4.3.1 Wildcard Examples

Wildcards can be used in the commands 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, prerequisite or command, wildcard expansion will take place at that time. To set objects to the expansion, instead use:

objects := $(wildcard *.o)

See section 4.3.3 The Function wildcard.