A union mount is a way of mounting that allows several filesystems or directories to be simultaneously mounted and visible through a single mount point, appearing to be one filesystem. Rather than mounting each filesystem at a different place in the directory hierarchy, a union mount overlays the filesystems, creating a unified hierarchy. Thus, any given directory (or "folder") in the resulting filesystem may contain files and subdirectories from any or all of the underlying filesystems.
Union mounts are usually implemented through union filesystems, such as UnionFS and AUFS. Generally, one of the constituent directories or filesystems will be treated as read-write, while other directories or filesystems are treated as read-only. If changes are written by applications to the union filesystem, they are recorded in the writeable overlay. As such, union mounts are frequently used by Live CDs.
Union mounts were invented circa 1990, appearing in 4.4BSD-Lite; the authors of the BSD implementation cite previous work on the 3-d filesystem and the Translucent File System (TFS) done at Bell Labs and Sun, respectively. They are also a central concept in Plan 9, which replaces several Unix staples with union mounts (e.g., several directories unioned together at a single
/bin directory replace the
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