.\" Copyright (c) 1994
.\" The Regents of the University of California. All rights reserved.
.\" This code is derived from software donated to Berkeley by
.\" Jan-Simon Pendry.
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.\" @(#)mount_union.8 8.6 (Berkeley) 3/27/94
.Dd March 27, 1994
.Dt MOUNT_UNION 8
.Os BSD 4.4
.Nd mount union filesystems
.Op Fl br
.Op Fl o Ar options
in such a way that the contents of both directory trees remain visible.
The options are as follows:
.Bl -tag -width indent
.It Fl b
Invert the default position, so that
becomes the lower layer and
becomes the upper layer.
remains the mount point.
.It Fl o
Options are specified with a
flag followed by a comma separated string of options.
.Xr mount 8
man page for possible options and their meanings.
.It Fl r
Hide the lower layer completely in the same way as mounting with
.Xr mount_null 8 .
To enforce filesystem security, the user mounting the filesystem
must be superuser or else have write permission on the mounted-on
Filenames are looked up in the upper layer and then in the
If a directory is found in the lower layer, and there is no entry
in the upper layer, then a
directory will be created in the upper layer.
It will be owned by the user who originally did the union mount,
(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
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
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
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
.Dv EROFS .
The union filesystem manipulates the namespace, rather than
The union operation applies recursively down the directory tree
now rooted at
.Ar uniondir .
Thus any filesystems which are mounted under
will take part in the union operation.
This differs from the
.Xr mount 8
which only applies the union operation to the mount point itself,
and then only for lookups.
.Bd -literal -offset indent
mount -t cd9660 -o ro /dev/cd0a /usr/src
mount -t union -o /var/obj /usr/src
mount the CD-ROM drive
and then attaches
For most purposes the effect of this is to make the
source tree appear writable
even though it is stored on a CD-ROM.
.Bd -literal -offset indent
mount -t union -o -b /sys $HOME/sys
attaches the system source tree below the
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
.Pa /sys .
.Sh SEE ALSO
.Xr intro 2 ,
.Xr mount 2 ,
.Xr unmount 2 ,
.Xr fstab 5 ,
.Xr mount 8 ,
.Xr mount_null 8
Without whiteout support from the filesystem backing the upper layer,
there is no way that delete and rename operations on lower layer
objects can be done.
is returned for this kind of operations along with any others
which would make modifications to the lower layer, such as
.Xr chmod 1 .
.Xr find 1
over a union tree has the side-effect of creating
a tree of shadow directories in the upper layer.
command first appeared in
.Bx 4.4 .