redo-stamp - detect if the current target has meaningfully changed


redo-stamp <$3


Normally, when redo(1) builds a target T, all the other targets that depend on T are marked as out of date. Even if the rebuilt T is identical to the old one, all its dependents need to be rebuilt.

By calling redo-stamp from your .do script, you can tell redo that even though the current target is building, its output may turn out to be unchanged. If it hasn't, redo may then be able to avoid building other targets that depend on this target.

redo-stamp marks the current target as changed or unchanged by comparing its stdin to the input that was provided last time redo-stamp was called for this target.

The stamp data can be anything you want. Some possibilities are:

  • the actual target file contents:

    redo-stamp <$3
  • a list of filenames:

    find -name '*.[ch]' | sort | redo-stamp
  • the contents of a web page:

    curl | redo-stamp

To ensure that your target gets checked every time, you might want to use redo-always(1).


While using redo-stamp is simple, the way it works is harder to explain. Deciding if a target is up to date or not is the job of redo-ifchange(1). Normally, a target is considered out of date when any of its dependencies (direct or indirect) have changed. When that happens, redo-ifchange runs the .do script for the target, which regenerates the entire dependency list, including rebuilding any dependencies as necessary.

Imagine that we have the following dependency chain:

T -> X -> Y -> Z

T depends on X, which depends on Y, which depends on Z. Now imagine someone has changed Z.

If someone runs redo-ifchange T, then redo-ifchange checks if X is up to date; to do that, it checks if Y is up to date; and to do that, it checks whether Z is up to date - which it isn't. Thus, Y is not up to date, which means X isn't, which means T isn't either, and so we need to run redo-ifchange won't directly redo X just because X is dirty; it redoes T, and might eventually call redo-ifchange X if it needs to.

When using redo-stamp, the behaviour is different. Let's say Y has used redo-stamp. In the above sequence, Y is outdated because Z has changed. However, we don't know yet if Y's stamp has changed, so we don't yet know if we need to redo X, and thus we don't know if we need to redo T. We can't just run redo T, because that could waste a lot of time if it turns out T didn't need to be rebuilt after all.

What we do instead is note whether the only dependencies of T that are out of date are 'stamped' targets. If any dependencies of T are normal, out-of-date redo targets, then T needs to be rebuilt anyway; we don't have to do anything special.

If the only dependency of T that has changed is Y (a 'stamped' target), then we need to redo Y automatically in order to determine if T needs to be rebuilt. This is the only time that redo ever rebuilds a target that hasn't been explicitly asked for as part of a running .do script.


Part of the redo(1) suite.


The original concept for redo was created by D. J. Bernstein and documented on his web site ( This independent implementation was created by Avery Pennarun and you can find its source code at


redo(1), redo-ifcreate(1), redo-ifchange(1), redo-always(1)