Check active window every second and run script when window class changed

.bashrc

# ....

# Run script if not running.
if ! pidof -x "catch_window_change.sh" > /dev/null; then
    (~/bin/catch_window_change.sh  > /dev/null 2>&1 &)
fi

catch_window_change.sh

#!/bin/bash

LAST_WIN=''

while true; do
        WIN_CLASS=$(xprop -id $(xprop -root _NET_ACTIVE_WINDOW | cut -d ' ' -f 5) WM_CLASS | cut -d "," -f2 | sed 's/\"//g') 
        if [ -z $LAST_WIN ]; then
                LAST_WIN=$WIN_CLASS
        fi

        if [ "$WIN_CLASS" != "$LAST_WIN" ]; then 
                # Window changed
                echo 'Window changed'
                LAST_WIN=$WIN_CLASS
        fi
        sleep 1;
done