Avery Pennarun 656c3c583c Docs: recommend adding redo-ifchange with a lot of targets at a time.

Some people don't know to do this, and their scripts are unnecessarily slow
because of it.

2011-05-08 04:25:51 -04:00

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% redo-ifchange(1) Redo %VERSION% % Avery Pennarun apenwarr@gmail.com % %DATE%

NAME

redo-ifchange - rebuild target files when source files have changed

SYNOPSIS

redo-ifchange [targets...]

DESCRIPTION

Normally redo-ifchange is run from a .do file that has been executed by redo(1). See redo(1) for more details.

redo-ifchange doesn't take any command line options other than a list of targets. To provide command line options, you need to run redo instead.

redo-ifchange performs the following steps:

it creates a dependency on the given targets. If any of those targets change in the future, the current target (the one calling redo-ifchange) will marked as needing to be rebuilt.
for any target that is out of date, it calls the equivalent of redo target.
for any target that is locked (because some other instance of redo or redo-ifchange is already building it), it waits until the lock is released.

redo-ifchange returns only after all the given targets are known to be up to date.

TIP 1

You don't have to run redo-ifchange before generating your target; you can generate your target first, then declare its dependencies. For example, as part of compiling a .c file, gcc learns the list of .h files it depends on. You can pass this information along to redo-ifchange, so if any of those headers are changed or deleted, your .c file will be rebuilt:

    redo-ifchange $1.c
    gcc -o $3 -c $1.c \
        -MMD -MF $1.deps
    read DEPS <$1.deps
    redo-ifchange ${DEPS#*:}

This is much less confusing than the equivalent autodependency mechanism in make(1), because make requires that you declare all your dependencies before running the target build commands.

TIP 2

Try to list as many dependencies as possible in a single call to redo-ifchange. Every time you run redo-ifchange, the shell has to fork+exec it, which takes time. Plus redo can only parallelize your build if you give it multiple targets to build at once. It's fine to have a couple of separate redo-ifchange invocations for a particular target when necessary (as in TIP 1 above), but try to keep it to a minimum. For example here's a trick for generating a list of targets, but redo-ifchanging them all at once:

for d in *.c; do
	echo ${d%.c}.o
done |
xargs redo-ifchange

REDO

Part of the redo(1) suite.

CREDITS

The original concept for redo was created by D. J. Bernstein and documented on his web site (http://cr.yp.to/redo.html). This independent implementation was created by Avery Pennarun and you can find its source code at http://github.com/apenwarr/redo.

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