I'm new to this and didn't know what my problem is called but here it is:

A program called "prepdata" is run which asks the user to enter in a <input> file for the data to be taken from. However, it won't accept that input filename as an argument:
$ prepdata <input> will NOT work

prepdata reads it from the user after prepdata is already running:
$ prepdata
Enter an input file:
<input>

$

I'd rather not attempt to edit the prepdata program for it to accept <input> as an argument. How can I get a script to "type" in the <input> filename when prepdata asks for it?
It's like I need the script to send raw keyboard input after a certain amount of time but I have no idea how to do that.

Pipe the data as stdin

or

Bah!
I looked at piping hours ago. Obviously I had it backwards.
Sorry for the trivial question, and thanks for the quick response.