XZ Utils

XZ Utils can compress and decompress the xz and lzma file formats. Since the LZMA format has been considered legacy,^[2] XZ Utils by default compresses to xz. In addition, decompression of the .lz format used by lzip is supported since version 5.3.4.^[3]

In most cases, xz achieves higher compression rates than alternatives like zip,^[4] 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.^[5][6]

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).

Both the behavior of the software and the properties of the file format have been designed to work similarly to those of the popular Unix compressing tools gzip and bzip2.

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/

xz has supported multi-threaded compression (with the -T flag)^[7] since 2014, version 5.2.0;^[3] since version 5.4.0 threaded decompression has been implemented. Threaded decompression requires multiple compressed blocks within a stream which are created by the threaded compression interface. 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.^[7]

An xz file is a sequence of one or more streams. There may be null bytes (padding) after each stream.

The xz format improves on lzma by allowing for preprocessing filters (BCJ and delta). 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. xz's RISC-V BCJ filter is its own addition.

Meta research from 2016 shows that xz has the highest compression ratio among lz4, zstd, zlib, and the slowest compression/decompression.^[8]

The author of lzip claims that the xz format is inadequate for long-term archiving.^[9]

Multi-byte values use little-endian.^[10]

def encode(num):
	if num >= 2**63:
		raise ValueError("num must not have more than 63 bits")

	buf = b""

	while num >= 0x80:
		buf += (0x80 | (num & 0x7f)).to_bytes(length=1)
		num >>= 7

	buf += num.to_bytes(length=1)

	return buf

def decode(buf):
    if len(buf) == 0:
        raise ValueError("buf must not be empty")

	num = 0

	for i, byte in enumerate(buf):
		if i > 8:
			raise ValueError("num must not have more than 63 bits")
		num |= (byte & 0x7f) << (i * 7)
		if byte & 0x80 == 0:
			return num

Development of XZ Utils took place within the Tukaani Project, a small group of developers who once maintained a Linux distribution based on Slackware. The chosen name "XZ" is not an abbreviation but instead appears to be a random given name for the data compressors, as there is no mention anywhere in the official specification on the meaning of "XZ".^[12] The .xz file format specification version 1.0.0 was officially released in January 2009.^[13]

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 GNU General Public License (GPL).^[1] The resulting software xz and liblzma binaries are public domain, unless the optional LGPL getopt implementation is incorporated.^[14]

Binaries are available for FreeBSD, NetBSD, 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,^[15] but as of 27 December 2019, packages are compressed with Zstandard compression.^[16] Fedora Linux also switched to compressing its RPM packages with Zstandard with Fedora Linux 31.^[17] The GNU FTP archive also uses xz.

On 29 March 2024, Andres Freund, a PostgreSQL developer working at Microsoft, announced that he had found a backdoor in XZ Utils, impacting versions 5.6.0 and 5.6.1. Malicious code for setting up the backdoor had been hidden in compressed test files, and the configure script in the tar files was modified to trigger the hidden code. Freund started his investigation "After observing a few odd symptoms around liblzma (part of the xz package)"; specifically that ssh logins using sshd were "taking a lot of CPU" and producing valgrind errors.^[18] The vulnerability received a Common Vulnerability Scoring System (CVSS) score of 10 (the highest).^[19]