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Section (2) umount

Linux manual pages Section 2  

Name

umount, umount2 — unmount filesystem

Synopsis

#include <sys/mount.h>
int umount( const char *target);
 
int umount2( const char *target,
  int flags);
 

DESCRIPTION

umount() and umount2() remove the attachment of the (topmost) filesystem mounted on target.

Appropriate privilege (Linux: the CAP_SYS_ADMIN capability) is required to unmount filesystems.

Linux 2.1.116 added the umount2() system call, which, like umount(), unmounts a target, but allows additional flags controlling the behavior of the operation:

MNT_FORCE (since Linux 2.1.116)

Ask the filesystem to abort pending requests before attempting the unmount. This may allow the unmount to complete without waiting for an inaccessible server, but could cause data loss. If, after aborting requests, some processes still have active references to the filesystem, the unmount will still fail. As at Linux 4.12, MNT_FORCE is supported only on the following filesystems: 9p (since Linux 2.6.16), ceph (since Linux 2.6.34), cifs (since Linux 2.6.12), fuse (since Linux 2.6.16), lustre (since Linux 3.11), and NFS (since Linux 2.1.116).

MNT_DETACH (since Linux 2.4.11)

Perform a lazy unmount: make the mount point unavailable for new accesses, immediately disconnect the filesystem and all filesystems mounted below it from each other and from the mount table, and actually perform the unmount when the mount point ceases to be busy.

MNT_EXPIRE (since Linux 2.6.8)

Mark the mount point as expired. If a mount point is not currently in use, then an initial call to umount2() with this flag fails with the error EAGAIN, but marks the mount point as expired. The mount point remains expired as long as it isn_zsingle_quotesz_t accessed by any process. A second umount2() call specifying MNT_EXPIRE unmounts an expired mount point. This flag cannot be specified with either MNT_FORCE or MNT_DETACH.

UMOUNT_NOFOLLOW (since Linux 2.6.34)

Don_zsingle_quotesz_t dereference target if it is a symbolic link. This flag allows security problems to be avoided in set-user-ID-root programs that allow unprivileged users to unmount filesystems.

RETURN VALUE

On success, zero is returned. On error, −1 is returned, and errno is set appropriately.

ERRORS

The error values given below result from filesystem type independent errors. Each filesystem type may have its own special errors and its own special behavior. See the Linux kernel source code for details.

EAGAIN

A call to umount2() specifying MNT_EXPIRE successfully marked an unbusy filesystem as expired.

EBUSY

target could not be unmounted because it is busy.

EFAULT

target points outside the user address space.

EINVAL

target is not a mount point.

EINVAL

umount2() was called with MNT_EXPIRE and either MNT_DETACH or MNT_FORCE.

EINVAL (since Linux 2.6.34)

umount2() was called with an invalid flag value in flags.

ENAMETOOLONG

A pathname was longer than MAXPATHLEN.

ENOENT

A pathname was empty or had a nonexistent component.

ENOMEM

The kernel could not allocate a free page to copy filenames or data into.

EPERM

The caller does not have the required privileges.

VERSIONS

MNT_DETACH and MNT_EXPIRE are available in glibc since version 2.11.

CONFORMING TO

These functions are Linux-specific and should not be used in programs intended to be portable.

NOTES

umount() and shared mount points

Shared mount points cause any mount activity on a mount point, including umount() operations, to be forwarded to every shared mount point in the peer group and every slave mount of that peer group. This means that umount() of any peer in a set of shared mounts will cause all of its peers to be unmounted and all of their slaves to be unmounted as well.

This propagation of unmount activity can be particularly surprising on systems where every mount point is shared by default. On such systems, recursively bind mounting the root directory of the filesystem onto a subdirectory and then later unmounting that subdirectory with MNT_DETACH will cause every mount in the mount namespace to be lazily unmounted.

To ensure umount() does not propagate in this fashion, the mount point may be remounted using a mount() call with a mount_flags argument that includes both MS_REC and MS_PRIVATE prior to umount() being called.

Historical details

The original umount() function was called as umount(device) and would return ENOTBLK when called with something other than a block device. In Linux 0.98p4, a call umount(dir) was added, in order to support anonymous devices. In Linux 2.3.99-pre7, the call umount(device) was removed, leaving only umount(dir) (since now devices can be mounted in more than one place, so specifying the device does not suffice).

SEE ALSO

mount(2), mount_namespaces(7), path_resolution(7), mount(8), umount(8)

COLOPHON

This page is part of release 4.16 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man−pages/.


  Copyright (C) 1993 Rickard E. Faith <faithcs.unc.edu>
and Copyright (C) 1994 Andries E. Brouwer <aebcwi.nl>
and Copyright (C) 2002, 2005 Michael Kerrisk <mtk.manpagesgmail.com>

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2008-10-06, mtk: Created this as a new page by splitting
    umount/umount2 material out of mount.2

Section (8) umount

Linux manual pages Section 8  

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 _zsingle_quotesz_busy_zsingle_quotesz_ - 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, but it may introduce another issues. See −−lazy description below.

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 _zsingle_quotesz_fakes_zsingle_quotesz_ 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).

Note that this option does not guarantee that umount command does not hang. It_zsingle_quotesz_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.

A system reboot would be expected in near future if you_zsingle_quotesz_re going to use this option for network filesystem or local filesystem with submounts. The recommended use-case for umount -l is to prevent hangs on shutdown due to an unreachable network share where a normal umount will hang due to a downed server or a network partition. Remounts of the share will not be possible.

−N, −−namespace ns

Perform umount in namespace specified by ns. ns is either PID of process running in that namespace or special file representing that namespace.

umount(8) switches to the namespace when it reads /etc/fstab, writes /etc/mtab (or writes to /run/mount) and calls umount(2) system call, otherwise it runs in the original namespace. It means that the target namespace does not have to contain any libraries or another requirements necessary to execute umount(2) command.

See namespaces(7) for more information.

−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.

−q, −−quiet

Suppress not mounted error messages.

−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. Note that 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_zsingle_quotesz_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] [−N namespace] [−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

SEE ALSO

umount(2), losetup(8), mount(8)

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


  Copyright (c) 1996 Andries Brouwer
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