Fix shellcheck warnings

What does this Merge Request do and why?

Address a number of shell scripts code quality raised via Code Quality (shellcheck) build.

Interestingly, the output of the warnings in Code Quality is different from running shellcheck build 🤔 We might need to investigate this in future MR.

• Before: Found 37 code quality issues
• After: Found 22 code quality issues

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.