GNU/Linux Command-Line Tools Summary
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Shutting Down/Rebooting the System

shutdown now

Shutdown the computer immediately (don't power down). Note that in UNIX systems this kind of shutdown means to go to “ single-user mode”. Single-user mode is a mode where only the administrator (root) has access to the computer, this mode is designed for maintenance and is often used for repairs.

For example this would take you to single user mode

shutdown now

shutdown -h now

Shutdown (-h = halt) the computer immediately. It begins the shutdown procedure, press CTRL-C (break-key) to stop it. After the end of the command you can also leave a message in quotation marks which will be broad-casted to all users, for example:

shutdown -h now "Warning system malfunction, self-destruct imminent"

This would halt the system and send the message to anyone who is currently logged in.

Shutting down at a particular time: You can also put a time that the system should shutdown instead of “now”. Typing “+x minutes” (any number of minutes is appropriate) or you can even set an exact time. For example to shutdown at 11:50 type:
shutdown -h 11:50

Shutdown -h vs poweroff: On some systems, shutdown -h and halt do not actually turn the system's power off. On systems that do not power off with these commands use the poweroff command

halt

The same as shutdown -h now doesn't take any options, this command simply shuts down immediately.

shutdown -r now

Shutdown ( -r = reboot) the computer immediately. It begins the reboot procedure, press CTRL-C (break-key) to stop it. After the end of the command you can also leave a message in quotation marks which will be broad-casted to all users, for example:

shutdown -r now "Warning system rebooting, all files will be destroyed"

This would reboot the system and send the message to anyone who was logged in.

Rebooting at a particular time: You can also put a time that the system should reboot instead of “now”. Typing “+x minutes” (any number of minutes is appropriate) or you can even set an exact time. For example to reboot at 11:50 type:
shutdown -r 11:50

reboot

The same as shutdown -r now, doesn't take any options, simply reboots the computer immediately.

CTRL-ALT-DEL

(key-combination) May be used from a terminal to reboot or shutdown, it depends on your system configuration. Note that this doesn't work from an xterminal. CTRL-ALT-DEL begins the reboot/shutdown immediately, the user does not have to be logged in.

You can change the behaviour of CTRL-ALT-DEL from rebooting: To disable CTRL-ALT-DEL from rebooting your computer (or to have it do something different), you can edit the /etc/inittab file (as root).
Here is how it looks on a Mandrake/Mandriva Linux system:
# Trap 
CTRL-ALT-DEL

ca::ctrlaltdel:/sbin/shutdown -t3 -r now 
Note that the # means a comment (and is not used). If you simply put a # (hash) before the command it would disable it (it would become a comment).
You could also change the command it runs for example if you changed the -r to a -h the computer would turn off instead of rebooting, or you could have it do anything you want. It's up to your creativity to make it do something interesting.