The problem is simple, but there is no explicit “method” when we are in front of a console, over SSH for example.

You have to use the basic functions of Linux, the same way to power off a PC under any of distributions :

# halt

… or :

# shutdown -s now

Wait until only one red LED still light on, and unplug the Raspberry’s electrical supply.

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On ArchLinux with Systemd, you can run:

# systemctl halt