static void unix_killProcesses(Collection<Int> pids, O... _) {
  if (empty(pids)) ret;
  L<S> pids2 = allToString(pids);
  S s = joinWithSpaces(pids2);
  print("Killing processes: " + s);
  Int level = cast optPar level(_);
  backtick("kill " + (level != null ? "-" + level + " " : "") + s);
}