How to execute a system file in procedure?

I would like execute a system command or my program or my shell
script in procedure. How do I do that?
For example,

CREATE FUNCTION myname() RETURN OPAQUE AS '
    BEGIN
        IF count(NEW.aColumn) >= 600
        THEN RAISE EXCEPTION ''Hello, it is time to re-index the
table.'';
        END IF;
        RETURN NEW;
    END; '
LANGUAGE 'plpgsql';

CREATE TRIGGER AFTER INSERT OR UPDATE ON mytable
FOR EACH ROW EXECUTE PROCEDURE myname();

But I prefer to execute a system command like mail

mail -s "Re-Index" system@host.domain < `echo "It is time to re-index
the table"`