sub invokeCmd {
  my ($cmd) = @_;

  # assuming full path for cmd, change path from
  # cmd area to results area
  my $curResultsFile=$cmd."res";
  # stat the file, check for existence, and 'last mod' value
  if ($too_old) {
    # re-run command and store results in $curResultsFile
  }
  my $res;
  open $res, $curResultsFile or die "could not open results: $!\n";
  return $res;
}