It can be useful to determine what processes or users are currently accessing certain files. Suppose, for example, you want to unmount a file system mounted at /mnt. umount returns "device is busy." The command fuser can then be used to determine what processes are accessing the device:

$ fuser -v /mnt/*

                     USER        PID ACCESS COMMAND
/mnt/notes.txt
                     jj        26597 f....  less

Following termination of the less process, which was running on another terminal, the file system can successfully be unmounted.