Section 7.  Permanent appointments and promotions in the classified state and city service shall be made only after certification by the appropriate department of civil service under a general system based upon merit, efficiency, fitness, and length of service, as ascertained by examination which, so far as practical, shall be competitive.  The number to be certified shall not be less than three; however, if more than one vacancy is to be filled, the name of one additional eligible for each vacancy may be certified.  Each commission shall adopt rules for the method of certifying persons eligible for appointment, promotion, reemployment, and reinstatement and shall provide for appointments defined as emergency and temporary appointments if certification is not required.