umount

Unmount a filesystem

Description

The umount command is used to unmount a filesystem that has already been mounted. You can unmount a filesystem using either its device name or its mount point; however, unmounting by mount point is recommended to avoid confusion when using bind mounts (where one device has multiple mount points).

Syntax

umount [options] [parameters]

Options

-a: Unmount all filesystems listed in /etc/mtab;
-h: Display help;
-n: Do not record the unmounting in /etc/mtab;
-r: If unmounting fails, attempt to remount the filesystem as read-only;
-t <fstype>: Unmount only the filesystems of the specified type;
-v: Verbose mode, displaying detailed information during execution;
-V: Display version information.

Parameters

Filesystem: Specifies the filesystem to be unmounted or its corresponding device filename.

Examples

The following two commands unmount a filesystem using the device name and the mount point, respectively, while providing detailed output:

Unmount by device name:

umount -v /dev/sda1
/dev/sda1 unmounted

Unmount by mount point:

umount -v /mnt/mymount/
/tmp/diskboot.img unmounted

If the device is busy, the unmounting will fail. A common reason for failure is an open shell whose current directory is within the mount point:

umount -v /mnt/mymount/
umount: /mnt/mymount: device is busy
umount: /mnt/mymount: device is busy

Sometimes it's difficult to find the reason why a device is busy. In such cases, you can use lsof to list open files and search the list for the mount point you're trying to unmount:

lsof | grep mymount         # Find open files in the mymount partition
bash   9341  francois  cwd   DIR   8,1   1024    2 /mnt/mymount

The output above shows that the mymount partition cannot be unmounted because a bash process with PID 9341, run by user francois, is using it.

Another way to handle a busy system file is to perform a lazy unmount:

umount -vl /mnt/mymount/     # Perform lazy unmount

A lazy unmount immediately detaches the filesystem from the directory tree and cleans up all associated resources as soon as the device is no longer busy. To unmount and eject removable storage media, use the eject command:

eject /dev/cdrom      # Unmount and eject CD