how to reset dual display to one screen from command line?

Question

How can I reset dual display to one screen from command line and set old second screen as main display? (12.04 + nvidia)

Answer 1

In 18.04 and earlier I would recommend installing disper, a command line display manager.

Then you can do disper -S to make secondary only, or disper -e -t left to extend to the left for example.