Execute a command using subprocess.Popen().

def _do_exec(self, cmd, args):
        """Execute a command using subprocess.Popen().
        """
        if not args:
            self.stderr.write("execute: empty command\n")
            return
        proc = subprocess.Popen(subprocess.list2cmdline(args),
                shell = True, stdout = self.stdout)
        proc.wait()