compress(1) General Commands Manual compress(1)
compress, uncompress, zcat - compress and expand data
compress [ -f ] [ -v ] [ -c ] [ -b bits ] [ name ... ]
uncompress [ -f ] [ -v ] [ -c ] [ name ... ]
zcat [ name ... ]
The command 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 modification times. If no files are specified, the
standard input is compressed to the standard output. Compressed files can be restored to their original form using or
The -f option will force compression of name, even if it does not actually shrink name, or if the corresponding name .Z file
already exists. If the -f option is omitted, the user is asked whether an existing name.Z file should be overwritten (unless is
run in the background under
The -c (cat) option makes compress/uncompress write to the standard output without changing any files. Neither -c nor alter files.
The command uses the modified Lempel-Ziv algorithm. 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). The bits must be between 9 and 16. The default can be changed in the source to allow to be run on a
After the bits limit is attained, periodically checks the compression ratio. If the ratio 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.
Note that the -b flag is omitted for since the bits parameter specified during compression is encoded within the output along with
a number that ensures that neither decompression of random data nor recompression of compressed data is attempted.
How much each file is compressed depends on the size of the input, the number of bits per code, and the distribution of common sub-
strings. Typically, text such as source code or English is reduced by 50-60%. Compression is generally much better than that
achieved by Huffman coding or adaptive Huffman coding, and takes less time to compute.
The -v option displays the percent reduction of each file.
If an error occurs, exit status is 1. However, if the last file was not compressed because it became larger, the status is 2.
Otherwise, the status is 0.
-f Forces compression of name.
-c Makes compress/uncompress write to the standard output.
-b Specifies the allowable bits limit. The default is 16.
-v Displays the percent reduction of each file.
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
The 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 compressed already. Rename the file and try again.
file already exists; do you wish to overwrite (y or n)?
Type y if you want the output file to be replaced; type n if you do not.
uncompress: corrupt input
A SIGSEGV violation was detected which usually means that the input file is corrupted.
Percent of the input saved by compression. (For the -v option only.)
-- not a regular file: unchanged
If the input file is not a regular file (for example, a directory), it remains unchanged.
-- has xx other links: unchanged
The input file has links; it is left unchanged. See for more information.
-- file unchanged
No savings is achieved by compression. The input remains unchanged.
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).