import org.apache.tools.ant.taskdefs.condition.Os
String cmdLine(String cmd, Writer out = new StringWriter(), Writer err = new StringWriter()) {
  (Os.isFamily(Os.FAMILY_WINDOWS) ? "cmd /c ${cmd}" : cmd).execute().waitForProcessOutput(out, err)
  assert err.toString().trim().size() == 0: "$err"
  out.toString()
}

task hello << {
  cmdLine("echo hello")
}