m4 copies the given files while expanding the macros that they contain. These macros are either built-in or user-defined and can take any number of arguments. Besides just doing macro expansion, m4 has built-in functions for including named files, running Unix commands, doing integer arithmetic, manipulating text in various ways, recursion, and so on. The m4 program can be used either as a front-end to a compiler or as a macro processor in its own right.