Getting Started with Makefiles

Basic Terminology

A rule is one or more commands to run paired with the inputs that the commands depend on. Makefiles are made up of rules.

A target is usually the name of the output of a rule. However, rules do not always have an output file. In the case that a rule does not have an output file, the target can be thought of as a name for what the rule does when run.

Prerequisites are the input files that the rule depends on.

A recipe consists of the commands that are run when the rule is executed.

The make Command

Running make in the shell will cause the shell to look for a Makefile in the current directory. If it finds one, it will attempt to create the first target listed in the Makefile.

You can also run make <target_name> to indicate exactly which rule we want to run.

Writing a Makefile

Your Makefile should be in a file called Makefile. The first line should tell the Makefile what shell to use. Since we are using bash, the first line should be:

SHELL = /usr/bin/env bash

Next you can start writing rules. Rules will look like this:

target: prerequisites
	recipe

Note that there is a tab on the second line before the recipe. This is very important!

Let’s look at some examples of rules.

print: whereami
	cat whereami | xargs echo "I am in"

whereami:
	pwd > whereami

The first rule has one prerequisite: a target called “whereami.” Given a file with that target name, the print rule will pass the contents of the file into echo, which will print out to the terminal. But when we first run make with this Makefile, we probably don’t have a file called “whereami!” Knowing that we need this file to process the first rule, the Makefile will look for a target called “whereami” and attempt to process that rule. Luckily, this is the second rule listed above.

The second rule creates a target called “whereami” and has no prerequisites. It will redirect the output of pwd into “whereami.”

Thus if we were in the same directory as the above Makefile and we ran make, this would be the equivalent of running

$ pwd > whereami && cat whereami | xargs echo "I am in"

Not only is typing make a lot easier than typing out the above oneliner, but it actually helps us be efficient if we were to run make a second time. If we now run make again, the Makefile will again attempt to process the first rule. We will again see that the print rule depends on the whereami target, so we go to the second rule. However, this time, we see that none of the prerequisites of whereami have changed (the whereami target has no prerequisites, and so there is nothing that could have changed), and so we don’t rerun the second rule; we can just run the first rule.

Now let’s add a third rule to this Makefile:

duplicate: whereami
	cp whereami duplicate

This rule is a bit different from the first two in that it has both a prerequisite (“whereami”) and an output file (“duplicate”). If we were to run make duplicate, the Makefile runs the rule and we end up with a file called “duplicate.” However, if we now run make duplicate a second time, we get a different result:

make: `duplicate` is up to date.

This is saying that since there were no changes to the prerequisite file “whereami,” there will be no changes to the output of running this rule. Thus, this target is already up to date. If we were to modify “whereami” and then rerun make duplicate, the rule would rerun since the prerequisite file has changed.

clean and .PHONY

Often times Makefiles include a target called clean that serves to clean up the output of running make:

clean:
	rm -f whereami duplicate

This rule has no prerequisites and removes the output files that the Makefile produces as a result of processing the other rules. However, this target does not create a file called “clean” as output. If we were to somehow end up with a file named “clean”, the Makefile would encounter a problem when we ran make clean: it would see the file “clean,” see that the prerequisites for the clean target have not changed (since there are no prerequisites, there is nothing that could have changed), and it will decide it does not need to rerun the rule. This is not what we intended! We want the Makefile to always run the rule when we type make clean. Thus we need to add the following line to the top of our Makefile:

.PHONY: clean

This says that clean is a phony target. With this line, even if we end up with a file called “clean,” make clean will still always result in running the rule as expected.

Resources

• Phony Targets
  • The manual on phony targets