XZ

File Format
Name	XZ
Ontology	Electronic File Formats Compression XZ
Extension(s)	.xz
MIME Type(s)	application/x-xz
PRONOM	fmt/1098
Wikidata ID	Q162839
Compression	lossless, mandatory
Magic Bytes	FD 37 7A 58 5A 00
Released	2009

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,
• initramfs,
• squashfs-tools
• As a specific switch -J or --xz in GNU tar under Linux.
• DWARFS - mainly via liblzma which on modern Linux installations, only LZMA2 is used in place of LZMA.

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.)

Software

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

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

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