There are quite a few ways on Linux to get a list of the users logged into the system and see what they are doing. The commands described in this article all provide very useful information.
The users command displays a simple list of logged-in users. In this example, one user is logged in twice and is, therefore, listed twice.
nemo popeye shs shs
Note that the users are listed in alphabetical order.
The who command provides additional information. The login terminal is identified along with the login date and time. The final field displays the terminal or the IP address of the connecting system.
Source:: Network World – Linux