How do I shut down or reboot from a terminal?

For shutdown:

sudo poweroff

For restart:

sudo reboot

Appendix: If your keyword is "lockup" so you can't enter a command like "reboot" which would run from "su" anyway, use the keyboard: hold down Alt + PrintScreen/SysRq, buttons and type "REISUB". It doesn't have to be capital letters. It will restart your computer gently. http://blog.kember.net/articles/reisub-the-gentle-linux-restart/

Open your terminal with CTRL+ALT+T and do these following commands

To shutdown the system:

sudo shutdown -h now 

To restart:

sudo reboot

& one more command for restart:

sudo shutdown -r now

Another way as one of the user mentioned.

For shutdown:

sudo halt

or:

sudo init 0 

For restart:

sudo init 6

You can get more info on the shutdown command by using one of the following:

• shutdown --help
• man shutdown

Hate passwords (sudo) and love one-liners?

For Ubuntu 15.04 and later

This is due to Ubuntu's shift in using systemd instead of Upstart

systemctl poweroff
systemctl reboot
systemctl suspend
systemctl hibernate
systemctl hybrid-sleep

Since hibernate is normally disabled by default in Ubuntu systems, you can enable this by checking this answer.

For Ubuntu 14.10 or earlier

Shutdown:

/usr/bin/dbus-send --system --print-reply --dest="org.freedesktop.ConsoleKit" /org/freedesktop/ConsoleKit/Manager org.freedesktop.ConsoleKit.Manager.Stop

Restart:

/usr/bin/dbus-send --system --print-reply --dest="org.freedesktop.ConsoleKit" /org/freedesktop/ConsoleKit/Manager org.freedesktop.ConsoleKit.Manager.Restart

Other commands you may like:

Suspend:

/usr/bin/dbus-send --system --print-reply --dest="org.freedesktop.UPower" /org/freedesktop/UPower org.freedesktop.UPower.Suspend

Hibernate: (if enabled on your system)

/usr/bin/dbus-send --system --print-reply --dest="org.freedesktop.UPower" /org/freedesktop/UPower org.freedesktop.UPower.Hibernate