scripts/bindhosts: validate input for custom_cron

This current code do not require bash (at least according to shellcheck) and the
simplistic checks seem to work as expected from my limited basic tests.

Basic standard expressions (e.g. * 12 * JAN 0)
Non-standard expressions (e.g. @daily, * * * * 7)
Complex expressions (e.g. 0,15,25 0-6 * * *, 0 0-2,2-24/4 * */2 *)
Expression logic?
	e.g. * * 31 FEB * should be invalid because Feb do not have day 31
	e.g. 1-59 * * * * is valid but not 59-1 * * * *

rebased from: https://github.com/backslashxx/bindhosts/pull/36

module/bindhosts.sh

@@ -80,11 +80,181 @@ run_crond() {
 	}
 }
 
+_month_txt2dec() {
+	local month="$1"
+	case "$(echo "$month" | tr "[:lower:]" "[:upper:]")" in
+		JAN) echo 1 ;;
+		FEB) echo 2 ;;
+		MAR) echo 3 ;;
+		APR) echo 4 ;;
+		MAY) echo 5 ;;
+		JUN) echo 6 ;;
+		JUL) echo 7 ;;
+		AUG) echo 8 ;;
+		SEP) echo 9 ;;
+		OCT) echo 10 ;;
+		NOV) echo 11 ;;
+		DEC) echo 12 ;;
+		*) echo $month ;; # Return unconverted if parameter is not JAN-DEC
+	esac
+}
+
+_day_txt2dec() {
+	local day="$1"
+	case "$(echo "$day" | tr "[:lower:]" "[:upper:]")" in
+		SUN) echo 0 ;;
+		MON) echo 1 ;;
+		TUE) echo 2 ;;
+		WED) echo 3 ;;
+		THU) echo 4 ;;
+		FRI) echo 5 ;;
+		SAT) echo 6 ;;
+		*) echo $day ;; # Return unconverted if parameter is not SUN-SAT
+	esac
+}
+
+_is_valid_cron_arg() { # return value: 0 = true, 1 = false
+(
+	x="$1"
+	valid_cron_arg="$2"
+	len=${#valid_cron_arg}
+	complex_check=false
+	[ "$len" -lt 1 ] && return 1 # End check if arg is empty
+	if [ "$len" -eq 1 ]; then
+		# End check if 1-char arg is not * or digit
+		[ "$(echo "$valid_cron_arg" | tr -d "\*[:digit:]" | wc -w)" -gt 0 ] && return 1
+		# Additional check if arg is a number
+		if (echo "$valid_cron_arg" | grep -q "[[:digit:]]"); then
+			case "$x" in
+				3) [ "$valid_cron_arg" -lt 1 ] && return 1 ;;
+				4) [ "$valid_cron_arg" -lt 1 ] && return 1 ;;
+				5) [ "$valid_cron_arg" -lt 0 ] || [ "$valid_cron_arg" -gt 7 ] && return 1 ;;
+			esac
+		fi
+	elif [ "$len" -eq 2 ]; then
+		# End check if 2-char arg is not a number
+		[ "$(echo "$valid_cron_arg" | tr -d "[:digit:]" | wc -w)" -gt 0 ] && return 1
+		case "$x" in
+			1) [ "$valid_cron_arg" -lt 0 ] || [ "$valid_cron_arg" -gt 59 ] && return 1 ;;
+			2) [ "$valid_cron_arg" -lt 0 ] || [ "$valid_cron_arg" -gt 23 ] && return 1 ;;
+			3) [ "$valid_cron_arg" -lt 1 ] || [ "$valid_cron_arg" -gt 31 ] && return 1 ;;
+			4) [ "$valid_cron_arg" -lt 1 ] || [ "$valid_cron_arg" -gt 12 ] && return 1 ;;
+			*) return 1 ;;
+		esac
+	elif [ "$len" -gt 2 ]; then
+		# Because the arg might be "1-6", "0,15,25" or even "1-7,7-21/3"
+		# We'll need extra check for these long arg (>= 3 char) if the arg is:
+		# - of the MONTH field but is not in JAN-DEC list
+		# - of the DAY_OF_WEEK field but is not in SUN-SAT list
+		# - of the MIN, HOUR or DAY_OF_MONTH fields
+		case "$x" in
+			4) (! echo "$valid_cron_arg" | grep -Eiqw "^(JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)$") && complex_check=true ;;
+			5) (! echo "$valid_cron_arg" | grep -Eiqw "^(SUN|MON|TUE|WED|THU|FRI|SAT)$") && complex_check=true ;;
+			*) complex_check=true ;;
+		esac
+	fi
+
+	if [ "$complex_check" = true ]; then
+		# End check if long arg (>= 3 char) is a number
+		[ "$(echo "$valid_cron_arg" | tr -d "[:digit:]" | wc -w)" -eq 0 ] && return 1
+
+		delimiter=""
+		# To split complex arg in the order from , > / > -
+		if (echo "$valid_cron_arg" | grep -q "\," > /dev/null 2>&1 ); then
+			delimiter=","
+		elif (echo "$valid_cron_arg" | grep -q "\/" > /dev/null 2>&1 ); then
+			delimiter="/"
+		elif (echo "$valid_cron_arg" | grep -q "\-" > /dev/null 2>&1 ); then
+			delimiter="-"
+		else
+			# End check if arg does not contain , / or -
+			return 1
+		fi
+
+		substring=""
+		prev_substring=0
+		while [ "$valid_cron_arg" != "$substring" ] ;do
+			# Extract the substring from start of arg up to delimiter
+			# Example: substring="JAN" from "JAN,MAR,JUN,SEP,DEC"
+			substring=${valid_cron_arg%%"$delimiter"*}
+			# Delete this first "substring" AND its delimiter, from arg
+			# Example: arg="MAR,JUN,SEP,DEC" from "JAN,MAR,JUN,SEP,DEC"
+			valid_cron_arg="${valid_cron_arg#"$substring""$delimiter"}"
+
+			is_arg_valid=false
+			# Diving in from full arg (e.g. "0-59/5") into
+			# individual value (e.g. "0", "59" "/2") for validation
+			_is_valid_cron_arg "$x" "$substring" && is_arg_valid=true
+
+			if [ "$delimiter" = "/" ] || [ "$delimiter" = "-" ]; then
+				[ "${#valid_cron_arg}" -lt 1 ] && return 1 # End check if no arg is behind / or -
+
+				# Test if arg behind / or - is JAN-DEC / SUN-SAT
+				# arg will be converted to its respective number
+				converted_arg="$valid_cron_arg"
+				[ "$x" -eq 4 ] && converted_arg=$(_month_txt2dec "$valid_cron_arg")
+				[ "$x" -eq 5 ] && converted_arg=$(_day_txt2dec "$valid_cron_arg")
+				# End check if the arg behind / or - is not a number or not in the JAN-DEC, SUN-SAT lists
+				[ "$(echo "$converted_arg" | tr -d "[:digit:]" | wc -w)" -gt 0 ] && return 1
+
+				if [ "$delimiter" = "/" ]; then
+					# Arg behind / must be a number larger than 0 or
+					# if arg is of the MONTH or DAY_OF_WEEK fields, must be in JAN-DEC, SUN-SAT lists
+					is_arg_valid=true
+					# End check if arg behind / is less than 1
+					[ "$converted_arg" -lt 1 ] && return 1
+				elif [ "$delimiter" = "-" ]; then
+					converted_substring="$substring"
+					[ "$x" -eq 4 ] && converted_substring=$(_month_txt2dec "$substring")
+					[ "$x" -eq 5 ] && converted_substring=$(_day_txt2dec "$substring")
+					# End check if arg in front of - is greater than the arg behind -
+					[ "$prev_substring" -gt "$converted_substring" ] && return 1
+					prev_substring="$converted_substring"
+				fi
+			fi
+
+			if [ "$is_arg_valid" = true ]; then
+				echo "[>] field #$x value [ $substring ] passed"
+			else
+				echo "[!] field #$x value [ $substring ] failed"
+				return 1
+			fi
+		done
+	fi
+	return 0
+)
+}
+
 custom_cron() {
 	shift
-	# c/o changhuapeng
-	if ( ! echo "$1" | grep -Eqw "^(@reboot|@hourly|@midnight|@daily|@weekly|@monthly|@annually|@yearly)$") && 
-		{ [ ! "$(echo "$1" | wc -w)" -eq 5 ] || [ ! -z "$2" ]; }; then
+	echo "[+] validating custom cron expression"
+	custom_cron_error=false
+	# Has only 1 arg and it starts with @
+	if [ "$(echo "$1" | wc -w)" -eq 1 ] && (echo "$1" | grep -q "^@"); then
+		# End check if arg is not these accepted @strings (case-sensitive)
+		(! echo "$1" | grep -Eqw "^(@reboot|@hourly|@midnight|@daily|@weekly|@monthly|@annually|@yearly)$") && custom_cron_error=true
+	# Has 5 args
+	elif [ "$(echo "$1" | wc -w)" -eq 5 ]; then
+		x=1
+		while [ $x -lt 6 ] ; do
+			custom_cron_arg=$(echo "$1" | cut -d ' ' -f "$x")
+			echo "[>] checking field #$x: [ $custom_cron_arg ]"
+			is_arg_valid=false
+			_is_valid_cron_arg "$x" "$custom_cron_arg" && is_arg_valid=true
+			if [ "$is_arg_valid" = true ]; then
+				echo "[+] field #$x passed"
+			else
+				echo "[!] field #$x failed"
+				break
+			fi
+			x=$((x+1))
+		done
+		[ "$x" -lt 6 ] && custom_cron_error=true
+	else
+		custom_cron_error=true
+	fi
+	# this atleast will catch globbed
+	if [ -z "$1" ] || [ ! -z "$2" ] || [ "$custom_cron_error" = true ]; then
 		# shoutout to native test and holmes
 		echo "[!] futile cronjob" 
 		echo "[!] syntax: --custom-cron \"0 2 * * *\" "