REDO-IFCHANGE(1)

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•   for any target that is out of date, it calls the equivalent of redo target.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 $2.c
        gcc -o $3 -c $2.c \
            -MMD -MF $2.deps
        read DEPS <$2.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.

SEE ALSO
redo(1), redo-ifcreate(1), redo-always(1), redo-stamp(1)

AUTHOR
Avery Pennarun <apenwarr@gmail.com>