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.\"	@(#)mount_union.8	8.7 (Berkeley) 5/1/95
.\"
.Dd February 5, 2008
.Dt MOUNT_UNION 8
.Os
.Sh NAME
.Nm mount_union
.Nd mount union file systems
.Sh SYNOPSIS
.Nm
.Op Fl b
.Op Fl o Ar options
.Ar directory
.Ar uniondir
.Sh DESCRIPTION
The
.Nm
command attaches
.Ar directory
above
.Ar uniondir
in such a way that the contents of both directory trees remain visible.
By default,
.Ar directory
becomes the
.Em upper
layer and
.Ar uniondir
becomes the
.Em lower
layer.
.Pp
Both
.Ar directory
and
.Ar uniondir
are converted to absolute paths before use.
.Pp
The options are as follows:
.Bl -tag -width indent
.It Fl b
Invert the default position, so that
.Ar directory
becomes the lower layer and
.Ar uniondir
becomes the upper layer.
However,
.Ar uniondir
remains the mount point.
.It Fl o
Options are specified with a
.Fl o
flag followed by a comma separated string of options.
See the
.Xr mount 8
man page for possible options and their meanings.
.El
.Pp
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
.Em shadow
directory will be created in the upper layer.
It will be owned by the user who originally did the union mount,
with mode
.Qq rwxrwxrwx
.Pq 0777
modified by the umask in effect at that time.
.Pp
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.
.Pp
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).
.Pp
Requests to create or modify objects in
.Ar 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
.Em 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
.Dv EROFS .
.Pp
The union file system manipulates the namespace, rather than
individual file systems.
The union operation applies recursively down the directory tree
now rooted at
.Ar uniondir .
Thus any file systems which are mounted under
.Ar uniondir
will take part in the union operation.
This differs from the
.Em union
option to
.Xr mount 8
which only applies the union operation to the mount point itself,
and then only for lookups.
.Sh EXAMPLES
The commands
.Bd -literal -offset indent
mount -t cd9660 -o ro /dev/cd0a /usr/src
mount -t union /var/obj /usr/src
.Ed
.Pp
mount the
.Tn CD-ROM
drive
.Pa /dev/cd0a
on
.Pa /usr/src
and then attaches
.Pa /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
.Tn CD-ROM .
.Pp
The command
.Bd -literal -offset indent
mount -t union -o -b /sys $HOME/sys
.Ed
.Pp
attaches the system source tree below the
.Pa 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
.Pa /sys .
.Sh SEE ALSO
.Xr intro 2 ,
.Xr mount 2 ,
.Xr unmount 2 ,
.Xr fstab 5 ,
.Xr fsck_ffs 8 ,
.Xr mount 8 ,
.Xr mount_null 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
command first appeared in
.Bx 4.4 .
.Sh 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
.Dv EOPNOTSUPP
.Pq Qq Operation not supported .
Whiteout support can be added to an existing FFS file system
by using the
.Fl c
option of
.Xr fsck_ffs 8 .
.Pp
Running
.Xr find 1
over a union tree has the side-effect of creating
a tree of shadow directories in the upper layer.