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The umount command is used to unlink a file system from the local host, usually a file system previously linked using the mount command.
Typically the umount command is used to release a CD or DVD-ROM drive so that it can be ejected, allow a USB memory key to be cleanly unmounted before unplugging it (Microsoft Windows has a similar function to safely remove devices) or any other file system linked to the local host.
An example pf the unmount command:
# umount /mnt/cdrom
This simply unlinks the CD-ROM drive (or rather the 'file system' stored on the physical media) from the local host.
The umount command will not unlink a file system that is in use. While this may sound obvious it can often catch-out even the best sysadmins. This could be caused by opening a file for editing or simply having the mount-point as the current directory, as per the following example:
# mount_cd9660 /dev/acd0 /mnt/cdrom/ # cd /mnt/cdrom/ # ls .cshrc INSTALL.HTM boot.catalog media sys .profile INSTALL.TXT cdrom.inf mnt tmp 6.3-RELEASE README.HTM dev packages tools COPYRIGHT README.TXT docbook.css proc usr ERRATA.HTM RELNOTES.HTM etc rescue var ERRATA.TXT RELNOTES.TXT floppies root HARDWARE.HTM bin lib sbin HARDWARE.TXT boot libexec stand # # umount /mnt/cdrom umount: unmount of /mnt/cdrom failed: Device busy
This example shows how a user could mount a CD-ROM file system and enter into the '/mnt/cdrom' mount-point before trying to unlink it. This creates an error noting the device (in this case, the CD-ROM drive) is busy. To resolve this problem the user would simply need to do the following:
# cd .. # umount /mnt/cdrom #
Which returns the user to the '/mnt' directory and releases the file system from use.
Other possible causes is mounting and entering into a linked file system as a general user and then using the su command to operate as the root user then, while as root, trying the umount command. The file system remains locked under the general user's session. In this case the user must exit out of the root user session and release the file system before trying again.
The fstat command can be used to identify open, or active, files on the locally attached file systems.