xpid: show pid of program that created an X11 window

xpid

#!/bin/bash
# https://www.linuxquestions.org/questions/linux-software-2/something-like-xkill-but-just-output-the-pid-or-resource-id-don%27t-kill-595638/#post2941613

wid=$(xwininfo  |grep 'Window id:' |cut -d" " -f4)
pid=$(xprop -id "$wid" |grep "^_NET_WM_PID" | cut -d" " -f3)

echo $pid |grep '^ *[0-9][0-9]* *$' && exit 0

cat <<EOF 1>&2
xpid ERROR: can't get PID for selected window.
            window ID:   $wid
            pid: $pid
EOF
exit 1