How do you restart from a terminal in Linux?

The "reboot" command or "init 6" or (On systemd machines.) "systemctl reboot."

Please note that to restart from a terminal you must have some way of using root permissions, either you're a sudoer or you know root's password and can su into root or log in as root directly.

Normal users actually do not, in Linux, have the permissions to affect runlevels, which includes shutting down or rebooting the computer. The reason a normal user can do this from a desktop is through the use of PolicyKit or systemd's logind infrastructure.