COMPRESS(1)
NAME
compress, uncompress, zcat - compress and expand data
(version 4.1)
SYNOPSIS
compress [ -f ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ name ...
]
uncompress [ -f ] [ -v ] [ -c ] [ -V ] [ name ... ]
zcat [ -V ] [ name ... ]
DESCRIPTION
Compress reduces the size of the named files using adap-
tive Lempel-Ziv coding. Whenever possible, each file is
replaced by one with the extension .Z, while keeping the
same ownership modes, access and modification times. If
no files are specified, the standard input is compressed
to the standard output. Compress will only attempt to
compress regular files. In particular, it will ignore
symbolic links. If a file has multiple hard links, com-
press will refuse to compress it unless the -f flag is
given.
If -f is not given and compress is run in the foreground,
the user is prompted as to whether an existing file should
be overwritten.
Compressed files can be restored to their original form
using uncompress or zcat.
uncompress takes a list of files on its command line and
replaces each file whose name ends with .Z and which
begins with the correct magic number with an uncompressed
file without the .Z. The uncompressed file will have the
mode, ownership and timestamps of the compressed file.
The -c option makes compress/uncompress write to the stan-
dard output; no files are changed.
zcat is identical to uncompress -c. zcat uncompresses
either a list of files on the command line or its standard
input and writes the uncompressed data on standard output.
zcat will uncompress files that have the correct magic
number whether they have a .Z suffix or not.
If the -r flag is specified, compress will operate recur-
sively. If any of the file names specified on the command
line are directories, compress will descend into the
directory and compress all the files it finds there.
The -V flag tells each of these programs to print its ver-
sion and patchlevel, along with any preprocessor flags
specified during compilation, on stderr before doing any
compression or uncompression.
Compress uses the modified Lempel-Ziv algorithm popular-
ized in "A Technique for High Performance Data Compres-
sion", 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 con-
tinues 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, com-
press continues to use the existing code dictionary. How-
ever, if the compression ratio decreases, compress dis-
cards 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 recompres-
sion 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 distribu-
tion of common substrings. Typically, text such as source
code or English is reduced by 50-60%. Compression is gen-
erally much better than that achieved by Huffman coding
(as used in pack), or adaptive Huffman coding (compact),
and takes less time to compute.
Under the -v option, a message is printed yielding the
percentage of reduction for each file compressed.
Exit status is normally 0; if the last file is larger
after (attempted) compression, the status is 2; if an
error occurs, exit status is 1.
SEE ALSO
pack(1) compact(1)
DIAGNOSTICS
Usage: compress [-dfvcVr] [-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 com-
pressed.
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 bits.
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 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 or directory: ignored
When the input file is not a regular file or
directory, (e.g. a symbolic link, socket, FIFO,
device file), it is left unaltered.
-- has xx other links: unchanged
The input file has links; it is left unchanged.
See ln(1) for more information. Use the -f flag to
force compression of multiply-linked files.
-- file unchanged
No savings is achieved by compression. The input
remains virgin.
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, as exhibited by the DEC PDP series, the Intel 80286,
etc.)
Invoking compress with a -r flag will occasionally cause
it to produce spurious error warnings of the form
"<filename>.Z already has .Z suffix - ignored"
These warnings can be ignored. See the comments in com-
press.c:compdir() for an explanation.