We can use the last command to find out who logged in to our computer, when, and in some cases over the network, where (IP addresses). You can also see if the are still logged in.

last on an Ubuntu system (vortex) `last` on an Ubuntu system (vortex)

last on an Linux Academy lab server `last` on an Linux Academy lab server

last on Mac OS X El Capitan `last` on Mac OS X El Capitan