I need to thank one of my friends for giving me one of his old laptops. We are now using it as a self-hosting server.

However, there is a problem around it: the charging port of the laptop is a little bit broken, which makes the connection to the charger loose. One day, when I accidentally touched the laptop, the charger disconnected from the laptop, and the laptop started discharging. I was not aware of that until I found my services stopped working because the laptop shut down after some time.

Because I may run some important tasks or services on this laptop, I need a way to get aware when the laptop starts discharging.

I found the way when I saw ntfy. It is a self-hostable pub-sub notification service. All I need to do is to host ntfy on my server and let my laptop publish a notification on it whenever it starts discharging.

To make my laptop run commands automatically when it switches the power source (AC or battery), I can write udev rules. For example, to run touch /tmp/discharging when the laptop starts discharging, I can write the following rule (write it to a file in /etc/udev/rules.d/ and run udevadm control --reload-rules to apply it):

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SUBSYSTEM=="power_supply", ATTR{online}=="0", RUN+="/usr/bin/touch /tmp/discharging"

However, I cannot just replace the touch command here with a ntfy command or a curl command because udev cannot run commands with access to the internet. Actually, we can find in systemd-udevd.service:

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[Unit]
IPAddressDeny=any

If you write any commands that try to access the internet in the udev rule, you will get errors of being unable to resolve hosts or connection time out.

Nevertheless, udev can trigger systemd services that can have access to the network. Write a systemd service file as such, and name it curl-ntfy.service for example:

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[Unit]
Description=Notify about discharging
After=network.target

[Service]
Type=oneshot
ExecStart=/opt/curl-ntfy/curl-ntfy

Here, /opt/curl-ntfy/curl-ntfy is the script that sends a notification to ntfy. The contents of the script are: 

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#!/usr/bin/env ruby
require 'yaml'
require 'uri'
require 'net/http'

CONFIG_FILENAME = ARGV[0] || File.join(__dir__, 'config.yml')
CONFIG = YAML.load_file CONFIG_FILENAME

PROTOCOL = CONFIG['use_https'] ? 'https' : 'http'
HOST = CONFIG['host']
TOPIC = CONFIG['topic']
HEADERS = CONFIG['headers']
CONTENTS = CONFIG['contents']

uri = URI "#{PROTOCOL}://#{HOST}/#{TOPIC}"
res = Net::HTTP.post uri, CONTENTS, HEADERS
# p res
# puts res.body

This is a Ruby script that basically reads the configuration from config.yml and sends a POST request to the ntfy server according to the configurations. The config.yml is, for example:

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host: ntfy.sh
topic: discharge-warning
use_https: true
headers:
  Title: Discharge warning
  Priority: urgent
contents: Your laptop is discharging

Finally, write a udev rule to make the systemd service triggered whenever the laptop starts discharging:

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SUBSYSTEM=="power_supply", ATTR{online}=="0", TAG+="systemd", ENV{SYSTEMD_WANTS}="curl-ntfy.service"

Hooray! Now, try unplugging the charger and see if you get a notification on your phone.