# Check for arrow keys
builtin echo "$NUM" | $GREP -q '^\[\[' && continue

++ builtin echo 6
++ grep -q '^\[\['
++ continue

		# Catch invalid inputs
			set -x
			# Keep reading if NUM is empty
			[[ -z "$NUM" ]] && continue
			# Also keep reading if NUM length = 0 ; is this even required?
			[[ ${#NUM} -eq 0 ]] && continue
			# Keep reading if NUM contains letters
			# Using builtin echo for speed and less disk usage
			builtin echo "$NUM" | $GREP -q [a-zA-Z]  && continue	# &2>/dev/zero
			# Check for arrow keys
			builtin echo "$NUM" | $GREP -q '^\[\[' && continue

[[ -z 5 ]]                                                                                              | #
++ [[ 1 -eq 0 ]]

++ builtin echo 5
++ grep -q '[a-zA-Z]'
++ builtin echo 5
++ grep -q '^\[\['
++ continue

[[ -z 4 ]]                                                                                              | #
++ [[ 1 -eq 0 ]]

++ builtin echo 4
++ grep -q '[a-zA-Z]'
++ continue