1.\" Copyright (c) 1994
2.\" The Regents of the University of California.  All rights reserved.
3.\"
4.\" This code is derived from software donated to Berkeley by
5.\" Jan-Simon Pendry.
6.\"
7.\" %sccs.include.redist.roff%
8.\"
9.\"	@(#)mount_union.8	8.7 (Berkeley) 05/01/95
10.\"
11.Dd
12.Dt MOUNT_UNION 8
13.Os BSD 4.4
14.Sh NAME
15.Nm mount_union
16.Nd mount union filesystems
17.Sh SYNOPSIS
18.Nm mount_union
19.Op Fl br
20.Op Fl o Ar options
21.Ar directory
22.Ar uniondir
23.Sh DESCRIPTION
24The
25.Nm mount_union
26command
27attaches
28.Ar directory
29above
30.Ar uniondir
31in such a way that the contents of both directory trees remain visible.
32By default,
33.Ar directory
34becomes the
35.Em upper
36layer and
37.Ar uniondir
38becomes the
39.Em lower
40layer.
41.Pp
42The options are as follows:
43.Bl -tag -width indent
44.It Fl b
45Invert the default position, so that
46.Ar directory
47becomes the lower layer and
48.Ar uniondir
49becomes the upper layer.
50However,
51.Ar uniondir
52remains the mount point.
53.It Fl o
54Options are specified with a
55.Fl o
56flag followed by a comma separated string of options.
57See the
58.Xr mount 8
59man page for possible options and their meanings.
60.It Fl r
61Hide the lower layer completely in the same way as mounting with
62.Xr mount_null 8 .
63.El
64.Pp
65To enforce filesystem security, the user mounting the filesystem
66must be superuser or else have write permission on the mounted-on
67directory.
68.Pp
69Filenames are looked up in the upper layer and then in the
70lower layer.
71If a directory is found in the lower layer, and there is no entry
72in the upper layer, then a
73.Em shadow
74directory will be created in the upper layer.
75It will be owned by the user who originally did the union mount,
76with mode
77.Dq rwxrwxrwx
78(0777) modified by the umask in effect at that time.
79.Pp
80If a file exists in the upper layer then there is no way to access
81a file with the same name in the lower layer.
82If necessary, a combination of loopback and union mounts can be made
83which will still allow the lower files to be accessed by a different
84pathname.
85.Pp
86Except in the case of a directory,
87access to an object is granted via the normal filesystem access checks.
88For directories, the current user must have access to both the upper
89and lower directories (should they both exist).
90.Pp
91Requests to create or modify objects in
92.Ar uniondir
93are passed to the upper layer with the exception of a few special cases.
94An attempt to open for writing a file which exists in the lower layer
95causes a copy of the
96.Em entire
97file to be made to the upper layer, and then for the upper layer copy
98to be opened.
99Similarly, an attempt to truncate a lower layer file to zero length
100causes an empty file to be created in the upper layer.
101Any other operation which would ultimately require modification to
102the lower layer fails with
103.Dv EROFS .
104.Pp
105The union filesystem manipulates the namespace, rather than
106individual filesystems.
107The union operation applies recursively down the directory tree
108now rooted at
109.Ar uniondir .
110Thus any filesystems which are mounted under
111.Ar uniondir
112will take part in the union operation.
113This differs from the
114.Em union
115option to
116.Xr mount 8
117which only applies the union operation to the mount point itself,
118and then only for lookups.
119.Sh EXAMPLES
120The commands
121.Bd -literal -offset indent
122mount -t cd9660 -o ro /dev/cd0a /usr/src
123mount -t union -o /var/obj /usr/src
124.Ed
125.Pp
126mount the CD-ROM drive
127.Pa /dev/cd0a
128on
129.Pa /usr/src
130and then attaches
131.Pa /var/obj
132on top.
133For most purposes the effect of this is to make the
134source tree appear writable
135even though it is stored on a CD-ROM.
136.Pp
137The command
138.Bd -literal -offset indent
139mount -t union -o -b /sys $HOME/sys
140.Ed
141.Pp
142attaches the system source tree below the
143.Pa sys
144directory in the user's home directory.
145This allows individual users to make private changes
146to the source, and build new kernels, without those
147changes becoming visible to other users.
148Note that the files in the lower layer remain
149accessible via
150.Pa /sys .
151.Sh SEE ALSO
152.Xr intro 2 ,
153.Xr mount 2 ,
154.Xr unmount 2 ,
155.Xr fstab 5 ,
156.Xr mount 8 ,
157.Xr mount_null 8
158.Sh BUGS
159Without whiteout support from the filesystem backing the upper layer,
160there is no way that delete and rename operations on lower layer
161objects can be done.
162.Dv EROFS
163is returned for this kind of operations along with any others
164which would make modifications to the lower layer, such as
165.Xr chmod 1 .
166.Pp
167Running
168.Xr find 1
169over a union tree has the side-effect of creating
170a tree of shadow directories in the upper layer.
171.Sh HISTORY
172The
173.Nm mount_union
174command first appeared in
175.Bx 4.4 .
176