Pass ShellCheck
Pass ShellCheck

ref #4

Silence all warnings produced by ShellCheck 0.4.5:

- SC2046
- SC2048
- SC2086
- SC2181 (new to 0.4.5)

Some quoting issues can be solved by replacing [ with the built-in [[.
That's more idiomatic than simply quoting.

DanskSupermarked/nvi