Phony Make Targets
It is often necessary to make things that are not files, a make target that does not create a file is a phony target.
clean: rm -rf temp .PHONY: clean
As the target
clean does not create a file of the same name, we mark it as
phony by adding it to the special
.PHONY target. A phony target will always
run the command even if a matching file exists.