Z (file format)

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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 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).

The LZW algorithm used in compress was patented by Sperry Research Center in 1983. The person who applied for the patent published an IEEE article on the algorithm in 1984^[1], without mentioning that the algorithm was patent pending. Spencer Thomas took this article and implemented compress in 1984, without realizing that the LZW algorithm was patent pending. (The GIF image format also incorporated LZW compression in this way and later Unisys claimed royalties on implementations of GIF.) Joseph M. Orost led the team and worked with Spencer 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. The patent on LZW compression was granted in 1985 and this is why compress could not be used without paying royalties to Sperry Research (which became Unisys). compress has fallen out of favor in particular user-groups because it makes use of the LZW algorithm, which is covered by UNISYS and IBM patents — 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 LZW patent expired in 2003, so it is now in the public domain in the United States.

'compress' for Mac OS X

There is also a Freeware Mac OS X application with the same name, but it is completely unrelated with the former. This program supports the zip, 7z, bzip2, gzip, and tar formats. A much earlier program dating from the 1980s, MacCompress, is a Mac OS GUI wrapper for compress, but has not been used for some time. The Darwin operating system includes a compress utility as described above.

See also

• Data compression
• Image compression
• List of Unix programs

External links

• ncompress - public domain compress/uncompress implementation for POSIX systems
• Compress for Mac OS X home page
• compress - original Unix compress (in a compress'd archive)
• compress - original Unix compress executable (gzip'd)
• Source Code for compress v4.0
• ZIP File containing a Windows port of the compress utility

References

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