Who

The command who typed at the command line in a terminal will show a list of people logged in at the moment. The format of this list is:


 * Login name,
 * Which terminal they are logged in to,
 * And at what date and time they logged in.

who -r will show you the current run level

Example
Mostly, you will find some unexplainable logins on your system, like the following if you are logged in once: $ who root    :0           Dec  3 11:31 root    pts/0        Dec  3 11:32 root    pts/1        Dec  3 11:36 $ These login needn't be hackers hijacking your computer. Find it out with who -Hi: $ who -Hi who: Warning: -i will be removed in a future release;  use -u instead NAME    LINE         TIME         IDLE          PID COMMENT root    :0           Dec  3 11:31   ? 3268 root    pts/0        Dec  3 11:32 00:13        3690 root    pts/1        Dec  3 11:36. 4320 $ ps -A | grep 3268 3268 ?       00:00:00 startkde $ ps -A | grep 3690 3690 ?       00:00:03 kded $ ps -A | grep 4320 4320 pts/1   00:00:00 bash In this case, all root logins can be explained by KDE and the bash. So, there is no hacker on your system.

More info
You can try a special variant of the command that produces different output on some distributions. $ who am i

Kick out users
Now we come to the fun part - kicking out users. For that, we just kill their login processes. In the following example, the user kicks out himself: $ who -Hi who: Warning: -i will be removed in a future release;  use -u instead NAME    LINE         TIME         IDLE          PID COMMENT root    pts/0        Nov 26 10:47. 19483 (p54a951f3.dip.t-dialin.net) root    pts/1        Nov 26 10:44   ? 1957 (p54a951f3.dip.t-dialin.net) $ kill -9 19483 Connection to myvps closed. scorpio:~ #

Provided by
Most (all?) Linux distributions incorporate this from the GNU Coreutils: and use its man page

Related Commands
All of these relate to user information.
 * id - dump UID and GID information.
 * logname - show the login name.
 * whoami - show effective user ID.
 * groups - show groups of the current user.
 * users - show who is logged in.