Command
who returns the active users on the machine.
"d" is the username of a registered user on this machine whom password I know (my Mom actually

).
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[Vorteks@thenudiebar ~]$ who
Vorteks :0 2011-08-24 23:00
Vorteks pts/0 2011-08-24 23:00
Vorteks pts/1 2011-08-24 23:59
[Vorteks@thenudiebar ~]$ su
Password:
[root@thenudiebar Vorteks]# who
Vorteks :0 2011-08-24 23:00
Vorteks pts/0 2011-08-24 23:00
Vorteks pts/1 2011-08-24 23:59
[root@thenudiebar Vorteks]# exit
exit
[Vorteks@thenudiebar ~]$ su d
Password:
[d@thenudiebar Vorteks]$ who
Vorteks :0 2011-08-24 23:00
Vorteks pts/0 2011-08-24 23:00
Vorteks pts/1 2011-08-24 23:59
[d@thenudiebar v]$ exit
exit
[Vorteks@thenudiebar ~]$
[Vorteks@thenudiebar ~]$ su
-Password:
[root@thenudiebar
~]# who
Vorteks :0 2011-08-24 23:00
Vorteks pts/0 2011-08-24 23:00
Vorteks pts/1 2011-08-24 23:59
[root@thenudiebar
~]# exit
[Vorteks@thenudiebar ~]$
=========================