Fix warnings reported by shellcheck

SC2006: Use $(...) notation instead of legacy backticked `...`.
SC2039: In POSIX sh, echo flags are undefined.
SC2039: In POSIX sh, string replacement is undefined.
SC2048: Use "$@" (with quotes) to prevent whitespace problems.
SC2086: Double quote to prevent globbing and word splitting.
SC2129: Consider using { cmd1; cmd2; } >> file instead of individual redirects.
SC2166: Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.
SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.
SC2206: Quote to prevent word splitting, or split robustly with mapfile or read -a.
SC2230: which is non-standard. Use builtin 'command -v' instead.
SC2231: Quote expansions in this for loop glob to prevent wordsplitting, e.g. "$dir"/*.txt .
SC2234: Remove superfluous (..) around test command.
SC2241: The exit status can only be one integer 0-255. Use stdout for other data.