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MOUNT_UNIONFS(8)	FreeBSD	System Manager's Manual	      MOUNT_UNIONFS(8)

NAME
     mount_unionfs -- mount union file systems

SYNOPSIS
     mount_unionfs [-br] [-o options] directory	uniondir

DESCRIPTION
     The mount_unionfs utility 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.

     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 sepa-
	     rated string of options.  See the mount(8)	man page for possible
	     options and their meanings.

     -r	     Hide the lower layer completely in	the same way as	mounting with
	     mount_nullfs(8).

     To	enforce	file system security, the user mounting	the file system	must
     be	superuser or else have write permission	on the mounted-on directory.

     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 unionfs /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 unionfs -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), mount(8), mount_nullfs(8)

BUGS
     THIS FILE SYSTEM TYPE IS NOT YET FULLY SUPPORTED (READ: IT	DOESN'T	WORK)
     AND USING IT MAY, IN FACT,	DESTROY	DATA ON	YOUR SYSTEM.  USE AT YOUR OWN
     RISK.  BEWARE OF DOG.  SLIPPERY WHEN WET.

     This code also needs an owner in order to be less dangerous - serious
     hackers can apply by sending mail to <hackers@FreeBSD.org>	and announcing
     their intent to take it over.

     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.  EROFS is returned for this kind of operations along with
     any others	which would make modifications to the lower layer, such	as
     chmod(1).

     Running find(1) over a union tree has the side-effect of creating a tree
     of	shadow directories in the upper	layer.

HISTORY
     The mount_unionfs utility first appeared in 4.4BSD.  It first worked in
     FreeBSD-(fill this	in).

FreeBSD	10.1			March 27, 1994			  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | SEE ALSO | BUGS | HISTORY

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