However, note that this is rather rude if there are users on the system at the time (who may be quite angry with you). The shutdown command provides a method for changing runlevels in a way that treats users reasonably. Similarly to the kill command's ability to send a variety of signals to a process, shutdown can be used to halt, reboot, or change to single-user mode. For example, to change to single-user mode in 5 minutes:

# shutdown 5
Broadcast message from root (pts/2) (Tue Jan 15 19:40:02 2002):
The system is going DOWN to maintenance mode in 5 minutes!

If you press control-c at this point, you can cancel the pending switch to single-user mode. The message above would appear on all terminals on the system, so users have a reasonable amount of time to save their work and log off. (Some might argue whether or not 5 minutes is "reasonable.")