Make (or GNU Make)’s basic value proposition is that it will generate files based on other files. How those files are generated is up to you, but that’s part of what’s great about Make: anything that you can do on the shell can be scripted in Make, so you are free to use whatever tools or languages you want.
Let’s look at an example of what we can do with Make. We’ll start with a folder structure like this:
- src - index.js - dest
dest is empty and we have one source file that we’d like to compile from the
src directory to the
dest directory. A simple Makefile that will “compile”
dest/index.js might look like this:
1dest/index.js: 2 cp src/index.js dest/index.js
make dest/index.js will execute the recipe and copy your source file
into the destination directory as-is. It’s not the most exciting thing ever, but
it’s a starting point.
Crucially, Make doesn’t know how often it should update the
target. If you make changes to your source file and re-run
you’ll see that your changes aren’t reflected in the destination file. We
haven’t told Make about the dependencies that the target has — its
Let’s teach Make what’s needed to keep
dest/index.js up to date. Since
dest/index.js is compiled from the source of
considered a prerequisite, or dependency, of
dest/index.js. Here’s an updated
version of our Makefile from above that lists
src/index.js as a prerequisite:
1dest/index.js: src/index.js 2 cp src/index.js dest/index.js
Make understands that if a prerequisite has been updated then any targets that
depend on that prerequisite need to be updated. Make looks at the modification
time of the files listed as prerequisites to determine if the target should be
rebuilt, so it only rebuilds what’s out of date. It also won’t rebuild targets
that exist but don’t have any prerequisites because it doesn’t know how to tell
if the target is out of date. In our example above, we’ve told Make that
dest/index.js should be rebuilt any time
src/index.js is updated.
The commands that are run to update a target in Make are called the
recipe and in our case, the recipe is pretty boring. Make knows what
file to generate (the target,
dest/index.js) and when to generate it (when the
src/index.js, have been changed), but it’s not really doing
anything interesting with the source file.
Let’s use terser to minify our source file on its way to
Assuming we’ve got a version of NodeJS installed that includes the
tool, the command to compress our source file and save it to our destination
folder looks like this:
1npx terser src/index.js --compress --output dest/index.js
You can change the options around however you’d like, but the idea remains the same: there’s an input file, some options, and an output file. Now let’s take that and use it in our Makefile’s recipe:
1dest/index.js: src/index.js 2 npx terser src/index.js --compress --output dest/index.js
Now, every time
src/index.js changes, our new recipe using terser will be run
and the source file will be compressed and output to Make’s target file. We’ve
taught Make how to ahem make our target!
More Than One Target
At this point, we’ve got Make compressing our source file and outputting it to
our destination. Let’s add a new
src/lib.js file to our project and teach Make
how to handle it.
The most naive approach to handling our new file would be to duplicate our existing target/recipe setup, changing only the file names:
1dest/index.js: src/index.js 2 npx terser src/index.js --compress --output dest/index.js 3 4dest/lib.js: src/lib.js 5 npx terser src/lib.js --compress --output dest/lib.js
1dest/%.js: src/%.js 2 npx terser '$<' --compress --output '$@'
The target, prerequisites, and recipe look very similar to before, but with some new stuff replacing the old file names. To explain a few things that are happening:
- The “%” in the target tells Make to match any non-empty string.
- The “%” in the prerequisite is filled in by Make with the same “stem” or
portion of the target name that was matched in the target. For example, if
dest/turkeys.jswas handed to this target, the “%” in the prerequisite would expand to
- The “$<“ in the recipe is an automatic variable populated by Make with the
name of the first prerequisite. When we run
make dest/index.js, the “%” matches the stem “index” and expands our first prerequisite to “src/index.js” which is then handed to the recipe, so “$<“ gets expanded to the same: “src/index.js”.
- The “$@“ in the recipe is another automatic variable populated by Make, but this time with the name of the target file.
Now if you run
make dest/index.js or
make dest/lib.js they’ll both be
handled by the same target and run the same recipe, generating the appropriate
file based on the corresponding prerequisite.