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.\" This code is derived from software contributed to Berkeley by
.\" James A. Woods, derived from original work by Spencer Thomas
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.\" @(#)compress.1 6.10 (Berkeley) 7/30/91
.Nd compress and expand data
reduces the size of the named files using adaptive Lempel-Ziv coding.
is replaced by one with the extension
while keeping the same ownership modes, access and modification times.
If no files are specified, the standard input is compressed to the
Compressed files can be restored to their original form using
even if it does not actually shrink
Except when run in the background under
is not given the user is prompted as to whether an existing
file should be overwritten.
writes to the standard output; no files are changed.
The nondestructive behavior of
Print the percentage reduction of each file.
uses the modified Lempel-Ziv algorithm popularized in
"A Technique for High Performance Data Compression",
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
flag is reached (default 16).
must be between 9 and 16. The default can be changed in the source to allow
to be run on a smaller machine.
periodically checks the compression ratio. If it is increasing,
continues to use the existing code dictionary. However,
if the compression ratio decreases,
discards the table of substrings and rebuilds it from scratch. This allows
the algorithm to adapt to the next "block" of the file.
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
per code, and the distribution of common substrings.
Typically, text such as source code or English
Compression is generally much better than that achieved by
Huffman coding (as used in the historical command
or adaptive Huffman coding (as
used in the historical command
and takes less time to compute.
If an error occurs, exit status is 1;
if the last file was not compressed because it became larger, the status
is 2; otherwise the status is 0.
.It "Usage: compress [-fvc] [-b maxbits] [file ...]"
Invalid options were specified on the command line.
.It Ar file : No "not in compressed format"
.Ar file : No "compressed with"
.Ar \&xx No "bits, can only handle"
was compressed by a program that could deal with
than the compress code on this machine.
Recompress the file with smaller
.It Ar file : No "already has .Z suffix -- no change"
The file is assumed to be already compressed.
Rename the file and try again.
.It Ar file : No "filename too long to tack on .Z"
The file cannot be compressed because its name is longer than
This message does not occur on
.It Ar file No "already exists; do you wish to overwrite (y or n)?"
Respond "y" if you want the output file to be replaced; "n" if not.
.It "uncompress: corrupt input"
violation was detected which usually means that the input file is
.It Compression: Em "xx.xx%"
Percentage of the input saved by compression.
.It "-- not a regular file: unchanged"
When the input file is not a regular file,
(e.g. a directory), it is
.It "-- has" Ar xx No "other links: unchanged"
The input file has links; it is left unchanged. See
No savings is achieved by
compression. The input remains virgin.
compressed file is file.Z
Although compressed files are compatible between machines with large memory,
should be used for file transfer to architectures with
a small process data space (64KB or less, as exhibited by the
series, the Intel 80286, etc.)
should be more flexible about the existence of the `.Z' suffix.
projects / unix-history / blob