Restrict access outside WordPress

Hello! I want to make a file that is located outside of my wordpress folder, but I still want that only registered users will be able to access it.

Something like:

..code here to check if user is registered and logged in...
...content that I want to show..
..else if user is not logged in or registered, redirect to main page...

what files or functions to i need to include to the file to make it work? Thank you!