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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.
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