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apenwarr-redo/Documentation/redo.md
Avery Pennarun 897c218f85 redo.md: explain .do file search-parent-dirs feature.
2011-01-16 16:33:22 -08:00

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% redo(1) Redo 0.00 % Avery Pennarun apenwarr@gmail.com % 2010-12-12

NAME

redo - rebuild target files when source files have changed

SYNOPSIS

redo [options...] [targets...]

DESCRIPTION

redo is a simple yet powerful tool for rebuilding target files, and any of their dependencies, based on a set of rules. The rules are encoded in simple sh(1) scripts called '.do scripts.'

redo supports GNU make(1)-style parallel builds using the -j option; in fact, redo's parallel jobserver is compatible with GNU Make, so redo and make can share build tokens with each other. redo can call a sub-make (eg. to build a subproject that uses Makefiles) or vice versa (eg. if a make-based project needs to build a redo-based subproject).

Unlike make, redo does not have any special syntax of its own; each target is built by running a .do file, which is simply a shell script that redo executes for you with a particular environment and command-line arguments.

If no targets are specified, redo pretends you specified exactly one target named all.

Note that redo always rebuilds the given targets (although it may skip rebuilding the targets' dependencies if they are up to date). If you only want to rebuild targets that are not up to date, use redo-ifchange(1) instead.

A .do script can call redo recursively to build its dependencies.

OPTIONS

-j, --jobs=maxjobs
execute at most maxjobs .do scripts in parallel. The default value is 1.
-d, --debug
print dependency checks as they happen. You can use this to figure out why a particular target is/isn't being rebuilt when your .do script calls it using redo-ifchange.
-v, --verbose
pass the -v option to /bin/sh when executing scripts. This normally causes the shell to echo the .do script lines to stderr as it reads them. Most shells will print the exact source line (eg. echo $3) and not the substituted value of variables (eg. echo mytarget.redo.tmp).
-x, --xtrace
pass the -x option to /bin/sh when executing scripts. This normally causes the shell to echo exactly which commands are being executed. Most shells will print the substituted variables (eg. echo mytarget.redo.tmp) and not the original source line (eg. echo $3).
-k, --keep-going
keep building as many targets as possible even if some of them return an error. If one target fails, any target that depends on it also cannot be built, of course.
--shuffle
randomize the order in which requested targets are built. Normally, if you run redo a b c, the targets will be built exactly in that order: first a, then b, then c. But if you use -j, they might end up being built in parallel, so it isn't safe to rely on this precise ordering. Using --shuffle, redo will build its targets in random order even without -j, which makes it easier to find accidental dependency problems of this sort. NOTE: if you really just want to guarantee that a is built, then b, then c, you can just run three redo commands consecutively. Because your .do script is just a script, it will not be accidentally parallelized.
--debug-locks
print messages about acquiring, releasing, and waiting on locks. Because redo can be highly parallelized, one instance may end up waiting for a target to be built by some other instance before it can continue. If you suspect this is causing troubles, use this option to see which instance is waiting and when.
--debug-pids
add the process id of the particular redo instance to each output message. This makes it easier to figure out which sub-instance of redo is doing what.

DISCUSSION

The core of redo is extremely simple. When you type redo targetname, then it will search for a matching .do file based on a simple algorithm. For example, given a target named mytarget.a.b.c.d, redo will look for a .do file in the following order:

  • mytarget.a.b.c.d.do
  • default.a.b.c.d.do
  • default.b.c.d.do
  • default.c.d.do
  • default.d.do
  • default.do

In all cases, the .do file must be in the same directory as the target file, or in one of the target's parent directories. For example, if given a target named ../a/b/xtarget.y, redo will look for a .do file in the following order:

  • $PWD/../a/b/xtarget.y
  • $PWD/../a/b/default.y.do
  • $PWD/../a/b/default.do
  • $PWD/../a/default.y.do
  • $PWD/../a/default.do
  • $PWD/../default.y.do
  • $PWD/../default.do

The first matching .do file is executed as a /bin/sh script. The .do script is always executed with the current working directory set to the directory containing the .do file. Because of that rule, the following two commands always have exactly identical behaviour:

redo path/to/target

cd path/to && redo target

(Note: in make(1), these commands have confusingly different semantics. The first command would look for a target named path/to/target in ./Makefile, while the second command would look for a target named target in ./path/to/Makefile. The two Makefiles might give completely different results, and it's likely that the first command would have incomplete dependency information. redo does not have this problem.)

The three arguments passed to the .do script are:

  • $1: the basename of the target (eg. mytarget.a.b)
  • $2: the extension of the target, if any (eg. .c.d)
  • $3: a temporary filename that the .do script should write its output to.

Instead of using $3, the .do script may also write the produced data to stdout.

If the .do file is in the same directory as the target, $1 and $3 are guaranteed to be simple filenames (with no path component). If the .do file is in a parent directory of the target, $1 and $3 will be relative paths (ie. will contain slashes).

redo is designed to update its targets atomically, and only if the do script succeeds (ie. returns a zero exit code). Thus, you should never write directly to the target file, only to $3 or stdout.

Normally, a .do script will call other .do scripts recursively, by running either redo (which will always build the sub-target) or redo-ifchange (which only rebuilds the sub-target if its dependencies have changed).

Running redo-ifchange is also the way your .do script declares dependencies on other targets; any target that is redo-ifchanged during your .do script's execution is both executed (if needed) and added as a dependency.

You may have heard that 'recursive make is considered harmful' (http://miller.emu.id.au/pmiller/books/rmch/). Unlike make(1), redo does correct locking, state management, and global dependency checking, so none of the arguments in that essay apply to redo. In fact, recursive redo is really the only kind of redo.

RELATED COMMANDS

When writing a .do script, it will probably need to run one or more of the following commands:

redo
to build a sub-target unconditionally.
redo-ifchange
to build a sub-target only if the sub-target's dependencies have changed.
redo-ifcreate
to tell redo that the current target must be rebuilt if a particular file gets created.
redo-always
to tell redo that the current target must always be rebuilt, even if someone calls it using redo-ifchange. (This might happen if the current target has dependencies other than the contents of files.)
redo-stamp
to tell redo that even though the current target has been rebuilt, it may not actually be any different from the previous version, so targets that depend on it might not need to be rebuilt. Often used in conjunction with redo-always to reduce the impact of always rebuilding a target.

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

sh(1), make(1), redo-ifchange(1), redo-ifcreate(1), redo-always(1), redo-stamp(1)