sS linux_switchToInternalMonitor_1() {
  S internal = linux_getInternalMonitorID();
  if (internal == null) ret "Unknown internal monitor ID";
  S external = linux_getExternalMonitorID();
  if (external == null)
    ret backtick_verbose("xrandr --output " + internal + " --auto");
  ret backtick_verbose("xrandr --output " + external + " --off --output " + internal + " --auto");
}

Began life as a copy of #1018311