Grabbing arguments and the start of the error checking is in place.

check_pf_interface_usage.sh

+#!/bin/sh
+
+status=""
+
+exitunknown() {
+	echo "UNKNOWN - $status"
+	exit 3
+}
+
+
+monthlogic() {
+
+}
+# GET ARGS
+while getopts r:i:w: o
+do	case "$o" in
+	r)	resetdate="$OPTARG";;
+	i)	interface="$OPTARG";;
+	w)	warning="$OPTARG";;
+	[?])	print >&2 " Usage: $0 -r date -i interface  [-w warning threshold]" \
+		"\n\n\tdate should be the time you want the counters to reset" \
+		"\n\tit will use the format <m|d>[DD]HHMM" \
+		"\n\tso if you want to reset at the start of the month use m010000" \
+		"\n\tor if you want to reset daily at 3:00 AM use d0300" \
+		"\n\twarning threshold is optional and measured in bytes."
+		exit 1;;
+	esac
+done
+shift $OPTIND-1
+
+period="$(echo "$resetdate" | cut -c1)"
+case "$period" in
+	m | M )	monthlogic;;
+	d | D ) periodsecs=86400;;
+	* ) 	status="You didn't specify a valid date"
+		exitunknown;;
+esac
+
+ifconfig $interface >/dev/null 2>&1
+rc=$?
+if [ $rc -ne 0 ]; then
+	status="The interface $interface isn't valid on this system."
+	exitunknown
+fi
+
+echo $resetdate
+echo $warning