---
name: "redo-ifchange"
section: "1"
platform: "linux"
package: "redo-python"
version: "0.42.d-6"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/redo-ifchange"
---

# redo-ifchange(1)

rebuild target files when source files have changed

## 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)](/redo/1.md).  See [**redo**(1)](/redo/1.md) 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  for any *target* that is out of date, it calls the equivalent of **redo target**.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)](/make/1.md), 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)](/redo/1.md) 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/1.md), [**redo-ifcreate**(1)](/redo-ifcreate/1.md), [**redo-always**(1)](/redo-always/1.md), [**redo-stamp**(1)](/redo-stamp/1.md)

## AUTHOR

Avery Pennarun \<apenwarr@gmail.com\>