root@ubuntu:/# exit
exit
ubuntu@ubuntu:~$ sudo umount /mnt/dev
ubuntu@ubuntu:~$ sudo umount /mnt/proc
ubuntu@ubuntu:~$ sudo umount /mnt/
umount: /mnt: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
ubuntu@ubuntu:~$ sudo umount /mnt/sys
ubuntu@ubuntu:~$ sudo umount /mnt/
umount: /mnt: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
ubuntu@ubuntu:~$