compress, uncompress, zcat - compress and expand data
compress [ -f ] [ -v ] [ -c ] [ -V ] [ -d ] [ -b bits ] [ name ... ]
uncompress [ -f ] [ -v ] [ -c ] [ -V ] [ name ... ]
zcat [ name ... ]
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 permission modes, access and
modification times. When invoked by the superuser the ownership modes
are also retained. 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. The -d option causes
compress to uncompress (they are the same program, normally looking at
the name with which they are invoked to determine whether to compress or
The -f option will force compression of name, even if it does not
actually shrink or the corresponding name.Z file already exists. Except
when run in the background under /bin/sh, if -f is not given the user is
prompted as to whether an existing name.Z file should be overwritten.
The -c (``cat'') option makes compress/uncompress write to the standard
output; no files are changed. The nondestructive behavior of zcat is
identical to that of uncompress -c.
Compress uses the modified Lempel-Ziv algorithm popularized in "A
Technique for High Performance Data Compression", Terry A. Welch, IEEE
Computer, vol. 17, no. 6 (June 1984), pp. 8-19. 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 limit specified by the -b flag is reached (default 16).
Bits 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 bits limit is attained, compress periodically checks the
compression ratio. If it is increasing, compress continues to use the
existing code dictionary. However, if the compression ratio decreases,
compress discards the table of substrings and rebuilds it from scratch.
This allows the algorithm to adapt to the next "block" of the file.
Note that the -b flag is omitted for uncompress, since the bits 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
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), or adaptive Huffman coding (compact), and takes less
time to compute.
The -v option causes the printing of the percentage reduction of each
file and additional error message(s) information. The -V option prints
the version of compress.
If an error occurs, exit status is 1, else if the last file was not
compressed because it became larger, the status is 2; else the status is
0. No error message is printed if the compression failed.
Usage: compress [-fvc] [-b maxbits] [file ...]
Invalid options were specified on the command line.
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
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 than 12
characters. Rename and try again. This message does not occur
on BSD systems.
file 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 for
-- not a regular file: unchanged
When the input file is not a regular file, (e.g. a directory), it
is left unaltered.
-- has xx other links: unchanged
The input file has links; it is left unchanged. See ln(1) for
-- file unchanged
No savings is achieved by compression. The input remains virgin.
(Relevant only for -v.)
The suffix .Z is used by the commands compress/uncompress. The suffix .z
is used by the commands pack/unpack. The suffix .gz is used by the
pack(1), unpack(1), gzip(1), gunzip(1), zcat(1).
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, as exhibited by the DEC PDP
series, the Intel 80286, etc.)
compress should be more flexible about the existence of the `.Z' suffix.
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