ar (Unix archive) file format

.a · .ar · archive (no compression) · introduced 1971 by AT&T (Unix)

What is a ar (Unix archive) file?

ar is the original Unix archive format, designed in 1971 to bundle object files into static libraries (.a). It is also the outer container of Debian .deb packages. ar is rarely used directly today outside of toolchains.

Most readers reach this page after double-clicking a .a file and finding their operating system unsure what to do with it. The good news: ar (Unix archive) is a well-understood format with mature tooling on every major platform. It is supported in one click by mainstream archivers on Windows, macOS, and Linux.

How to extract a ar (Unix archive) file

Opening a .a file is straightforward on every modern operating system. The exact steps depend on which platform you are on:

On Windows

Locate the .a file in File Explorer.
Right-click the file. If you have 7-Zip installed, choose 7-Zip → Extract Here. On Windows 11 the built-in extractor handles ZIP, 7z, RAR and several other formats directly through Extract All…
If prompted, pick a destination folder and click Extract.
The extracted files appear in a folder next to the original archive.

On macOS

Find the .a file in Finder.
Install The Unarchiver from the Mac App Store, or Keka for both creating and extracting.
Right-click the file → Open With → The Unarchiver (or just double-click after Unarchiver is set as the default).
The extracted folder appears next to the archive.

On Linux

Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).

On the command line, run one of:

7z x archive.a    # works for almost any format

The files appear in the current directory.

How to create a ar (Unix archive) file

To pack files into a .a archive, you typically need an archiver that supports writing this format. Not every tool can create every format — for example, only WinRAR can create real RAR files. The sidebar lists the software known to read and write ar (Unix archive).

On the command line, the canonical create command is:

# Use 7-Zip, PeaZip, or the format's reference tool to create .a files.

If you regularly create archives at scale, a dedicated archive automation suite can wrap this command in scheduled jobs, integrity checks, and offsite replication.

Strengths

Trivial format
Built into every Unix toolchain

Weaknesses and limitations

No compression
No subdirectories
Filename length limitations in classic format

Typical use cases

Static C/C++ libraries (.a)
Debian package wrapper
Linker input

Technical details

ar (Unix archive) uses the None algorithm and is identified on disk by the magic byte sequence 21 3C 61 72 63 68 3E 0A. The standard MIME type is application/x-archive. The format is an open specification and is free for any use.

Frequently asked questions

Is ar (Unix archive) safe to open?

Archive files are containers — they are only as safe as what they contain. A .a from a trusted source is fine; one received from a stranger can hold malicious executables, scripts, or files engineered to exploit bugs in your archiver. Always keep your archiver updated, and never run unknown executables that come out of an archive. A safe workflow is to extract into a sandbox and scan the contents before opening anything.

What is the maximum file size?

Modern implementations support archives in the multi-terabyte range. The practical limit is your filesystem (FAT32 caps at 4 GB; ext4, NTFS, APFS, and exFAT all comfortably handle multi-TB files).

Can I password-protect a ar (Unix archive)?

Some archivers can wrap ar (Unix archive) files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does ar (Unix archive) compare to other formats?

See our format comparison pages such as ZIP vs 7z, ZIP vs RAR, and tar.gz vs tar.xz for side-by-side feature tables.