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ar (Unix archive)

ar is one of the oldest Unix archive formats — originally used to pack object files into static libraries (libfoo.a). It is not a compression format: it just concatenates members with a small fixed-size header per file. Debian and Ubuntu reuse it as the outer wrapper for .deb packages.

Layout

An ar archive starts with the 8-byte magic !<arch>\n followed by zero or more members. Each member has a 60-byte ASCII header:

Offset Size Field Notes
0 16 Filename Space-padded
16 12 Modification time Decimal Unix timestamp
28 6 Owner UID Decimal
34 6 Group GID Decimal
40 8 Mode Octal
48 10 Size Decimal bytes
58 2 End marker `\n

The file content follows, padded to an even byte boundary. No compression; no deduplication; no central index.

How .deb uses it

A .deb archive contains exactly three members, in fixed order:

debian-binary       (format version, always "2.0\n")
control.tar.*       (package metadata + maintainer scripts)
data.tar.*          (the payload tree to extract)

The .tar.* members are compressed: .tar.xz (modern, most common), .tar.gz (older releases), .tar.bz2 (wheezy era), or .tar.zst (experimental). The outer ar wrapper is uncompressed. flatroot detects the compression from the member name and decompresses accordingly.

Why ar and not tar

Historical choice. .deb predates tar's ubiquity in the Linux packaging world, and the three-member layout is fixed enough that the minimalism of ar is a feature rather than a limitation. Moving .deb to a single tar today would break every existing tool for no functional gain.

Further reading

  • Appendix / Formats / cpio (RPM payload) — cpio is a Unix archive format like tar — a stream of (header, filename, content) triples with no central index. It is older than tar, less widespread today, and still used in...
    appendix formats
  • Appendix / Formats / RFC 822 records — RFC 822 is the 1982 internet standard for electronic mail message headers — the Key: Value format every email header line uses. Debian adopted this format (with minor...
    appendix formats
  • Appendix / Formats / DwarFS — DwarFS — Deduplicating Warp-speed Advanced Read-only File System — is a compressed read-only filesystem optimized for extreme compression while preserving fast random access....
    appendix formats export