Shutdown Pressing Power Button
First of all, make sure that
button module is loaded (check the output of
lsmod). If it is not, load it manually and set it to load at boot.
logind handles the event from pressing the power key, and the behaviour can be configured in
/etc/systemd/logind.conf. See https://wiki.archlinux.org/index.php/Systemd#ACPI_power_management for more information.