Make scripts POSIX compliant

[saccarosium]

There is no reason to prefer /bin/bash when none of its unique features are used. Using /bin/sh lets you run the script even on those system without bash installed (e.g. Alpine Linux, BSD etc)

Also note that using the keyword function for function declaration is not standard and you should prefer declare functions by name().

[robherley]

Thanks for POSIX-ing!