XZ

Latest revision as of 10:58, 16 August 2025

XZ is a stream compression format with built-in integrity checks. It uses LZMA2 compression. Compressed streams are able to be concatenated and still be decompressible like a single-stream file.

XZ is a successor to LZMA_Alone format, and an alternative to Lzip. The compression format is offered as an option among the likes of:

• vmlinuz^[2],
• initramfs^[3][4][5],
• squashfs-tools^[6]
• As a specific switch -J or --xz in GNU tar^[7] under Linux.
• DWARFS - mainly via liblzma^[8] which on modern Linux installations, only LZMA2 is used in place of LZMA.

[edit] Identification

XZ files always have lengths that are multiples of 4 bytes, and they begin with a 6-byte "magic" sequence of (hex) FD 37 7A 58 5A 00 and end with footer "magic bytes" of 59 5A. (The ending bytes can be verified as a sign that the file has not been truncated in transmission.)

[edit] Software

• 7-Zip
• LZMA SDK
• XZ Utils:
  • xz, unxz, xzcat, etc.
  • xzdec
  • liblzma
• XZ Embedded

[edit] Sample files

• dexvert samples — archive/xz
• Search results of files with .xz extension, belongs to archive family and is the format xz - Discmaster.textfiles.com

[edit] Links

• Current spec
• Official site (has historical specs with other info)
• Wikipedia article
• Xz format inadequate for long-term archiving - Analysis of weaknesses and design errors in the xz format, by the author of another format

[edit] References

1. ↑ The .xz File Format - Version 1.0.0 (2009-01-14) - tukaani.org
2. ↑ extract-vmlinux shell script (line 52) - Linus Torvalds' kernel - GitHub
3. ↑ dracut.conf.5.adoc - dracut-ng manual pages - GitHub
4. ↑ mkinitcpio.conf (lines 61-67) - mkinitcpio - Arch Linux GitLab
5. ↑ manpage.md (line 57) - booster - GitHub
6. ↑ compressor.c (lines 31-77) - squashfs-tools - GitHub
7. ↑ 8.1.1 Creating and Reading Compressed Archives - GNU tar 1.35 online manual (single page) - gnu.org
8. ↑ lzma.cpp - DWARFS - GitHub