Make shell scripts POSIX-compliant

[j8r]

Purpose

Fixes #878
It converts bash scripts to POSIX compliant ones, in order to run them in any other OS without bash like Alpine (even if we can install it).

Description

Bashisms were removed or converted to POSIX-compliant syntax.
I have also added minor improvements, mainly stylistic (but not opinionated). I can explain if there are questions about them.

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test

[edwardloveall]

@j8r Thanks for trying to make this more accessible! I'm not s POSIX expert and had a couple of questions that I was hoping you could answer. In general, I approve of this change, I'm just mostly surprised that the set / pwd syntax needed to change.

[edwardloveall]

Thanks!