Should use more portable shebang

Hey, hope you dont mind me adding this issue.

I pulled the code down onto my mac (running vanilla terminal with vanilla bash [5.0.18]) and the code would not work due to a bash substring issue on line 110, which was caused because my machine was running the script with bash due to the shebang not working for my machine.

So according to this link (https://stackoverflow.com/questions/10376206/what-is-the-preferred-bash-shebang) the preffered shebang for portability across machines/OS's/etc... is
```#!/usr/bin/env bash```

So I have created this ticket to change that and I hope its ok ill just quickly get on with it as well if thats alright?