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31 .\" @(#)mount_union.8 8.6 (Berkeley) 3/27/94
39 .Nd mount union file systems
53 in such a way that the contents of both directory trees remain visible.
64 The options are as follows:
65 .Bl -tag -width indent
67 Invert the default position, so that
69 becomes the lower layer and
71 becomes the upper layer.
74 remains the mount point.
76 Options are specified with a
78 flag followed by a comma separated string of options.
81 man page for possible options and their meanings.
83 Hide the lower layer completely in the same way as mounting with
87 To enforce file system security, the user mounting the file system
88 must be superuser or else have write permission on the mounted-on
93 variable must be set to 1 to permit file system mounting by ordinary users.
95 Filenames are looked up in the upper layer and then in the
97 If a directory is found in the lower layer, and there is no entry
98 in the upper layer, then a
100 directory will be created in the upper layer.
101 It will be owned by the user who originally did the union mount,
104 (0777) modified by the umask in effect at that time.
106 If a file exists in the upper layer then there is no way to access
107 a file with the same name in the lower layer.
108 If necessary, a combination of loopback and union mounts can be made
109 which will still allow the lower files to be accessed by a different
112 Except in the case of a directory,
113 access to an object is granted via the normal file system access checks.
114 For directories, the current user must have access to both the upper
115 and lower directories (should they both exist).
117 Requests to create or modify objects in
119 are passed to the upper layer with the exception of a few special cases.
120 An attempt to open for writing a file which exists in the lower layer
123 file to be made to the upper layer, and then for the upper layer copy
125 Similarly, an attempt to truncate a lower layer file to zero length
126 causes an empty file to be created in the upper layer.
127 Any other operation which would ultimately require modification to
128 the lower layer fails with
131 The union file system manipulates the namespace, rather than
132 individual file systems.
133 The union operation applies recursively down the directory tree
136 Thus any file systems which are mounted under
138 will take part in the union operation.
139 This differs from the
143 which only applies the union operation to the mount point itself,
144 and then only for lookups.
147 .Bd -literal -offset indent
148 mount -t cd9660 -o ro /dev/cd0a /usr/src
149 mount -t unionfs /var/obj /usr/src
152 mount the CD-ROM drive
159 For most purposes the effect of this is to make the
160 source tree appear writable
161 even though it is stored on a CD-ROM.
164 .Bd -literal -offset indent
165 mount -t unionfs -o -b /sys $HOME/sys
168 attaches the system source tree below the
170 directory in the user's home directory.
171 This allows individual users to make private changes
172 to the source, and build new kernels, without those
173 changes becoming visible to other users.
174 Note that the files in the lower layer remain
187 utility first appeared in
190 .Fx Ns -(fill this in) .
192 THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT DOESN'T WORK)
193 AND USING IT MAY, IN FACT, DESTROY DATA ON YOUR SYSTEM.
199 This code also needs an owner in order to be less dangerous - serious
200 hackers can apply by sending mail to
201 .Aq hackers@FreeBSD.org
203 their intent to take it over.
205 Without whiteout support from the file system backing the upper layer,
206 there is no way that delete and rename operations on lower layer
209 is returned for this kind of operations along with any others
210 which would make modifications to the lower layer, such as
215 over a union tree has the side-effect of creating
216 a tree of shadow directories in the upper layer.