Custom Build Systems

While the built-in build systems should meet your needs for the vast majority of packages, some packages provide custom build scripts. This guide is intended for the following use cases:

  • Packaging software with its own custom build system

  • Adding support for new build systems

If you want to add support for a new build system, a good place to start is to look at the definitions of other build systems. This guide focuses mostly on how Spack’s build systems work.

In this guide, we will be using the perl and cmake packages as examples. perl’s build system is a hand-written Configure shell script, while cmake bootstraps itself during installation. Both of these packages require custom build systems.

Base class

If your package does not belong to any of the built-in build systems that Spack already supports, you should inherit from the Package base class. Package is a simple base class with a single phase: install. If your package is simple, you may be able to simply write an install method that gets the job done. However, if your package is more complex and installation involves multiple steps, you should add separate phases as mentioned in the next section.

If you are creating a new build system base class, you should inherit from PackageBase. This is the superclass for all build systems in Spack.


The most important concept in Spack’s build system support is the idea of phases. Each build system defines a set of phases that are necessary to install the package. They usually follow some sort of “configure”, “build”, “install” guideline, but any of those phases may be missing or combined with another phase.

If you look at the perl package, you’ll see:

phases = ["configure", "build", "install"]

Similarly, cmake defines:

phases = ["bootstrap", "build", "install"]

If we look at the cmake example, this tells Spack’s PackageBase class to run the bootstrap, build, and install functions in that order. It is now up to you to define these methods.

Phase and phase_args functions

If we look at perl, we see that it defines a configure method:

def configure(self, spec, prefix):
    configure = Executable("./Configure")

There is also a corresponding configure_args function that handles all of the arguments to pass to Configure, just like in AutotoolsPackage. Comparatively, the build and install phases are pretty simple:

def build(self, spec, prefix):

def install(self, spec, prefix):

The cmake package looks very similar, but with a bootstrap function instead of configure:

def bootstrap(self, spec, prefix):
    bootstrap = Executable("./bootstrap")

def build(self, spec, prefix):

def install(self, spec, prefix):

Again, there is a boostrap_args function that determines the correct bootstrap flags to use.


Occasionally, you may want to run extra steps either before or after a given phase. This applies not just to custom build systems, but to existing build systems as well. You may need to patch a file that is generated by configure, or install extra files in addition to what make install copies to the installation prefix. This is where @run_before and @run_after come in.

These Python decorators allow you to write functions that are called before or after a particular phase. For example, in perl, we see:

def install_cpanm(self):
    spec = self.spec

    if spec.satisfies("+cpanm"):
        with working_dir(join_path("cpanm", "cpanm")):
            perl = spec["perl"].command

This extra step automatically installs cpanm in addition to the base Perl installation.


The run_before/run_after logic discussed above becomes particularly powerful when combined with the @on_package_attributes decorator. This decorator allows you to conditionally run certain functions depending on the attributes of that package. The most common example is conditional testing. Many unit tests are prone to failure, even when there is nothing wrong with the installation. Unfortunately, non-portable unit tests and tests that are “supposed to fail” are more common than we would like. Instead of always running unit tests on installation, Spack lets users conditionally run tests with the --test=root flag.

If we wanted to define a function that would conditionally run if and only if this flag is set, we would use the following line:



Let’s put everything together and add unit tests to be optionally run during the installation of our package. In the perl package, we can see:

def test(self):

As you can guess, this runs make test after building the package, if and only if testing is requested. Again, this is not specific to custom build systems, it can be added to existing build systems as well.


The order of decorators matters. The following ordering:


works as expected. However, if you reverse the ordering:


the tests will always be run regardless of whether or not --test=root is requested. See for more information

Ideally, every package in Spack will have some sort of test to ensure that it was built correctly. It is up to the package authors to make sure this happens. If you are adding a package for some software and the developers list commands to test the installation, please add these tests to your

For more information on other forms of package testing, refer to Checking an installation.