shutdown brings the system down in a secure way. All logged-in users
are notified that the system is going down, and login(1) is blocked.
It is possible to shut the system down immediately or after a specified
delay. All processes are first notified that the system is going down
by the signal SIGTERM. This gives programs like vi(1) the time to save
the file being edited, mail and news processing programs a chance to
exit cleanly, etc. shutdown does its job by signalling the init pro‐
cess, asking it to change the runlevel.