compress (software)
This article needs additional citations for verification. (June 2012) |
Original author(s) | Spencer Thomas |
---|---|
Initial release | February 1985 |
Operating system | Unix, Unix-like, IBM i |
Type | Command |
Filename extension |
.Z |
---|---|
Internet media type |
application/x-compress |
Developed by | Spencer Thomas |
Type of format | data compression |
compress is a
The uncompress utility will restore files to their original state after they have been compressed using the compress utility. If no files are specified, the
In the upcoming
Description
Files compressed by compress are typically given the
-Z
". (The tar program in its own does not compress; it just stores multiple files within one tape archive.)
Files can be returned to their original state using uncompress. The usual action of uncompress is not merely to create an uncompressed copy of the file, but also to restore the timestamp and other attributes of the compressed file.
For files produced by compress on other systems, uncompress supports 9- to 16-bit compression.
History
The LZW algorithm used in compress was patented by
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,
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
In the up-coming
Special output format
Output binary consists of bit groups. Each bit group consists of codes with fixed amount of bits (9-16). Each group (except last) should be aligned by amount of bits multiplied by 8 and right padded with zeroes. Last group should be aligned by 8 and padded with zeroes. You can find more information in ncompress issue.
Example:
- You want to output ten 9-bit codes, five 10-bit codes and thirteen 11-bit codes. You now have three groups of bits that you want to output: 90 bits, 50 bits and 143 bits.
- First group should then be 90 bits of data + 54 zero bits of padding in order to be aligned to 72 bits (9 bits × 8).
- Second group should then be 50 bits of data + 30 zero bits of padding in order to be aligned to 80 bits (10 bits × 8).
- Third group should then be 143 bits of data + 1 zero bit of padding in order to be aligned to 8 bits (1 byte only, since this is the last group in the output).
It is actually a bug. LZW doesn't require any alignment. This bug is a part of original UNIX compress, ncompress, gzip and even windows port. It exists more than 35 years. All application/x-compress files were created using this bug. So we have to include it in output specification.
Some compress implementations write random bits from uninitialized buffer as alignment bits. There is no guarantee that alignment bits will be zeroes. So in terms of 100% compatibility decompressor have to just ignore alignment bit values.
Standardization and availability
compress was standardized in X/Open CAE Specification in 1994,[7] and further in The Open Group Base Specifications, Issue 6 and 7.[8] Linux Standard Base does not requires compress.[9]
compress is often not installed by default in Linux distributions, but can be installed from an additional package.[10] compress is available for FreeBSD, OpenBSD, MINIX, Solaris and AIX.
compress is allowed for
The gunzip utility is able to decompress .Z files.[11]
See also
- Data compression
- Image compression
- List of Unix commands
- gzip
References
- ^ Frysinger, Mike. "ncompress: a public domain project". Retrieved 2014-07-30.
Compress is a fast, simple LZW file compressor. Compress does not have the highest compression rate, but it is one of the fastest programs to compress data. Compress is the de facto standard in the UNIX community for compressing files.
- ^ Gommans, Luc. "compression - What's the difference between gzip and compress?". Unix & Linux Stack Exchange.
- ^ "Large Text Compression Benchmark". mattmahoney.net.
compress 4.3d....
- ^ "0001041: Encourage implementations to include better integrity checksum, compression and decompression utilities if possible". Austin Group Bug Tracker. Retrieved 2017-11-23.
- S2CID 2055321.
- ^ IBM. "IBM System i Version 7.2 Programming Qshell" (PDF). IBM. Retrieved 2020-09-05.
- ^ X/Open CAE Specification Commands and Utilities Issue 4, Version 2 (pdf), 1994, opengroup.org
- The Single UNIX Specification, Version 3 from The Open Group
- ^ Chapter 17. Commands and Utilities in Linux Standard Base Core Specification 5.0.0, linuxfoundation.org
- ^ ncompress, pkgs.org
- ^ "GNU Gzip". The GNU Operating System and the Free Software Movement. 2023-02-05. Retrieved 2024-04-03.
gunzip can currently decompress files created by gzip, zip, compress or pack. The detection of the input format is automatic.
External links
- The Single UNIX Specification, Version 4 from The Open Group
: compress data – Shell and Utilities Reference,
- Version 8 Unix Programmer's Manual
–
- FreeBSD General Commands Manual –
- OpenBSD General Commands Manual –
- Solaris 11.4 User Commands Reference Manual
–
- ncompress - public domain compress/uncompress implementation for POSIX systems
- compress - original Unix compress (in a compress'd archive)
- compress - original Unix compress executable (gzip'd)
- Source Code for compress v4.0 (gzip'd sharchives)
- ZIP File containing a Windows port of the compress utility
- source code to the current version of fcompress.c from compress
- bit groups alignment - Explanation of bit groups alignment.
- lzws - New library and CLI, implemented without legacy code.
- ruby-lzws - Ruby bindings with streaming support.
- compress.com - official website for file compression.