Julia Evans

ninja: a simple way to do builds

Hello! Every so often I find a new piece of software I really like, and today I want to talk about one of my recent favourites: ninja!

incremental builds are useful

I do a lot of small projects where I want to set up incremental builds – for example, right now I’m writing a zine about bash, and I have one .svg file for each page of the zine. I need to convert the SVGs to PDFs, and I’d been doing it something like this:

for i in *.svg
    svg2pdf $i $i.pdf # or ${i/.svg/.pdf} if you want to get really fancy

This works fine, but my svg2pdf script is a little slow (it uses Inkscape), and it’s annoying to have to wait 90 seconds or whatever to rebuild all the PDFs when I’ve just updated 1 page.

build systems are confusing

In the past I’ve been pretty put off by using a Build System like make or bazel for my small projects because bazel is this Big Complicated Thing and make feels a little arcane to me. I don’t really know how to use either of them.

So for a long time I’ve just written a bash script or something for my builds and resigned myself to just waiting for a minute sometimes.

ninja is an EXTREMELY SIMPLE build system

But ninja is not complicated! Here is literally everything I know about ninja build file syntax: how to create a rule and a build:

a rule has a command and description (the description is just for humans to read so you can tell what it’s doing when it’s building your code)

rule svg2pdf
  command = inkscape $in --export-text-to-path --export-pdf=$out
  description = svg2pdf $in $out

the syntax for build is build output_file: rule_name input_files. Here’s one using the svg2pdf rule. The output goes in $out in the rule and the input goes in $in.

build pdfs/variables.pdf: svg2pdf variables.svg

That’s it! If you put those two things in a file called build.ninja and then run ninja, ninja will run inkscape variables.svg --export-text-to-path --export-pdf=pdfs/variables.pdf. And then if you run it again, it won’t run anything (because it can tell that you’ve already built pdfs/variables.pdf and you’re up to date)

Ninja has a few more features than this (see the manual), but I haven’t used them yet. It was originally built for Chromium, so even with a small feature set it can support large builds.

ninja files are usually automatically generated

The magic of ninja is that instead of having to use some confusing Build Language that’s hard to remember because you use it so infrequently (like make), instead the ninja language is SUPER simple, and if you want to do something complicated then you can generate the build file you want using any programming language you want. I find this this a lot easier because I know Python, but I can never remember anything about how the make language works.

I like to make a build.py file or that looks something like this, that creates the ninja build file and then runs ninja:

with open('build.ninja', 'w') as ninja_file:
    # write some rules
rule svg2pdf
  command = inkscape $in --export-text-to-path --export-pdf=$out
  description = svg2pdf $in $out
    # some for loop with every file I need to build
    for filename in things_to_convert:
build {filename.replace('svg', 'pdf')}: svg2pdf {filename}

# run ninja
import subprocess

I’m sure there are a bunch of ninja best practices, but I don’t know them and for my small projects I find this works well.

meson is a build system that generates ninja files

I don’t know too much about Meson yet, but recently I was building a C program (plocate, a faster alternative to locate) and I noticed that instead of the usual ./configure; make; make install, there were different build instructions:

meson builddir
cd builddir

It seems like Meson is a build system for C/C++/Java/Rust/Fortran that can use ninja as a backend.

that’s all!

I’ve been using ninja for a few months now. I really like it and it’s caused me approximately 0 build-related headaches which feels pretty magical to me.

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