compress

This article is about the data compression utility. For other uses, see Compression.

Compress

Filename extension	.Z
Internet media type	application/x-compress
Developed by	Spencer Thomas
Type of format	data compression

Compress is a UNIX compression program based on the LZC compression method, which is an LZW implementation using variable size pointers as in LZ78.

Description of program

Files compressed by compress are typically given the extension .Z. Most tar programs will pipe their data through compress when given the command line option -Z (tar in itself does not compress, it just stores multiple files within one tape archive file).

Files can be returned to their original state using uncompress. The usual action of the program is not merely to create an uncompressed copy of the file, but to uncompress the compressed file (so that the uncompressed version retains the timestamp and other attributes of the compressed file).

History

The LZW algorithm used in compress was patented by Sperry Research Center in 1983. Terry Welch published an IEEE article on the algorithm in 1984,^[1] but failed to note that he had applied for a patent on the algorithm. Spencer Thomas took this article and implemented compress in 1984, without realizing that a patent was pending on the LZW algorithm. The GIF image format also incorporated LZW compression in this way, and Unisys later claimed royalties on implementations of GIF. Joseph M. Orost led the team and worked with Thomas et al. to create the 'final' (4.0) version of compress and published it as free software to the 'comp.sources.unix' USENET group in 1985. U.S. Patent 4,558,302 was granted in 1985, and this is why compress could not be used without paying royalties to Sperry Research, which was eventually merged into Unisys. compress has fallen out of favor in particular user-groups because it makes use of the LZW algorithm, which was covered by a Unisys patent — because of this, gzip and bzip2 increased in popularity on Linux-based operating systems due to their alternative algorithms. compress has, however, maintained a presence on Unix and BSD systems. The US LZW patent expired in 2003, so it is now in the public domain in the United States. All patents on the LZW worldwide have also expired (see Graphics Interchange Format#Unisys and LZW patent enforcement).

See also

• Data compression
• Image compression
• List of Unix programs

External links

• compress: compress data – Commands & Utilities Reference, The Single UNIX® Specification, Issue 7 from The Open Group
• ncompress - public domain compress/uncompress implementation for POSIX systems
• compress(1) – Linux User Commands Manual
• compress - original Unix compress (in a compress'd archive)
• compress - original Unix compress executable (gzip'd)
• Source Code for compress v4.0 (gzip'd sharchive)
• ZIP File containing a Windows port of the compress utility
• source code to the current version of fcompress.c from compress

References

1. ^ Welch, Terry A., "A technique for high performance data compression", IEEE Computer, 17(6), pp.8-19, June 1984.