger after
(attempted) compression, the status is 2; if an error oc-
curs, exit status is 1.
DIAGNOSTICS
This section lists the error messages displayed on the
screen followed by a description of how to recover from the
error.
Usage: compress [-dfvcV] [-b maxbits] [file...]
Invalid options were specified on the command line.
Missing maxbits
maxbits must follow -b.
file: not in compressed format
The file specified to uncompress has not been compressed.
file: compressed with xx bits,
can only handle yy bits
file was compressed by a program that could deal with more
bits 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
The file cannot be compressed because its name is longer
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compress(1) compress(1)
than 12 characters. Rename and try again.
file 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 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
The input file has links; it is left unchanged. See ln(1)
for more information.
-- file unchanged
No savings is achieved by compression. The input remains
virgin.
DISCLAIMER
This reference manual entry describes a utility that Apple
understands to have been released into the public domain by
its author or authors. Apple has included this public-
domain utility for your convenience and for use at your own
discretion.
The source code is normally found in /usr/src or is made
available through the Apple Programmer's and Developer's As-
sociation (APDA). This source code should also be used at
your own risk and without support from Apple.
FILES
/usr/ucb/compress
/usr/ucb/uncompress
/usr/ucb/zcat
SEE ALSO
compact(1), pack(1).
Terry A. Welch, ``A Technique for High Performance Data
April, 1990 3
Compression,'' IEEE Computer, Vol. 17, No. 6 (June 1984),
pages 8-19.
BUGS
Although compressed files are compatible between machines
with large memory, -b 12 should be used for file transfer to
architectures with a small process data space (64K or less,
as exhibited by the DEC PDP series, the Intel 80286, etc.).
4 April, 1990