Five Minutes of Make

If you’ve ever compiled a large software project, you’ve probably used make. If you’re like me, you probably think that make is some arcane tool that only fifty-year-olds with giant beards know how to use. This summer, I learned I was totally wrong, and make is actually really simple and really useful. I’m still not an expert, but all of the tutorials I could find online would have taken me five hours, and I think knowing five minutes¹ of make now is way more useful than knowing five hours of make “someday”.

As an example, we’ll write a simple Makefile for a hypothetical site using Jekyll. In the project root, make a new file called Makefile, and put the following² in it:

build:
  jekyll build

Now to build, just run make build. Now let’s say that we add a Javascript file some-js.js, and we want to use browserify to build it. We just add another rule:

scripts:
  mkdir -p js
  browserify some-js.js -o js/bundle.js

Now make scripts will build our Javascript and stick it in js/bundle.js. Except now we have a problem: we need to run two commands, make scripts and make build, whenever we update things. That’s silly, and make has a solution. We just tell it that in order to make build it had better make scripts first, by modifying the first rule to read:

build: scripts
  jekyll build

This, though, has the disadvantage that we always have to recompile the Javascript even if we didn’t modify it. Here’s where make really shines: instead of calling our make target scripts, we just name it after the file that’s generated, and put the Javascript source file as a dependency.

build: js/bundle.js
  jekyll build

js/bundle.js: some-js.js
  mkdir -p js
  browserify some-js.js -o js/bundle.js

Now the Javascript bundle will only get updated when it needs to be. Lastly, to be safe, we should tell make that build is not an actual file, and therefore it need not check for a file of that name. We do this with the fake target .PHONY:

.PHONY: build

Lastly, make has variables that work somewhat similarly to shell variables. This is useful to make it easy to add more build dependencies. We use them like this:

JSFILES=some-js.js some-more-js.js

js/bundle.js: $(JSFILES)
  mkdir -p js
  browserify $(JSFILES) -o bundle.js

Now if we add another javascript file to be compiled, we don’t need to list it separately. So here’s our complete Makefile:

JSFILES=some-js.js some-more-js.js

build: js/bundle.js
  jekyll build

js/bundle.js: $(JSFILES)
  mkdir -p js
  browserify $(JSFILES) -o bundle.js

.PHONY: build

There’s a lot more to learn about make: there are default rules that, for example, let one specify that all files of a particular extension should be treated the same way, and various other useful macros, but hopefully this is enough to be useful and enough to learn more.