Linux / X-Windows: Which process owns this window?

Once in a while, there’s a piece of junk on the desktop, and the question is who should be blamed for it.

The short answer is:

$ xwininfo

and fetch the window’s ID from the line at the beginning saying e.g.

xwininfo: Window id: 0x860000a "xclock"

And next, fetch the alleged process ID:

$ xprop -id 0x860000a | grep _NET_WM_PID
_NET_WM_PID(CARDINAL) = 58637

Note that it’s the duty of the program that generated the window to set this value correctly, so it may be absent or wrong. If the window belongs to a client on another machine, this process ID might be misleading, as it’s on the process table of the client (check out WM_CLIENT_MACHINE, also given by xprop).

If _NET_WM_PID isn’t helpful, try to look for other hints in xprop’s answer, or the correct process can be found with the rather complicated method described on this page.