mount

Category

Linux Command

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Usage

mount [-fnrsvw] [-t vfstype] [-o options] device dir

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Manual

All files accessible in a Unix system are arranged in one big tree, the
file hierarchy, rooted at /. These files can be spread out over sev-
eral devices. The mount command serves to attach the filesystem found
on some device to the big file tree. Conversely, the umount(8) command
will detach it again.

The standard form of the mount command, is

mount -t type device dir

This tells the kernel to attach the filesystem found on device (which
is of type type) at the directory dir. The previous contents (if any)
and owner and mode of dir become invisible, and as long as this
filesystem remains mounted, the pathname dir refers to the root of the
filesystem on device.

The listing and help.
Three forms of invocation do not actually mount anything:

mount -h
prints a help message

mount -V
prints a version string

mount [-l] [-t type]
lists all mounted filesystems (of type type). The option
-l adds the labels in this listing. See below.

The device indication.
Most devices are indicated by a file name (of a block special
device), like /dev/sda1, but there are other possibilities. For
example, in the case of an NFS mount, device may look like
knuth.cwi.nl:/dir. It is possible to indicate a block special
device using its volume LABEL or UUID (see the -L and -U options
below).

The recommended setup is to use LABEL= or UUID=
tags rather than /dev/disk/by-{label,uuid} udev symlinks in the
/etc/fstab file. The tags are more readable, robust and
portable. The mount(8) command internally uses udev symlinks, so
use the symlinks in /etc/fstab is not advantage over
LABEL=/UUID=. For more details see libblkid(3).

The proc filesystem is not associated with a special device, and
when mounting it, an arbitrary keyword, such as proc can be used
instead of a device specification. (The customary choice none
is less fortunate: the error message ‘none busy’ from umount can
be confusing.)

The /etc/fstab, /etc/mtab and /proc/mounts files.
The file /etc/fstab (see fstab(5)), may contain lines describing
what devices are usually mounted where, using which options.

The command

mount -a [-t type] [-O optlist]

(usually given in a bootscript) causes all filesystems mentioned
in fstab (of the proper type and/or having or not having the
proper options) to be mounted as indicated, except for those
whose line contains the noauto keyword. Adding the -F option
will make mount fork, so that the filesystems are mounted simul-
taneously.

When mounting a filesystem mentioned in fstab or mtab, it suf-
fices to give only the device, or only the mount point.


The programs mount and umount maintain a list of currently
mounted filesystems in the file /etc/mtab. If no arguments are
given to mount, this list is printed.

The mount program does not read the /etc/fstab file if device
(or LABEL/UUID) and dir are specified. For example:

mount /dev/foo /dir

If you want to override mount options from /etc/fstab you have
to use:

mount device|dir -o

and then the mount options from command line will be appended to
the list of options from /etc/fstab. The usual behaviour is
that the last option wins if there is more duplicated options.

When the proc filesystem is mounted (say at /proc), the files
/etc/mtab and /proc/mounts have very similar contents. The for-
mer has somewhat more information, such as the mount options
used, but is not necessarily up-to-date (cf. the -n option
below). It is possible to replace /etc/mtab by a symbolic link
to /proc/mounts, and especially when you have very large numbers
of mounts things will be much faster with that symlink, but some
information is lost that way, and in particular using the "user"
option will fail.

The non-superuser mounts.
Normally, only the superuser can mount filesystems. However,
when fstab contains the user option on a line, anybody can mount
the corresponding system.

Thus, given a line

/dev/cdrom /cd iso9660 ro,user,noauto,unhide

any user can mount the iso9660 filesystem found on his CDROM
using the command

mount /dev/cdrom

or

mount /cd

For more details, see fstab(5). Only the user that mounted a
filesystem can unmount it again. If any user should be able to
unmount, then use users instead of user in the fstab line. The
owner option is similar to the user option, with the restriction
that the user must be the owner of the special file. This may be
useful e.g. for /dev/fd if a login script makes the console user
owner of this device. The group option is similar, with the
restriction that the user must be member of the group of the
special file.


The bind mounts.
Since Linux 2.4.0 it is possible to remount part of the file
hierarchy somewhere else. The call is
mount --bind olddir newdir
or shortoption
mount -B olddir newdir
or fstab entry is:
/olddir /newdir none bind

After this call the same contents is accessible in two places.
One can also remount a single file (on a single file).

This call attaches only (part of) a single filesystem, not pos-
sible submounts. The entire file hierarchy including submounts
is attached a second place using
mount --rbind olddir newdir
or shortoption
mount -R olddir newdir

Note that the filesystem mount options will remain the same as
those on the original mount point, and cannot be changed by
passing the -o option along with --bind/--rbind. The mount
options can be changed by a separate remount command, for exam-
ple:

mount --bind olddir newdir
mount -o remount,ro newdir


The move operation.
Since Linux 2.5.1 it is possible to atomically move a mounted
tree to another place. The call is
mount --move olddir newdir
or shortoption
mount -M olddir newdir
This will cause the contents which previously appeared under
olddir to be accessed under newdir. The physical location of
the files is not changed.

Note also that moving a mount residing under a shared mount is
invalid and unsupported (in the other words the parent of the
olddir has to use private propagation flag). See
/proc/self/mountinfo for the current propagation flags.

The shared subtrees operations.
Since Linux 2.6.15 it is possible to mark a mount and its sub-
mounts as shared, private, slave or unbindable. A shared mount
provides ability to create mirrors of that mount such that
mounts and umounts within any of the mirrors propagate to the
other mirror. A slave mount receives propagation from its mas-
ter, but any not vice-versa. A private mount carries no propa-
gation abilities. A unbindable mount is a private mount which
cannot cloned through a bind operation. Detailed semantics is
documented in Documentation/sharedsubtree.txt file in the kernel
source tree.

mount --make-shared mountpoint
mount --make-slave mountpoint
mount --make-private mountpoint
mount --make-unbindable mountpoint

The following commands allows one to recursively change the type
of all the mounts under a given mountpoint.

mount --make-rshared mountpoint
mount --make-rslave mountpoint
mount --make-rprivate mountpoint
mount --make-runbindable mountpoint

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