How does one exit the X server?
So I need to turn off Unity GUI and get back to the text user interface, how do I do that in Ubuntu 12.04 LTS?
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use Ctrl + Alt + F1 for text user interface
and When you want to back Unity GUI then use Ctrl + Alt + F7
If you're looking to edit your run levels, it's not too easy in Ubuntu, but here is a great reference page with all the information you need for 12.04 lts.
I've you're looking to save system resources for server running, or just want the pure CLI experience, you should switch to a much more open distro like Fedora, Debian, cENTos, or RHEL.
Or if you just want access to a command line, just use Terminal!