[Discuss] Please help with a BASH puzzle

[bill at horne.net (Bill Horne)]

Thanks for reading this: I appreciate your time.

I'm trying to do something that should be very easy to do, and yet I 
can't remember how to do it, and I'm asking for help.

I have an alpha-numeric, sorted file, that looks like this:

01AA

01AB

01AC

01AE

01AF

.. etc.

I'm trying to remember what BASH utility, script, or? command would flag 
the missing value (in this case, "01AD"). There are, of course, any 
number of ways to program a solution, but I can't remember which of the 
BASH utilities will do it. All suggestion welcome, and thanks in advance.

Bill Horne