4.4.1 Wildcard Examples

Wildcards can be used in the recipe of a rule, where they are expanded by the shell. For example, here is a rule to delete all the object files:

clean:
        rm -f *.o

Wildcards are also useful in the prerequisites of a rule. With the following rule in the makefile, ‘make print’ will print all the ‘.c’ files that have changed since the last time you printed them:

print: *.c
        lpr -p $?
        touch print

This rule uses print as an empty target file; see Empty Target Files to Record Events. (The automatic variable ‘$?’ is used to print only those files that have changed; see Automatic Variables.)

Wildcard expansion does not happen when you define a variable. Thus, if you write this:

objects = *.o

then the value of the variable objects is the actual string ‘*.o’. However, if you use the value of objects in a target or prerequisite, wildcard expansion will take place there. If you use the value of objects in a recipe, the shell may perform wildcard expansion when the recipe runs. To set objects to the expansion, instead use:

objects := $(wildcard *.o)

See The Function wildcard.