Apply suggested fixes from ShellCheck in sh scripts

[ishanjalan]

https://github.com/koalaman/shellcheck/

[Pathoschild]

List of included fixes per discussion with @ishanjalan:

level	suggested fix
error	Argument mixes string and array. Use * or separate argument.
error	Double quote array expansions to avoid re-splitting elements.
warn	Declare and assign separately to avoid masking return values.
warn	Use ... || exit or ... || return in case command fails.
warn	Quote to prevent word splitting.
hint	read without -r will mangle backslashes.
hint	Use $(...) notation instead of legacy backticked `...`.
hint	Double quote to prevent globbing and word splitting.
hint	Use ./*glob* or -- *glob* so names with dashes won't become options.

[Pathoschild]

@ishanjalan I'll close this PR since it changes too many different things at once, and it still has some unresolved comments. Feel free to submit smaller PRs that change specific things so they can be discussed separately. Sorry for the inconvenience!