magic numbers? (teach me, please)

[K. Richard Magill]

Two questions. I presume they are related. Answers or pointers to doc would
be appreciated.

	1)  How does the shell (exec?) know whether the command I just typed
		is a shell script or one of several possible types of
		executable?

	2)  Presuming the answer to #1 above has something to do with
		magic numbers, who issues them?  is there a common
		(definitive) base of them or does each
		manufacturer/environment make up their own set?

K. Richard Magill
(someplace between an advanced user & a guru)