who command

who command gives the list of the users currently working on various terminals.
$who
vsc tty01 Dec 25 09:35
atmiya tty02 Dec 25 08:50
student tty03 Dec 25 09:45

$who –Hu

USER LINE LOGIN-TIME IDLE PID COMMENTS

vsc tty01 Dec 25 09:35 . 24
student tty02 Dec 25 09:40 0:30 25

-H is for header and –u option provides more detailed list
The idle column shows that student seems to be idling for last6 30 minutes. PID shows the process identification number.