The
mount_union command attaches
directory above
uniondir in such a way that the contents of both directory trees remain visible. By default,
directory becomes the
upper layer and
uniondir becomes the
lower layer.
Both
directory and
uniondir are converted to absolute paths before use.
The options are as follows:
-b
Invert the default position, so that directory becomes the lower layer and uniondir becomes the upper layer. However, uniondir remains the mount point.
-o
Options are specified with a
-o flag followed by a comma separated string of options. See the
mount(8) man page for possible options and their meanings.
Filenames are looked up in the upper layer and then in the lower layer. If a directory is found in the lower layer, and there is no entry in the upper layer, then a
shadow directory will be created in the upper layer. It will be owned by the user who originally did the union mount, with mode “rwxrwxrwx” (0777) modified by the umask in effect at that time.
If a file exists in the upper layer then there is no way to access a file with the same name in the lower layer. If necessary, a combination of loopback and union mounts can be made which will still allow the lower files to be accessed by a different pathname.
Except in the case of a directory, access to an object is granted via the normal filesystem access checks. For directories, the current user must have access to both the upper and lower directories (should they both exist).
Requests to create or modify objects in
uniondir are passed to the upper layer with the exception of a few special cases. An attempt to open for writing a file which exists in the lower layer causes a copy of the
entire file to be made to the upper layer, and then for the upper layer copy to be opened. Similarly, an attempt to truncate a lower layer file to zero length causes an empty file to be created in the upper layer. Any other operation which would ultimately require modification to the lower layer fails with
EROFS.
The union filesystem manipulates the namespace, rather than individual filesystems. The union operation applies recursively down the directory tree now rooted at
uniondir. Thus any filesystems which are mounted under
uniondir will take part in the union operation. This differs from the
union option to
mount(8) which only applies the union operation to the mount point itself, and then only for lookups.