I am logged into Ubuntu server via ssh and I would like to know if another user is logged in via SSH as well. Is there a command that I can run to accomplish this?
Just type who:
~$ who
(unknown) tty7 2013-12-06 13:37 (:0)
john pts/0 2013-12-08 00:16 (192.168.0.100)
jane pts/1 2013-12-08 00:17 (192.168.0.101)
Regarding your bonus question :)
To see login history for users you can use last -i. This will show all logins and IP addresses since start of current logfile /var/log/wtmp.
Depending on your logrotate you can show previous logs with last -f /var/log/wtmp.1, too.
To only show the very last login for each user use last -i | sort -r | uniq -w 16
~$ last -i | sort -r | uniq -w 20
wtmp begins Sun Dec 1 16:54:49 2013
john pts/0 192.168.0.100 Sun Dec 8 18:49 still logged in
jane pts/1 192.168.0.101 Sun Dec 8 00:17 - 00:34 (00:16)
