COMPRESS(1) General Commands Manual COMPRESS(1)
compress, uncompress, zcat - compress or expand files, display expanded
compress [ -cfv ] [ -b bits ] [ filename... ]
uncompress [ -cv ] [ filename... ]
zcat [ filename... ]
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 cod-
ing (as used in sys-unconfig(8)), or adaptive Huffman coding (old-com-
pact(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 uncom-
zcat produces uncompressed output on the standard output, but leaves
the compressed .Z file intact.
-c Write to the standard output; no files are changed. The nonde-
structive behavior of zcat is identical to that of `uncompress
-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 com-
Set the upper limit (in bits) for common substring codes. bits
must be between 9 and 16 (16 is the default).
ln(1V), old-compact(1), sh(1), sys-unconfig(8)
A Technique for High Performance Data Compression, Terry A. Welch, com-
puter, vol. 17, no. 6 (June 1984), pp. 8-19.
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
Usage: compress [-fvc] [-b maxbits] [filename...]
Invalid options were specified on the command line.
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
uncompress: corrupt input
A SIGSEGV violation was detected, which usually means that
the input file is corrupted.
Percentage of the input saved by compression. (Relevant only
-- not a regular file: unchanged
When the input file is not a regular file, (such as a direc-
tory), 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
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.
9 September 1987 COMPRESS(1)