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.