Unionfs is a stackable unification file system
which can appear to merge the contents of several
directories (branches), while keeping their
physical content separate. Unionfs is useful for
unified source tree management, merging the
contents of a split CD-ROM, merging separate
software package directories, data grids, and
more. Unionfs allows any mix of read-only and
read-write branches, as well as insertion and
deletion of branches anywhere in the fan-out. To
maintain Unix semantics, Unionfs handles
elimination of duplicates, partial-error
conditions, and more.
