https://dataswamp.org/~solene/2025-05-31-linux-killswitch-on-power-disconnect.html

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About me: My name is Solene Rapenne, pronouns she/her. I like
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How to trigger a command on Linux when disconnected from power

Written by Solene, on 31 May 2025.
Tags: #security #linux

Comments on Fediverse/Mastodon

Table of contents

  * 1. Introduction
  * 2. Setup
  * 3. Testing
  * 4. Script ideas
  * 5. Conclusion

1. Introduction SS

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

2. Setup SS

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}=="Mains", RUN+="/usr/local/bin/power_supply_off"

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

3. Testing SS

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.

4. Script ideas SS

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

5. Conclusion SS

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.

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