Output apparatus and method of a printer, a display, or the like for receiving a group of commands from a host computer and for changing subsequent data processes. The output apparatus comprises: a substituting circuit for substituting another discrimination information for discrimination information to be changed in accordance with an input of instruction information to instruct a change in discrimination information stored; and an executing circuit for executing a predetermined operation shown by the discrimination information before it is substituted by the substituting circuit in accordance with an input of the discrimination information substituted by the substituting circuit. A command table of an output unit is rewritten by a special command from the host computer, thereby enabling a desired outputting process to be selectively executed while the host computer easily discriminates the output unit.