Use '$' arithmetic substitution, not old '`' (backtick)

vaporget

@@ -87,7 +87,7 @@ while getopts m: f; do
 		;;
 	esac
 done
-shift `expr $OPTIND - 1`
+shift $(( $OPTIND - 1 ))
 
 # Use our custom handler for the TERM signal
 trap handle_term TERM