compress, uncompress, zcat - compress and expand data (version 4.1)
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 ownership modes, access and modi‐
fication times. If no files are specified, the standard input is com‐
pressed 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, 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 uncom‐
press 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 standard 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 uncom‐
pressed 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 recursively. If any
of the file names specified on the command line are directories, com‐
press will descend into the directory and compress all the files it
The -V flag tells each of these programs to print its version and
patchlevel, along with any preprocessor flags specified during compila‐
tion, on stderr before doing any compression or uncompression.
Compress uses the modified Lempel-Ziv algorithm popularized in "A Tech‐
nique for High Performance Data Compression", Terry A. Welch, IEEE Com‐
puter, 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 com‐
pression 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 parame‐
ter 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 cod‐
ing (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 ALSOpack(1), compact(1)DIAGNOSTICS
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
uncompress: corrupt input
A SIGSEGV violation was detected which usually means that the
input file has been corrupted.
Percentage of the input saved by compression. (Relevant only
-- 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 unal‐
-- 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 mul‐
-- file unchanged
No savings is achieved by compression. The input remains vir‐
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 compress.c:compdir()
for an explanation.