|Developer||Open Source Community|
|Full name||Unification File System|
|File system permissions||POSIX|
|Transparent encryption||No (provided at the block device level)|
|Supported operating systems||Linux, FreeBSD and NetBSD|
UnionFS is a filesystem service for Linux, FreeBSD and NetBSD which implements a union mount for other file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system. Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.
When mounting branches, the priority of one branch over the other is specified. So when both branches contain a file with the same name, one gets priority over the other.
The different branches may be both read-only and read-write file systems, so that writes to the virtual, merged copy are directed to a specific real file system. This allows a file system to appear as writeable, but without actually allowing writes to change the file system, also known as copy-on-write. This may be desirable when the media is physically read-only, such as in the case of Live CDs.
In Knoppix, a union between the file system on the CD-ROM or DVD and a file system contained in an image file called knoppix.img (knoppix-data.img for knoppix 7) on a writable drive (such as a USB memory stick) can be made, where the writable drive has priority over the read-only filesystem. This allows the user to change any of the files on the system, with the new file stored in the image and transparently used instead of the one on the CD.
Other implementations 
Unionfs for Linux has two versions. Version 1.x is a standalone one that can be built as a module. Version 2.x is a newer, redesigned, and reimplemented one. Version 2.x is the smallest implementation of unioning for Linux, it is heavily tested and inspected by many kernel developers, and it is more efficient.
UnionFsFuse is an independent project, implemented as an user space filesystem program, instead of a kernel module or patch. Like UnionFS, it supports copy-on-write and read-only or read-write branches.
Plan 9 from Bell Labs operating system uses union mounts extensively to build custom namespaces per user/processes. The same concept, implemented as "union mount", has also been available in BSD, since at least 1995.
mhddfs works like UnionFS but permits balancing files over drives with the most free space available. It is implemented as a user space filesystem.
Sun Microsystems introduced the first implementation of a stacked, layered file system with copy-on-write, whiteouts (hiding files in lower layers from higher layers), etc. as the Translucent File Service in SunOS 3, circa 1986.
- C. P. Wright et al. (2004). "Versatility and Unix Semantics in a Fan-Out Unification File System (pdf)." (Discusses UnionFS implementations and semantics.)
- Persistent disk image section on knoppix wiki
- "aufs Homepage". Retrieved 27 April 2012.
- "USENIX 1995 Technical Conference Proceedings". 16–20 January 1995. Retrieved 7 March 2012.
- Sun/Release 3.0. vol. 1. Interleaf Technical Publishing. 1988. pp. 15‑1–18, 16‑1–19.
- Waisybabu (19 June 2011). "Comex Working On Faster Jailbreaking Process for JailbreakMe 3.0; iPad 2 Jailbreak Coming Soon!". Redmond Pie. Retrieved 24 September 2011.
- Official Linux Unionfs home page
- Bugzilla bug database for Linux Unionfs
- UnionFsFuse FUSE-based alternative implementation of UnionFS
- AuFS is an alternative unionfs for linux.
- FunionFS (Another advanced FUSE based implementation of UnionFS with log, control...)
- Kernel Korner - Unionfs: Bringing Filesystems Together - article from LinuxJournal
- The new implementation of union fs for FreeBSD
- On implementation of a union fs
- LUFS-based unionfs
- Linux Implementations - excellent write-up on history of union file systems for Linux
- The Translucent File Service (white paper), David Hendricks, Sun Microsystems (gzip PostScript)
- mini_fo: overlay filesystem (rw) for read-only rootfs
- FreeBSD mount_union man page