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A *nix command usually found in /usr/sbin/ that allows a privileged user to change system state.

The syntax is:
shutdown -what when message

`what' is usually h (for halt) or r (for reboot) and when is a number of minutes. The message is printed to every system console before it activates.

shutdown -h now prepares the system for power off instantly.
shutdown -r 20 Rebooting at 5pm will reboot the computer 20 minutes later and display the message on everybody's screen.