How do I restart Linux (Ubuntu) from the command-line?

There's a few ways:

sudo reboot

sudo init 6

sudo shutdown -r now

If you've got freedesktop-compliant session manager, you can use DBus to invoke restart from inside the X session. The command goes:

dbus-send --system --dest=org.freedesktop.Hal --type=method_call \
    --print-reply /org/freedesktop/Hal/devices/computer \
    org.freedesktop.Hal.Device.SystemPowerManagement.Reboot int32:0

(this is probably more than necessary; works for me). I use this in a shell script. You don't need to run this from root, but you need to run it from inside an X session (f.e. in a terminal). You can find more on this topic at https://wiki.ubuntu.com/DebuggingGNOMEPowerManager

I found this page on computerhope.com:

The command is reboot. There is also halt, poweroff & shutdown.

nothing wrong with any of the above, but you can also install gnome-do and enable the session management plugin, which lets you easily shutdown,reboot,suspend from gnome-do