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When you press Control Alt Delete in a Terminal, your system reboots. This does not prompt for authentication to reboot. However, running sudo reboot does require authentication as a sudoer to execute. These commands appear to be doing the same thing. Why does one of them require superuser privileges, and the other does not?

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If logged in to a desktop you don't need su permissions either to reboot or halt the system. Even if not logged in at all you can reboot/halt from LightDM login screen. This is just a configuration which you you can customize to your needs. In the old days everything was controlled by root: no connection to a WiFi without root-password. Not convenient for users on a laptop. However, for a Ubuntu server installation it could be a problem if a normal user reboots/halts the system by accidentally pressing CTRL+ALT+DEL (most (Ex)Windows-users have an unintentional reflex for this combination). –  Andre Dec 11 '12 at 2:20
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up vote 5 down vote accepted

Ctrl-Alt-Del combination only works when you're at the physical keyboard connected to the machine (as opposed to being connected to a remote machine via an SSH session). So it only works if you have physical access to the machine. In this case, asking for a password makes no sense - you can just use Reset button or even unplug the machine from the wall socket or pour a cup of coffee inside it - all those actions will effectively shut it down without asking for a password.

You can disable this feature if you want: see Console Security in Ubuntu wiki

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Ah! Thanks for clarifying this for me. –  Aaron Dec 11 '12 at 2:24
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