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December 2010
January 2011

2010-12-14 »

The only build system that might someday replace make... djb redo.

There are only two problems. In order of increasing difficulty:

  1. you've never heard of it.

  2. it doesn't exist.

Well, I just solved problem #1. Progress!

Problem #2 is a lot tougher. You see, the page linked above talks about a theoretically great build system, which perhaps D. J. Bernstein (djb), maker of qmail and djbdns, has implemented part of. But he doesn't link to an implementation, probably because his own code isn't up to his (rather exacting) standards yet, or he's gotten busy with other things and hasn't finished thinking through all the details.

I would like to tell you, in a concise way, exactly why redo is literally the most amazingly groundbreaking build system since the original invention of 'make', but I don't know how. So lacking conciseness, I'll just make a few grandiose claims:

  • it can do everything make can do;
  • with no baked-in assumptions about what you're building;
  • with much less code;
  • with much greater parallelism;
  • with finer-grained dependencies;
  • with much less syntax (actually nothing but /bin/sh);
  • while supporting recursion and full dependency information simultaneously (no Recursive Make Considered Harmful crap);
  • yet build scripts are highly modular and readable;
  • and you can checksum your targets instead of using timestamps;
  • and your build scripts run linearly instead of an orderless "ruleset";
  • with no implicit rules required;
  • and implementing C header autodependencies is completely sane;
  • and dependency checks involve no forking or parsing so it's crazy fast;
  • and you can incrementally convert parts of your project;
  • because it can play well with other build systems;
  • including jobserver compatibility with make -j;
  • oh, and you can write a plug-compatible toy
    implementation in 100 lines of shell.

Sounds awesome? You bet it is. Well, except for the not existing part.

So I wrote it

Yes. djb's design is so simple that I thought I could do it in a couple of days. Actually I did have a working version in a couple of days. And normally, I'm a big fan of "release early, release often." But when it comes to build systems, well, you know, it's kind of a crowded market. I figured I'm only going to get one chance to convince you that this is the most fabulous thing ever.

That's why I've spent more than a month - days, evenings, and weekends - of my so-called vacation1 working on this stupid thing. It has man pages. It has a ridiculously huge README+FAQ. It has an installer. It has parallelism. It has unit tests, oh god, the unit tests. It has locking. In fact, it has so much parallelism and locking that it uncovered a race condition in MacOS X's fcntl(). It has a GNU make compatible jobserver, so that 'redo -j5' and 'make -j5' can call each other recursively and "just work." It has a few extensions that djb didn't talk about, like checksum-based dependencies. For testing, I converted a pretty huge build system (one that builds the Linux kernel and a bunch of libraries for two different target platforms), with some of my targets depending on more than 12000 files. redo can check every single fine-grained dependency in the whole project in about 4 seconds.

Version 0.01 actually delivers on every one of those grandiose claims.

As I'm writing this, it's 1574 lines of python. That's a lot smaller than the source for GNU make.

And for fun, I also wrote a super-stripped-down "forwards compatible" implementation (without support for actual dependencies; it just assumes everything is always dirty) in 100 lines of shell. That's less than 2 kbytes, and it works with any input file that works with full-sized redo (modulo any as-yet-undiscovered bugs), because djb's design is that awesome.

And you know what? I almost certainly still, after all that effort, screwed up and it probably still won't work for you. Dammit. That's why I paranoidly called it version 0.01 instead of 1.00. But please give it a try:

apenwarr's redo on github

(The super oversized README is visible at that link. Or you can read the man pages.)


Because redo is currently written in python (and it does a lot of fork-exec calls), it runs a little slower than I would like. Someday rewriting it in C would make it faster - probably at least 10x faster. But it's reasonably fast already, and the increased parallelism and faster dependency checking often makes up for the slowness.

The non-optimal performance is the only significant flaw I'm aware of at the moment... but I'm sure you'll try it out and tell me otherwise, right?

You should join the redo mailing list

You can find the mailing list on Google Groups at

Of course the mailing list archives are currently empty, because this is the first time I've announced it.

It might not look like it, but yes, you can subscribe without having a Google Account. Just send a message here: a href=""


1 I told the guard at the U.S./Canada border that I was "between jobs." He offered his sympathies. I said, "No, I mean, literally." He said, "Oh, normally people use that as a euphemism." Anyway, it's now painfully clear that I suck at vacations.

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