xfocusnotify is an X11-tool which exits when a window is focused and prints the respective window-id. It's meant for scripting where for some reason you need to know when a new window gets focused and you want to do something with it. It follows EWMH-specifications and uses _NET_ACTIVE_WINDOW to determine the focused window.
sudo make install
The following shell-code prints the window-id each time a new window is focused:
while window=$(xfocusnotify); do
echo "focused $window"
done
If it doesn't report any focused windows it's probably because your window-manager doesn't set _NET_ACTIVE_WINDOW. Verify that it's set by running:
xprop -root
You should see the following line (the window-id might be different):
_NET_ACTIVE_WINDOW(WINDOW): window id # 0xe00005
If you don't see it then _NET_ACTIVE_WINDOW isn't set and you should ask the developers of your window-manager to update it whenever a new window is focused.