xkill -version (return code: 0)
xkill 1.0.4
xkill --help (return code: 1)
usage: xkill [-option ...]
where options include:
-display displayname X server to contact
-id resource resource whose client is to be killed
-frame don't ignore window manager frames
-button number specific button to be pressed to select window
-all kill all clients with top level windows
-version print version and exit