Mine

#!/bin/bash

last_state=`cat /proc/acpi/battery/BAT1/state`
echo "current_state: $last_state"

while true
do
state=`cat /proc/acpi/battery/BAT1/state`
if [[ "$state" == "$last_state" ]];
then
    echo "state not changed don't do anything"
else
    echo "state changed to $last_state , do something"
    
    last_state=`echo $state`
fi

sleep 5
done