Title: How to trigger a command on Linux when disconnected from power

	Title: How to trigger a command on Linux when disconnected from power
	Author: Solène
	Date: 31 May 2025
	Tags: security linux
	Description: In this blog post, you will learn how to trigger a command
	when a laptop is disconnected from power supply

	# Introduction

	After thinking about BusKill product that triggers a command once the
	USB cord disconnects, I have been thinking at a simple alternative.

	BusKill official project website

	When using a laptop connected to power most of the time, you may want
	it to power off once it gets disconnected, this can be really useful if
	you use it in a public area like a bar or a train. The idea is to
	protect the laptop if it gets stolen while in use and unlocked.

	Here is how to proceed on Linux, using a trigger on an udev rule
	looking for a change in the power_supply subsystem.

	For OpenBSD users, it is possible to use apmd as I explained in this
	article:

	=> Rarely known OpenBSD features: apmd daemon hooks

	In the example, the script will just power off the machine, it is up to
	you to do whatever you want like destroy the LUKS master key or trigger
	the coffee machine :D

	# Setup

	Create a file `/etc/udev/rules.d/disconnect.rules`, you can name it how
	you want as long as it ends with `.rules`:

	```
	SUBSYSTEM=="power_supply", ENV{POWER_SUPPLY_ONLINE}=="0", ENV{POWER_SUPPLY_TYPE…
	```

	Create a file `/usr/local/bin/power_supply_off` that will be executed
	when you unplug the laptop:

	```
	#!/bin/sh
	echo "Going off because power supply got disconnected" \| systemd-cat
	systemctl poweroff
	```

	This simple script will add an entry in journald before triggering the
	system shutdown.

	Mark this script executable with:
	```
	chmod +x /usr/local/bin/power_supply_off
	```

	Reload udev rules using the following commands:

	```
	udevadm control --reload-rules
	udevadm trigger
	```

	# Testing

	If you unplug your laptop power, it should power off, you should find
	an entry in the logs.

	If nothing happens, looks at systemd logs to see if something is wrong
	in udev, like a syntax error in the file you created or an incorrect
	path for the script.

	# Script ideas

	Depending on your needs, here is a list of actions the script could do,
	from gentle to hardcore:

	* Lock user sessions
	* Hibernate
	* Proper shutdown
	* Instant power off (through sysrq)
	* Destroy LUKS master key to make LUKS volume unrecoverable + Instant
	power off

	# Conclusion

	While BusKill is an effective / unusual product that is certainly
	useful for a niche, protecting a running laptop against thieves is an
	extra layer when being outside.

	Obviously, this use case works only when the laptop is connected to
	power.