compress, uncompress, zcat - compress and expand data
compress [ -f ] [ -v ] [ -c ] [ -V ] [ -r ] [ -b bits ] [ name ... ]
uncompress [ -f ] [ -v ] [ -c ] [ -V ] [ name ... ]
zcat [ -V ] [ 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 own-
ership 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 regu-
lar files. In particular, it will ignore symbolic links. If a file has multiple hard
links, compress 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
The -c option makes compress/uncompress write to the standard output; no files are
zcat is identical to uncompress -c. zcat uncompresses either a list of files on the com-
mand 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
If the -r flag is specified, compress will operate recursively. 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 version and patchlevel, along with
any preprocessor flags specified during compilation, on stderr before doing any compres-
sion or uncompression.
Compress uses the modified Lempel-Ziv algorithm popularized in "A Technique for High Per-
formance 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
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.
Under the -v option, a message is printed yielding the percentage of reduction for each
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.
Usage: compress [-dfvcVr] [-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 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
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.
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 compress42.c:compdir() in the source
distribution for an explanation.