{{Short description|Compression shell command based on the LZW compression algorithm}} {{redirect|compress|the cloth used in medicine|Compress (medical)|other uses|Compression (disambiguation)}} {{More citations needed|date=June 2012}} {{lowercase title}} {{Infobox software | name = compress / uncompress | logo = | screenshot = | screenshot size = | caption = | author = Spencer Thomas | developer = | released = {{Start date and age|1985|2}} | latest release version = | latest release date = | operating system = Unix, Unix-like, IBM i | genre = Command | license = | website = }} {{Infobox file format | name = compress .Z | extension = .Z | mime = application/x-compress | owner = Spencer Thomas | genre = data compression | website = }} <code>'''compress'''</code> is a shell command for compressing data based on the LZW algorithm.<ref>{{cite web |url=https://ncompress.sourceforge.net/ |title=ncompress: a public domain project |last=Frysinger |first=Mike |access-date=2014-07-30 |quote=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. }}</ref> <code>'''uncompress'''</code> is a companion shell command that restores files to their original state (both content and metadata) from a file created with {{code|compress}}.

Although once popular, {{code|compress}} has fallen out of favor because it uses the patented LZW algorithm. Its use has been replaced by commands such as <code>gzip</code> and <code>bzip2</code> that use other algorithms and provide better data compression. Compared to <code>gzip</code> at its fastest setting, {{code|compress}} is slightly slower at compression, slightly faster at decompression, and has a significantly lower compression ratio.<ref>{{cite web |first=Luc|last=Gommans|title=compression - What's the difference between gzip and compress? |url=https://unix.stackexchange.com/a/412573 |website=Unix & Linux Stack Exchange |language=en}}</ref> 1.8 MiB of memory is used to compress the Hutter Prize data, slightly more than {{code|gzip}} at its slowest setting.<ref>{{cite web |title=Large Text Compression Benchmark |url=http://mattmahoney.net/dc/text.html |website=mattmahoney.net |quote=compress 4.3d....}}</ref>

{{code|compress}} and {{code|uncompress}} have maintained a presence on Unix and BSD systems and have been ported to IBM i.<ref>{{cite web |title=IBM System i Version 7.2 Programming Qshell |language=en |author=IBM |website=IBM |author-link=IBM |url=https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_74/rzahz/rzahzpdf.pdf?view=kc |access-date=2020-09-05 }}</ref>

{{code|compress}} was standardized in X/Open CAE Specification in 1994,<ref>[https://pubs.opengroup.org/onlinepubs/009656399/toc.pdf X/Open CAE Specification Commands and Utilities Issue 4, Version 2] (pdf), 1994, opengroup.org</ref> and further in The Open Group Base Specifications, Issue 6 and 7.<ref>{{man|cu|compress|SUS6}}</ref> Linux Standard Base does not require {{code|compress}}.<ref>[https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/command.html Chapter 17. Commands and Utilities] in Linux Standard Base Core Specification 5.0.0, linuxfoundation.org</ref>

{{code|compress}} is often excluded from the default installation of a Linux distribution but can be installed from a separate package.<ref>[https://pkgs.org/download/ncompress ncompress], pkgs.org</ref> {{code|compress}} is available for FreeBSD, OpenBSD, MINIX, Solaris and AIX.

{{code|compress}} is allowed for Point-to-Point Protocol in {{IETF RFC|1977}} and for HTTP/1.1 in {{IETF RFC|9110}}, though it is rarely used in modern deployments as the better deflate/gzip is available.

== Use == Files compressed by {{code|compress}} are typically named with extension ".Z" and therefore sometimes called .Z files. The extension derives from the earlier pack program which used extension ".z".

Most <code>tar</code> implementations support compression by piping data through {{code|compress}} when given the <code>-Z</code> command line option.

{{code|gunzip}} can decompress .Z files.<ref>{{cite web | title=GNU Gzip | website=The GNU Operating System and the Free Software Movement | date=2023-02-05 | url=https://www.gnu.org/software/gzip/manual/gzip.html | access-date=2024-04-03 |quote=gunzip can currently decompress files created by gzip, zip, compress or pack. The detection of the input format is automatic. }}</ref>

== Algorithm == The LZW algorithm used in {{code|compress}} was patented by Sperry Research Center in 1983. Terry Welch published an IEEE article on the algorithm in 1984,<ref>{{cite journal|author=Welch, Terry A.|author-link=Terry Welch|url=https://www.cs.duke.edu/courses/spring03/cps296.5/papers/welch_1984_technique_for.pdf|title=A technique for high performance data compression|journal=IEEE Computer|volume=17|issue=6|pages=8–19|year=1984|doi=10.1109/MC.1984.1659158|s2cid=2055321 }}</ref> but failed to note that he had applied for a patent on the algorithm. Spencer Thomas of the University of Utah took this article and implemented {{code|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 {{code|compress}} and published it as free software to the net.sources USENET group in 1985. {{US patent|4558302}} was granted in 1985 {{endash}} making {{code|compress}} unusable without paying royalties to Sperry Research (which later merged into Unisys).

The US LZW patent expired in 2003, so it is now in the public domain in the United States. Today, all LZW patents worldwide are expired (see Graphics Interchange Format#Unisys and LZW patent enforcement).

As of POSIX.1-2024 {{code|compress}} supports the DEFLATE algorithm used in gzip.<ref>{{cite web |title=compress |url=https://pubs.opengroup.org/onlinepubs/9799919799/utilities/compress.html |publisher=opengroup |access-date=2 November 2024}}</ref>

== File format == The compressed output consists of bit groups. Each bit group consists of codes with fixed amount of bits (9&ndash;16). Each group, except the last group, is aligned to the number of bits per code multiplied by 8 and right padded with zeroes. The last group is aligned to 8 bit octets and padded with zeroes. More information can be found at an issue on the ''ncompress'' GitHub repository.<ref>{{Cite web |title=compression with 9 bits don't work · Issue #5 · vapier/ncompress |url=https://github.com/vapier/ncompress/issues/5 |access-date=2024-09-17 |website=GitHub |language=en}}</ref>

Example: : Suppose the output has ten 9-bit codes, five 10-bit codes, and thirteen 11-bit codes. There are three groups to output containing 90 bits, 50 bits, and 143 bits of data. :* First group will be 90 bits of data + 54 zero bits of padding in order to be aligned to 72 bits (9 bits × 8). :* Second group will be 50 bits of data + 30 zero bits of padding in order to be aligned to 80 bits (10 bits × 8). :* Third group will be 143 bits of data + 1 zero bit of padding in order to be aligned to 8 bits (since this is the last group in the output).

The existence of padding bits is actually a bug, as LZW does not require any alignment. This bug existed for more than 35 years and was in the original UNIX ''compress'', ''ncompress'', ''gzip'' and the Windows port. All ''application/x-compress'' files were created using this bug.<!--What is "specification" here?: So we have to include it in output specification.-->

Some {{code|compress}} implementations write random bits from uninitialized buffer in paddings. There is no guarantee that the paddings will be zeroes. The decompressor must ignore the values in the paddings for compatibility.

== See also == * Image compression * List of POSIX commands

== References == {{Reflist}}

== External links == * {{man|cu|compress|SUS|compress data}} * {{man|1|compress|v8}} * {{man|1|compress|FreeBSD}} * {{man|1|compress|OpenBSD}} * {{man|1|compress|Solaris}} * [https://ncompress.sourceforge.net/ ncompress] - public domain compress/uncompress implementation for POSIX systems * [http://ibiblio.org/pub/linux/utils/compress/compress.tar.Z compress] - original Unix compress (in a compress'd archive) * [http://ibiblio.org/pub/linux/utils/compress/compress.z compress] - original Unix compress executable (gzip'd) * [http://ftp.sunet.se/mirror/archive/ftp.sunet.se/pub/usenet/ftp.uu.net/comp.sources.unix/volume2/compress4.0/ Source Code for compress v4.0] (gzip'd sharchives) * [http://www.willus.com/archive/unixcmds.zip ZIP File containing a Windows port of the compress utility] * [https://svnweb.freebsd.org/base/head/usr.bin/compress/ source code to the current version of fcompress.c from compress] * [https://github.com/vapier/ncompress/issues/5 bit groups alignment] - Explanation of bit groups alignment. * [https://github.com/andrew-aladev/lzws lzws] - New library and CLI, implemented without legacy code. * [https://github.com/andrew-aladev/ruby-lzws ruby-lzws] - Ruby bindings with streaming support.

{{Compression Software Implementations}}{{Archive formats}} Category:Data compression software Category:Unix archivers and compression-related utilities Category:Standard Unix programs Category:Unix SUS2008 utilities Category:IBM i Qshell commands