A while ago I wrote about getting notified on battery low. It is quite useful when you leave your laptop unattended for a while, to get a notification on your phone and/or smartwatch when that laptop is about to run out of juice. It has saved me a few hard shutdowns (or interrupted tasks because of forced hibernation).

The udev rule in that article should still work (if your device/battery emits those kinds of events at least), but for me the PushBullet method used there stopped working.

As I recently started using PushOver next to PushBullet, I decided to redo the little setup with a cronjob and a PushOver shell script. The cronjob works around the battery subsystem not emitting an event when you need it, the shell script has less dependencies than a Python script.

An example with the udev rule could look like this:

/etc/udev/rules.d/99-lowbat.rules

# Suspend the system when battery level drops to 5% or lower
SUBSYSTEM=="power_supply", ATTR{status}=="Discharging", ATTR{capacity}=="[0-5]", RUN+="/home/youruser/bin/powerlow_notification.sh"

(Rule based on a snippet found on this handy wikipage)

Myself, I wrote a cronjob to be run every 5 minutes (you can use whatever interval you prefer of course), which looks like this:

#!/bin/bash

BATTERYLEVEL=$(cat /sys/class/power_supply/BAT0/capacity)

if [ $BATTERYLEVEL -lt 15 ]; then
    /home/youruser/workspace/pushover.sh/pushover.sh -t "Power low on $HOSTNAME" "Battery power now is ${BATTERYLEVEL}%"
fi

This takes the current battery level from the udev system path, checks whether the value is less than 15 (percent) and sends a message through PushOver if it is. It will do so every 5 minutes until it's over 15 again, but that's fine with me (and a small price to pay on a system without the battery events). It might be made a bit more clever with keeping track of 'did I just send a message already' state files, but that exercise is left for the reader :)

Then, the crontab entry looks thusly:

# Check battery level every five minutes, PushOver message when below a certain percentage
*/5 * * * * /home/youruser/bin/cron/check_battery_level

N.B.: the little pushover.sh) script that I used, has a config file, located in ${HOME}/.config/pushover.conf, which takes your PushOver application token, your user token and optional CURL options:

TOKEN="your application's token here"
USER="your user/group key here"
CURL_OPTS="options to pass to curl"

Your user token is on the homepage of PushOver if you are logged in. The Application Token ('TOKEN') you can create by scrolling down that page and create a new application in Your Applications. There you can give it a nice icon and such too.