Detecting when an X Window has been closed

Question

I'm running Ubuntu 20.04.5 LTS with the XFCE4 window manager.

I'm looking for a way for a shell script to be fired off when a particular X Window has been closed.

I know that I can create a program to run wmctrl or xdotool over and over again in a loop that checks for the existence of the X Window in question, and then executes a shell script when the presence of that X Window is no longer detected.

But I'm wondering whether there might be some sort of event-based utility that I can run which can do some or all of this for me, instead of my having to write this busy-wait-based program.

Any suggestions?

Thank you in advance.

Answer 1

A simple way to do this is with the xprop standard X11 utility. You provide a means of identifying the window, by window id, window name, or interactively by clicking on it, and it shows the X11 properties of that window. Adding the option -spy will make it loop waiting for changed-property events, which it will display. This is not a busy loop, but uses the standard X11 mechanism to wait for asked-for events. When the window dies, the event queue gets closed, and xprop ends cleanly. For example,

xterm -title mytest -e 'sleep 5' &
sleep 1
xprop -spy -name mytest
echo "rc=$? xterm done"

Typically, only a few properties ever change, such as when the window is iconised.