Shell Scripting problem

So I am new to this shell scripting stuff and I got a task set to me to create a shell script which takes in two files, and compares them with each other (similar to the diff command) except you cannot use diff.

I am completely stuck and have no idea how to even attempt this problem.

EDIT:开发者_如何学C sorry, I can only use nl, join and sort

---

use comm

comm compares 2 files line by line and supports different output schemes (only uniques to a file, only common lines, etc)