Script that reads input from terminal. How to tell the script to print to both tty and stdout in case it is called with a stdout redirection?

I have a script which asks users for input in the terminal, therefore in case the script is called with stdout redirected to a file, the user has no context on the prompt. So I would like to duplicate stdout to both sites. The script outputs a calculus so it still makes sense to allow stdout redirection. But if stdin is provided (kind of yes command or non-interactive context like a cron), it makes sense the output IS perfectly allowed to not being required in the tty. So I would like to do some kind of logic like: If (test -t 0 && ! test -t 1); // Tell stdout to print everything to tty as well Is this possible? Or is it fundamentally inappropriate?