NAME
mount_union —
mount union file
systems
SYNOPSIS
mount_union |
[-b]
[-o
options] directory
uniondir |
DESCRIPTION
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
file system 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 file system manipulates the namespace, rather than individual file
systems. The union operation applies recursively down the directory tree now
rooted at
uniondir. Thus any file systems 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.
EXAMPLES
The commands
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.
SEE ALSO
intro(2),
mount(2),
unmount(2),
fstab(5),
fsck_ffs(8),
mount(8),
mount_null(8),
sysctl(8)
HISTORY
The
mount_union command first appeared in
4.4BSD.
BUGS
Without whiteout support from the file system backing the upper layer, there is
no way that delete and rename operations on lower layer objects can be done.
An attempt to mount a union directory under one which does not have whiteout
support will return
EOPNOTSUPP
(“Operation not
supported”). Whiteout support can be added to an existing FFS file
system 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.