XZ Utils

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XZ Utils
Original author(s)Lasse Collin
Developer(s)The Tukaani Project
Stable release
5.2.5 / March 17, 2020 (2020-03-17)[1]
Repository
Written inC
Operating systemCross-platform
TypeData compression
LicensePublic domain.[2] (but see details in Development and adoption)
Websitetukaani.org/xz/

XZ Utils (previously LZMA Utils) is a set of free software command-line lossless data compressors, including lzma and xz, for Unix-like operating systems and, from version 5.0 onwards, Microsoft Windows.

In most cases, xz achieves higher compression rates than alternatives like gzip and bzip2. Decompression speed is higher than bzip2, but lower than gzip. Compression can be much slower than gzip, and is slower than bzip2 for high levels of compression, and is most useful when a compressed file will be used many times.[3][4]

XZ Utils consists of two major components:

  • xz, the command-line compressor and decompressor (analogous to gzip)
  • liblzma, a software library with an API similar to zlib

Various command shortcuts exist, such as lzma (for xz --format=lzma), unxz (for xz --decompress; analogous to gunzip) and xzcat (for unxz --stdout; analogous to zcat)

XZ Utils can compress and decompress both the xz and lzma file formats, but since the LZMA format is now legacy,[5] XZ Utils compresses by default to xz.

Usage[]

Both the behavior of the software as well as the properties of the file format have been designed to work similarly to those of the popular Unix compressing tools gzip and bzip2. It consists of a Unix port of Igor Pavlov's LZMA-SDK that has been adapted to fit seamlessly into Unix environments and their usual structure and behavior.

xz has supported multi-threaded compression (with the -T flag)[6] since 2014, version 5.2.0.;[7] as of 2019 threaded decompression has not been implemented yet.[6] The number of threads can be less than defined if the file is not big enough for threading with the given settings or if using more threads would exceed the memory usage limit.[6]

Just like gzip and bzip, xz and lzma can only compress single files (or data streams) as input. They cannot bundle multiple files into a single archive – to do this an archiving program is used first, such as tar.

Compressing an archive: 

xz   my_archive.tar    # results in my_archive.tar.xz
lzma my_archive.tar    # results in my_archive.tar.lzma

Decompressing the archive: 

unxz    my_archive.tar.xz      # results in my_archive.tar
unlzma  my_archive.tar.lzma    # results in my_archive.tar

Version 1.22 or greater of the GNU implementation of tar has transparent support for tarballs compressed with lzma and xz, using the switches --xz or -J for xz compression, and --lzma for LZMA compression.

Creating an archive and compressing it: 

tar -c --xz   -f my_archive.tar.xz   /some_directory    # results in my_archive.tar.xz
tar -c --lzma -f my_archive.tar.lzma /some_directory    # results in my_archive.tar.lzma

Decompressing the archive and extracting its contents: 

tar -x --xz   -f my_archive.tar.xz      # results in /some_directory
tar -x --lzma -f my_archive.tar.lzma    # results in /some_directory

Single-letter tar example for archive with compress and decompress with extract using short suffix: 

tar cJf keep.txz keep   # archive then compress the directory ./keep/ into the file ./keep.txz
tar xJf keep.txz        # decompress then extract the file ./keep.txz creating the directory ./keep/

The xz format[]

The xz format improves on lzma by allowing for preprocessing filters. The exact filters used are similar to those used in 7z, as 7z's filters are available in the public domain via the LZMA SDK.

Development and adoption[]

Development of XZ Utils took place within the Tukaani Project, which was led by Mike Kezner, by a small group of developers who once maintained a Linux distribution based on Slackware.

All of the source code for xz and liblzma has been released into the public domain. The XZ Utils source distribution additionally includes some optional scripts and an example program that are subject to various versions of the GPL.[2]

Specifically, the full list of GPL scripts and sources distributed with the XZ Utils software include:

  • An optional implementation of a common libc function, getopt (GNU GPL v2 and GNU LGPL v2.1)
  • An m4 script for pthread detection (GNU GPL v3)
  • Some nonessential wrapper scripts (xzgrep, etc) (GNU GPL v2)
  • And the example program scanlzma, which is not integrated with the build system

The resulting software xz and liblzma binaries are public domain, unless the optional LGPL getopt implementation is incorporated.[8]

Binaries are available for FreeBSD, Linux systems, Microsoft Windows, and FreeDOS. A number of Linux distributions, including Fedora, Slackware, Ubuntu, and Debian use xz for compressing their software packages. Arch Linux previously used xz to compress packages,[9] but as of December 27, 2019, packages are compressed with Zstandard compression.[10] The GNU FTP archive also uses xz.

References[]

  1. ^ "XZ Utils Release Notes". [git repo]. Lasse Collin. 2020-03-17. Retrieved 2020-03-30.
  2. ^ a b Licensing on tukaani.org "The most interesting parts of XZ Utils (e.g. liblzma) are in the public domain. You can do whatever you want with the public domain parts. Some parts of XZ Utils (e.g. build system and some utilities) are under different free software licenses such as GNU LGPLv2.1, GNU GPLv2, or GNU GPLv3."
  3. ^ Henry-Stocker, Sandra (2017-12-12). "How to squeeze the most out of Linux file compression". Network World. Retrieved 2020-02-09.
  4. ^ "Gzip vs Bzip2 vs XZ Performance Comparison". RootUsers. 2015-09-16. Retrieved 2020-02-09.
  5. ^ LZMA Utils, retrieved 2011-01-25
  6. ^ a b c https://man.cx/xz
  7. ^ https://git.tukaani.org/?p=xz.git;a=blob;f=NEWS;hb=HEAD
  8. ^ "In what cases is the output of a GPL program covered by the GPL too?". GNU.org. Retrieved 21 August 2019.
  9. ^ Pierre Schmitz (2010-03-23). "News: Switching to xz compression for new packages".
  10. ^ "Arch Linux - News: Now using Zstandard instead of xz for package compression". www.archlinux.org. Retrieved 2020-01-07.

External links[]

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