Bash script that notify if the battery level is too low. Initially made for a cronjob

batt_percentage.sh

#!/bin/bash

STATE=$(upower -i /org/freedesktop/UPower/devices/battery_BAT0 | head -n 12 | tail -n1 | awk '{print $2}')
if [ "$STATE" == "charging" ]; then
    exit 0
fi

PERCENT=$(upower -i /org/freedesktop/UPower/devices/battery_BAT0 | head -n 21 | tail -n1 | awk '{print $2}' | cut -d '%' -f1)
if [ $PERCENT -lt 20 ]; then
    export DISPLAY=:0
    /usr/bin/notify-send -u critical "Battery is low ($PERCENT)%"
fi

Corresponding cronjob:

*/2 * * * * /path/to/batt_percentage.sh

This will execute the script every 2 minutes.