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UMOUNT

Section: System Administration (8)
Updated: July 2014
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NAME

umount - unmount file systems

SYNOPSIS

umount -a [-dflnrv] [-t fstype] [-O option...]

umount [-dflnrv] {directory|device}...

umount -h|-V

DESCRIPTION

The umount command detaches the mentioned file system(s) from the file hierarchy. A file system is specified by giving the directory where it has been mounted. Giving the special device on which the file system lives may also work, but is obsolete, mainly because it will fail in case this device was mounted on more than one directory.

Note that a file system cannot be unmounted when it is 'busy' - for example, when there are open files on it, or when some process has its working directory there, or when a swap file on it is in use. The offending process could even be umount itself - it opens libc, and libc in its turn may open for example locale files. A lazy unmount avoids this problem.

OPTIONS

-a, --all: All of the filesystems described in /proc/self/mountinfo (or in deprecated /etc/mtab) are unmounted, except the proc, devfs, devpts, sysfs, rpc_pipefs and nfsd filesystems. This list of the filesystems may be replaced by --types umount option.
-A, --all-targets: Unmount all mountpoints in the current namespace for the specified filesystem. The filesystem can be specified by one of the mountpoints or the device name (or UUID, etc.). When this option is used together with --recursive, then all nested mounts within the filesystem are recursively unmounted. This option is only supported on systems where /etc/mtab is a symlink to /proc/mounts.
-c, --no-canonicalize: Do not canonicalize paths. The paths canonicalization is based on stat(2) and readlink(2) system calls. These system calls may hang in some cases (for example on NFS if server is not available). The option has to be used with canonical path to the mount point.
For more details about this option see the mount(8) man page. Note that umount does not pass this option to the /sbin/umount.type helpers.
-d, --detach-loop: When the unmounted device was a loop device, also free this loop device. This option is unnecessary for devices initialized by mount(8), in this case "autoclear" functionality is enabled by default.
--fake: Causes everything to be done except for the actual system call or umount helper execution; this 'fakes' unmounting the filesystem. It can be used to remove entries from the deprecated /etc/mtab that were unmounted earlier with the -n option.
-f, --force: Force an unmount (in case of an unreachable NFS system). (Requires kernel 2.1.116 or later.)
Note that this option does not guarantee that umount command does not hang. It's strongly recommended to use absolute paths without symlinks to avoid unwanted readlink and stat system calls on unreachable NFS in umount.
-i, --internal-only: Do not call the /sbin/umount.filesystem helper even if it exists. By default such a helper program is called if it exists.
-l, --lazy: Lazy unmount. Detach the filesystem from the file hierarchy now, and clean up all references to this filesystem as soon as it is not busy anymore. (Requires kernel 2.4.11 or later.)
-n, --no-mtab: Unmount without writing in /etc/mtab.
-O, --test-opts option...: Unmount only the filesystems that have the specified option set in /etc/fstab. More than one option may be specified in a comma-separated list. Each option can be prefixed with no to indicate that no action should be taken for this option.
-R, --recursive: Recursively unmount each specified directory. Recursion for each directory will stop if any unmount operation in the chain fails for any reason. The relationship between mountpoints is determined by /proc/self/mountinfo entries. The filesystem must be specified by mountpoint path; a recursive unmount by device name (or UUID) is unsupported.
-r, --read-only: When an unmount fails, try to remount the filesystem read-only.
-t, --types type...: Indicate that the actions should only be taken on filesystems of the specified type. More than one type may be specified in a comma-separated list. The list of filesystem types can be prefixed with no to indicate that no action should be taken for all of the mentioned types. Notethat umount reads information about mounted filesystems from kernel (/proc/mounts) and filesystem names may be different than filesystem names used in the /etc/fstab (e.g. "nfs4" vs. "nfs").
-v, --verbose: Verbose mode.
-V, --version: Display version information and exit.
-h, --help: Display help text and exit.

LOOP DEVICE

The umount command will automatically detach loop device previously initialized by mount(8) command independently of /etc/mtab.

In this case the device is initialized with "autoclear" flag (see losetup(8) output for more details), otherwise it's necessary to use the option --detach-loop or call losetup -d <device>. The autoclear feature is supported since Linux 2.6.25.

EXTERNAL HELPERS

The syntax of external unmount helpers is:

: umount.suffix {directory|device} [-flnrv] [-t type.subtype]

where suffix is the filesystem type (or the value from a uhelper= or helper= marker in the mtab file). The -t option can be used for filesystems that have subtype support. For example:

: umount.fuse -t fuse.sshfs

A uhelper=something marker (unprivileged helper) can appear in the /etc/mtab file when ordinary users need to be able to unmount a mountpoint that is not defined in /etc/fstab (for example for a device that was mounted by udisks(1)).

A helper=type marker in the mtab file will redirect all unmount requests to the /sbin/umount.type helper independently of UID.

Note that /etc/mtab is currently deprecated and helper= and another userspace mount options are maintained by libmount.

FILES

/etc/mtab: table of mounted filesystems (deprecated and usually replaced by symlink to /proc/mounts)
/etc/fstab: table of known filesystems
/proc/self/mountinfo: table of mounted filesystems generated by kernel.

ENVIRONMENT

LIBMOUNT_FSTAB=<path>: overrides the default location of the fstab file (ignored for suid)
LIBMOUNT_MTAB=<path>: overrides the default location of the mtab file (ignored for suid)
LIBMOUNT_DEBUG=all: enables libmount debug output

HISTORY

A umount command appeared in Version 6 AT&T UNIX.

AVAILABILITY

The umount command is part of the util-linux package and is available from Linux Kernel Archive