redo: a recursive, general-purpose build system

redo is a competitor to the long-lived, but sadly imperfect, make program. Unlike other such competitors, redo captures the essential simplicity and flexibility of make, while avoiding its flaws. It manages to do this while being simultaneously simpler than make, more flexible than make, and more powerful than make, and without sacrificing performance - a rare combination of features.

The original design for redo comes from Daniel J. Bernstein (creator of qmail and djbdns, among many other useful things). He posted some terse notes on his web site at one point (there is no date) with the unassuming title, "Rebuilding target files when source files have changed." Those notes are enough information to understand how the system is supposed to work; unfortunately there's no code to go with it. I wrote this implementation of redo from scratch, based on that design.

After I found out about djb redo, I searched the Internet for any sign that other people had discovered what I had: a hidden, unimplemented gem of brilliant code design. I found only one interesting link: Alan Grosskurth, whose Master's thesis at the University of Waterloo was about top-down software rebuilding, that is, djb redo. He wrote his own (admittedly slow) implementation in about 250 lines of shell script, which gives an idea for how straightforward the system is.

My implementation of redo is called redo for the same reason that there are 75 different versions of make that are all called make. It's somehow easier that way.

I also provide an extremely minimal pure-POSIX-sh implementation, called do, in the minimal/ directory of this repository.

(Want to discuss redo? See the bottom of this file for information about our mailing list.)

What's so special about redo?

The theory behind redo sounds too good to be true: it can do everything make can do, but the implementation is vastly simpler, the syntax is cleaner, and you have even more flexibility without resorting to ugly hacks. Also, you get all the speed of non-recursive make (only check dependencies once per run) combined with all the cleanliness of recursive make (you don't have code from one module stomping on code from another module).

(Disclaimer: my current implementation is not as fast as make for some things, because it's written in python. Eventually I'll rewrite it an C and it'll be very, very fast.)

The easiest way to show it is to jump into an example. Here's one for compiling a C++ program.

Create a file called default.o.do:

redo-ifchange $2.c
gcc -MD -MF $2.d -c -o $3 $2.c
read DEPS <$2.d
redo-ifchange ${DEPS#*:}

Create a file called myprog.do:

DEPS="a.o b.o"
redo-ifchange $DEPS
gcc -o $3 $DEPS

Of course, you'll also have to create a.c and b.c, the C language source files that you want to build to create your application.

In a.c:

#include <stdio.h>
#include "b.h"

int main() { printf(bstr); }

In b.h:

extern char *bstr;

In b.c:

char *bstr = "hello, world!\n";

Now you simply run:

$ redo myprog

And it says:

redo  myprog
redo    a.o
redo    b.o

Now try this:

$ touch b.h
$ redo myprog

Sure enough, it says:

redo  myprog
redo    a.o

Did you catch the shell incantation in default.o.do where it generates the autodependencies? The filename default.o.do means "run this script to generate a .o file unless there's a more specific whatever.o.do script that applies."

The key thing to understand about redo is that declaring a dependency is just another shell command. The redo-ifchange command means, "build each of my arguments. If any of them or their dependencies ever change, then I need to run the current script over again."

Dependencies are tracked in a persistent .redo database so that redo can check them later. If a file needs to be rebuilt, it re-executes the whatever.do script and regenerates the dependencies. If a file doesn't need to be rebuilt, redo figures that out just using its persistent .redo database, without re-running the script. And it can do that check just once right at the start of your project build, which is really fast.

Best of all, as you can see in default.o.do, you can declare a dependency after building the program. In C, you get your best dependency information by trying to actually build, since that's how you find out which headers you need. redo is based on this simple insight: you don't actually care what the dependencies are before you build the target. If the target doesn't exist, you obviously need to build it.

Once you're building it anyway, the build script itself can calculate the dependency information however it wants; unlike in make, you don't need a special dependency syntax at all. You can even declare some of your dependencies after building, which makes C-style autodependencies much simpler.

redo therefore is a unique combination of imperative and declarative programming. The initial build is almost entirely imperative (running a series of scripts). As part of that, the scripts declare dependencies a few at a time, and redo assembles those into a larger data structure. Then, in the future, it uses that pre-declared data structure to decide what work needs to be redone.

(GNU make supports putting some of your dependencies in include files, and auto-reloading those include files if they change. But this is very confusing - the program flow through a Makefile is hard to trace already, and even harder when it restarts from the beginning because an include file changes at runtime. With redo, you can just read each build script from top to bottom. A redo-ifchange call is like calling a function, which you can also read from top to bottom.)

What projects use redo?

Some larger proprietary projects are using it, but unfortunately they can't easily be linked from this document. Here are a few open source examples:

  • Liberation Circuit is a straightforward example of a C++ binary (a game) compiled with redo.

  • WvStreams uses a more complex setup producing several binaries, libraries, and scripts. It shows how to produce output files in a different directory than the source files.

  • WvBuild can cross-compile several dependencies, like openssl and zlib, and then builds WvStreams using those same libraries. It's a good example of redo/make interop and complex dependencies.

  • There's an experimental variant of Buildroot that uses redo in order to clean up its dependency logic.

  • You can also find some curated examples in the Documentation/cookbook/ subdir of the redo project itself.

  • A github search for all.do shows an ever-growing number of projects using redo.

If you switch your program's build process to use redo, please let us know and we can link to it here for some free publicity.

(Please don't use the integration testing code in the redo project's t/ directory as serious examples of how to use redo. Many of the tests are doing things in intentionally psychotic ways in order to stress redo's code and find bugs.)

How does this redo compare to other redo implementations?

djb never released his version, so other people have implemented their own variants based on his published specification.

This version, sometimes called apenwarr/redo, is probably the most advanced one, including support for parallel builds, resilient timestamps and checksums, build log linearization, and helpful debugging features. It's currently written in python for easier experimentation, but the plan is to eventually migrate it to plain C. (Some people like to call this version "python-redo", but I don't like that name. We shouldn't have to rename it when we later transliterate the code to C.)

Here are some other redo variants (thanks to Nils Dagsson Moskopp for many of these links):

  • Alan Grosskurth's redo thesis and related sh implementation. (Arguably, this paper is the one that got all the rest of us started.)

  • Nils Dagsson Moskopp's redo-sh is a completely self-sufficient sh-based implementation.

  • apenwarr's minimal/do is included with this copy of redo. It's also sh-based, but intended to be simple and failsafe, so it doesn't understand how to "redo" targets more than once.

  • Christian Neukirchen's redo-c, a C implementation.

  • Jonathan de Boyne Pollard's fork of Alan Grosskurth's redo (another sh-based implementation).

  • Jonathan de Boyne Pollard's redo rewritten in C++

  • Gyepi Sam's redux in Go

  • jekor's redo in Haskell

  • Shanti Bouchez-Mongard√© (mildred)'s fork of apenwarr's redo in python

  • Tharre's redo in C

  • catenate's credo, a (very rearchitected) variant written for the Inferno Shell.

The original redo design is so simple and elegant that many individuals have been inspired to (and able to) write their own version of it. In the honoured tradition of Unix's make, they (almost) all just use the same name, redo. Unfortunately, many of these implementations are unmaintained, slightly incompatible with the "standard" redo semantics, and/or have few or no automated tests.

At the time of this writing, none of them except apenwarr/redo (ie. this project) support parallel builds (redo -j) or log linearization (redo-log). For large projects, parallel builds are usually considered essential.

The automated tests in this version of redo are almost, but not quite, appropriate for testing any redo implementation. If you really must write a new version of redo, we invite you to thoroughly test it against the existing test suite to ensure compatibility. You can also steal our tests (with attribution, of course) and include them in your own source package. We'd also love it it you contribute more automated tests when you find a bug, or send us patches if you find a test which is accidentally incompatible (as opposed to finding a real bug) with other redo implementations.