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 Wildcard Function.