How To Have A Program Return A Value

I am making a graphical shell and i have a log in function. I can use it as a function just fine but it brought up the question of could i have a log in program ran from the main program that returned if the log in was valid? i was wondering how to do this but i couldn't find a way to make programs return a value. is it possible to do something like shell.run("/returntest"); and have it return a value in the program returntest. (I know you cant use shell.run because that returns true or false depending on if it was successful) Is there a function to do this?

This is what APIs are for: Set the login program up as one, load it, call its functions, then unload it if you so choose.

Thats a good idea thank you!