COMPRESS(1)

NAME
compress, uncompress, zcat - compress and expand data

SYNOPSIS
compress [ -f ] [ -v ] [ -c ] [ -b bits ] [ name ... ]
uncompress [ -f ] [ -v ] [ -c ] [ name ... ]
zcat [ name ... ]

DESCRIPTION
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 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 uncompress or zcat.

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.

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.

DIAGNOSTICS
Usage: compress [-fvc] [-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 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 is corrupted.
Compression: xx.xx%
Percentage of the input saved by compression. (Relevant only for -v.)
-- 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 more information.
-- 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.)

compress should be more flexible about the existence of the `.Z' suffix.