The most common way to use this facility is to pass extra flags to compilers. For example, in a properly written makefile, the variable, CFLAGS, is included in each command that runs the C compiler. So, a file, ‘foo.c’, would be compiled using something like: cc -c $(CFLAGS) foo.c.

Thus, whatever value you set for CFLAGS affects each compilation that occurs.

The makefile probably specifies the usual value for CFLAGS, like: CFLAGS=-g.

Each time you run make, you can override this value if you wish. For example, if you say ‘make CFLAGS=‘-g -O’’, each C compilation will be done with ‘cc -c -g -O’. (This illustrates how you can use quoting in the shell to enclose spaces and other special characters in the value of a variable when you override it.)

The variable, CFLAGS, is only one of many standard variables that exist just so that you can change them this way. See Variables used by implicit rules for a complete list.

You can also program the makefile to look at additional variables of your own, giving the user the ability to control other aspects of how the makefile works by changing the variables.

When you override a variable with a command argument, you can define either a recursively-expanded variable or a simply-expanded variable. The examples shown previously make a recursively-expanded variable; to make a simply-expanded variable, write ‘:=’ instead of ‘=’. Unless you want to include a variable reference or function call in the value that you specify, it makes no difference which kind of variable you create.

There is one way that the makefile can change a variable that you have overridden. This is to use the override directive, which is a line using something like ‘override variable=value’ (see The override directive).