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make in action

By default, make starts with the first target whose name does not start with .. This is called the default goal. (Goals are the targets that make tries to update.)

In the simple example of the previous section, the default goal is to update the executable program edit; therefore, we put that rule first.

Thus, when you give the command: 

make

make reads the makefile in the current directory and begins by processing the first rule. In the example, this rule is for relinking edit; but before make can fully process this rule, it must process the rules for the files that edit depends on, which in this case are the object files. Each of these files is processed according to its own rule. These rules say to update each .o file by compiling its source file. The recompilation must be done if the source file, or any of the header files named as prerequisites, is more recent than the object file, or if the object file does not exist.

The other rules are processed because their targets appear as prerequisites of the goal. If some other rule is not depended on by the goal (or anything that the goal depends on, and so forth), then that rule is not processed, unless you tell make to do so (with a command such as make clean.

Before recompiling an object file, make considers updating its prerequisites (the source file and header files). This makefile does not specify anything to be done for them--the .c and .h files are not the targets of any rules--so make does nothing for these files. But make can update automatically generated C programs, such as those made by Bison or Yacc, by defining the .c and .h files as targets and specifying how to create them with Bison, Yacc, or whatever other program generated them.

After recompiling the appropriate object files, make decides whether to link edit. This must be done if the file edit does not exist, or if any of the object files are newer than it is. If an object file was just recompiled, it is now newer than edit, so edit is relinked.

Thus, if we change the file insert.c and run make, then make will recompile that file, update insert.o, and then link edit. If we change the file command.h and run make, make will recompile the object files kbd.o, command.o and files.o, and then link the file edit.