Sometimes, when an attempt is made to umount a device, it is still in use by another account. umount will report ``device busy'' under these circumstances. So, ``Who has it?'', is the question which must be answered before that account can be contacted and asked to relinquish the device. fuser is the answer to this question.
Assume that a CD-ROM device has been mounted and now the material has been transferred, or installed, and it is time to unmount the device, but the umount command fails because the device is still busy. Very often the System Administrator has forgotten that some virtual console has the current working path somewhere on the CD. When memory doesn't resolve the issue it is possibly some other process and user account that are still using the device. The command:
fuser -muv /cdrom
will list each account using the device mounted on /cdrom, along with the PID of the command that is currently executing in that device path. Once the account is identified, a message can be sent to the session indicating the System Administrator's desire to mount something else on that device.