Lint script (shellcheck)

[JeanMax]

Hey!

It would be nice to clean the script, using shellcheck for instance. I'm kinda annoyed with all the backticks :D There are also a few other warnings that could be fixed, nothing serious though.

But in the long run, it would help catching mistake easily! On the other hand, this is going to create huge chunks of diffs, and it will be a pain for everybody trying to maintain a fork...

Let me know!