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Summary:
To find the PID of a running UNIX/XWindow process:
$ xprop _NET_WM_PID
If that fails:
$ ps -ww -fp <PID>
$ pwdx <PID>
Detailed explanation:
So How can you find the PID (not Window ID!) of a running UNIX/XWindow process?
Method 1 (simple):
$ xprop […]