+if [ ! -n "$selector" ]; then
+ cat << EOF
+Usage: $0 SELECTOR [KEYFILE] - set up DKIM system and configuration
+
+If existing KEYFILE is given, set up DKIM to use SELECTOR and apply key from
+KEYFILE.
+
+If existing KEYFILE is not given, generate KEYFILE and DNS TXT file for
+SELECTOR.
+EOF
+ exit
+fi
+