How to get a specific PID #

[Ralf Mardorf]

>xkill

If the OP wants to know what PID belongs to what window, then wmctrl is
the way to go. There are exceptions, when wmctrl does not provide the
PID, but usually it does provide it. However, I doubt that xkill would
work, if a program doesn't provide a real window anymore, but wmctrl
usually allows to at least determine the PID of the instances that
still work as expected, so the OP just needs to kill the remaining PID
by htop, kill or another command line tool.