About Restart and Shutdown to arduino

How Can I restart and shut down to Arduino with a button?

Thanks for now.

Yes, one between the powersupply and the Arduino.

Optionally you could have one button that starts an Arduino which in turn starts a relay between power and itself. If you press another button the Arduino can shut off the relay and itself. Then the code decides when it stops e.g. not in the middle of a log statement and it can close all files to SD etc.