def get_presig(self, target, source, env, executor=None):
        """Return the signature contents of this action's command line.

        This strips $(-$) and everything in between the string,
        since those parts don't affect signatures.
        """
        from SCons.Subst import SUBST_SIG
        cmd = self.cmd_list
        if is_List(cmd):
            cmd = ' '.join(map(str, cmd))
        else:
            cmd = str(cmd)
        if executor:
            return env.subst_target_source(cmd, SUBST_SIG, executor=executor)
        else:
            return env.subst_target_source(cmd, SUBST_SIG, target, source)