Close Idle Tmux Panes

I have a two-line prompt, the second line of which is $, so:

for p in $(tmux list-panes -a -F '#{pane_id}'); do
  if [[ "$(tmux capture-pane -t $p -p -S $(tmux display-message -t $p -p '#{cursor_y}') -E $(tmux display-message -t $p -p '#{cursor_y}'))" =~ ^\$\ *$ ]]; then
		tmux send-keys -t $p "exit" Enter
	fi
done