The command who will return a list of all the accounts currently logged into the system, the terminal device being used, and the time of login. With the fuser command we might learn that fred is the account using the /cdrom path. The who command would then tell us fred is logged into terminal ttyS3. You can then send fred a message by typing the following:
write fred ttyS3
How long do you need to use the /cdrom mount point?
I would like to mount a new CD.
<ctrl-D>
When the first line is entered, something like the following is printed at Fred's terminal:
Message from root@your.machine.net on ttyp1 at 14:52
How long do you need to use the /cdrom mount point?
I would like to mount a new CD.
Fred may then send a message back telling of his intention to comply.