From Linux Man Pages
umount - unmount file systems
SYNOPSIS
umount [-hV]
umount -a [-dflnrv] [-t vfstype] [-O options]
umount [-dflnrv] dir | device [...]
DESCRIPTION
The umount command detaches the file system(s) mentioned 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work, but is obsolete, mainly because it will fail in case this device was mounted on more than one directory.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 for the umount command:
-V Print version and exit.
-h Print help message and exit.
-v Verbose mode.
-n Unmount without writing in /etc/mtab.
-r In case unmounting fails, try to remount read-only.
-d In case the unmounted device was a loop device, also free this loop device.
-i Don't call the /sbin/umount.<filesystem> helper even if it exists. By default /sbin/umount.<filesystem>
helper is called if one exists.
-a All of the file systems described in /etc/mtab are unmounted. (With umount version 2.7 and later: the proc
filesystem is not unmounted.)
-t vfstype
Indicate that the actions should only be taken on file systems of the specified type. More than one type
may be specified in a comma separated list. The list of file system types can be prefixed with no to
specify the file system types on which no action should be taken.
-O options
Indicate that the actions should only be taken on file systems with the specified options in /etc/fstab.
More than one option type may be specified in a comma separated list. Each option can be prefixed with no
to specify options for which no action should be taken.
-f Force unmount (in case of an unreachable NFS system). (Requires kernel 2.1.116 or later.)
-l Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the
filesystem as soon as it is not busy anymore. (Requires kernel 2.4.11 or later.)
THE LOOP DEVICE
The umount command will free the loop device (if any) associated with the mount, in case it finds the option
`loop=...' in /etc/mtab, or when the -d option was given. Any pending loop devices can be freed using `losetup
-d', see losetup(8).
FILES
/etc/mtab table of mounted file systems
RELATED
umount(2), mount(8), losetup(8).
HISTORY
A umount command appeared in Version 6 AT&T UNIX.
CATEGORY