, but basically that is not a good method, if your that external file is changeable by every other users. Then you have very big problem on hand(`rm -rf /` is also possible there, how you would check it?).
I think your requirement can be solved easily by adopting some other methods, not the one which you are talking about.
Vivek
-- In accordance with the prarabdha of each, the One whose function it is to ordain makes each to act. What will not happen will never happen, whatever effort one may put forth. And what will happen will not fail to happen, however much one may seek to prevent it. This is certain. The part of wisdom therefore is to stay quiet.