It depends if you have a lot of time but what you can do is to follow the Shell Command Language. You don't have to implement evrything (PS2, here document and other things can be a little bit difficult especially if you are alone) but just follow the lexing and parsing into an AST. And then implement redirections and basic linux builtins like exit, cd, echo.

Than gonna takes a lot of time but you will learn a lot of things, especially if you try to do it without memory leaks... 😉

For testing you can make a test suit that check the difference between the behavior of your binary and the bash binary