Ubuntu :: How To Tell What Shell I'm Using

I wrote a script a while back that is supposed to be run in bash. It includes a hashpling (#!/bin/bash). Some people who are using the script are having problems with it however because they are forcing it to run in sh. They are doing things like:

Code:

sh script.sh

and

Code:

cat script.sh | sh

which ignores the hashpling and runs the script in sh, making the script fail half way through. How do I make my script determine the shell it's being run in and stop running if it's in /bin/sh?

PS: No, it's not

Code:

echo $SHELL

which has nothing to do with the shell you are currently in. No it's not

Code:

echo $0

which can be used in an interactive shell but does not work the same way in a script. No it's not

Code:

ps -p $$

as this will not work inside of a script the same way as it does in an interactive shell either. Those are the 3 methods that people keep describing when searching for the answer on google, and none of those works for what I need.