Both directory and uniondir are converted to absolute paths before use.
The options are as follows:
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.
mount -t cd9660 -o ro /dev/cd0a /usr/src
mount -t union /var/obj /usr/src
mount the
CD-ROM
drive
/dev/cd0a
on
/usr/src
and then attaches
/var/obj
on top.
For most purposes the effect of this is to make the
source tree appear writable
even though it is stored on a
CD-ROM.
The command
mount -t union -o -b /sys $HOME/sys
attaches the system source tree below the
sys
directory in the user's home directory.
This allows individual users to make private changes
to the source, and build new kernels, without those
changes becoming visible to other users.
Note that the files in the lower layer remain
accessible via
/sys
.
EOPNOTSUPP
("Operation not supported).
Whiteout support can be added to an existing FFS filesystem
by using the
-c
option of
fsck_ffs(8).
Running find(1) over a union tree has the side-effect of creating a tree of shadow directories in the upper layer.