Like Perl, Python, and R, Ruby has its own build system for installing Ruby gems.


The RubyBuilder and RubyPackage base classes provide the following phases that can be overridden:

  1. build - build everything needed to install

  2. install - install everything from build directory

For packages that come with a *.gemspec file, these phases run:

$ gem build *.gemspec
$ gem install *.gem

For packages that come with a Rakefile file, these phases run:

$ rake package
$ gem install *.gem

For packages that come pre-packaged as a *.gem file, the build phase is skipped and the install phase runs:

$ gem install *.gem

These are all standard gem commands and can be found by running:

$ gem help commands

For packages that only distribute *.gem files, these files can be downloaded with the expand=False option in the version directive. The build phase will be automatically skipped.

Important files

When building from source, Ruby packages can be identified by the presence of any of the following files:

  • *.gemspec

  • Rakefile

  • setup.rb (not yet supported)

However, not all Ruby packages are released as source code. Some are only released as *.gem files. These files can be extracted using:

$ gem unpack *.gem


The *.gemspec file may contain something like:

summary = "An implementation of the AsciiDoc text processor and publishing toolchain"
description = "A fast, open source text processor and publishing toolchain for converting AsciiDoc content to HTML 5, DocBook 5, and other formats."

Either of these can be used for the description of the Spack package.


The *.gemspec file may contain something like:

homepage = ""

This should be used as the official homepage of the Spack package.

Build system dependencies

All Ruby packages require Ruby at build and run-time. For this reason, the base class contains:


The *.gemspec file may contain something like:

required_ruby_version = ">= 2.3.0"

This can be added to the Spack package using:

depends_on("ruby@2.3.0:", type=("build", "run"))

Ruby dependencies

When you install a package with gem, it reads the *.gemspec file in order to determine the dependencies of the package. If the dependencies are not yet installed, gem downloads them and installs them for you. This may sound convenient, but Spack cannot rely on this behavior for two reasons:

  1. Spack needs to be able to install packages on air-gapped networks.

    If there is no internet connection, gem can’t download the package dependencies. By explicitly listing every dependency in the, Spack knows what to download ahead of time.

  2. Duplicate installations of the same dependency may occur.

    Spack supports activation of Ruby extensions, which involves symlinking the package installation prefix to the Ruby installation prefix. If your package is missing a dependency, that dependency will be installed to the installation directory of the same package. If you try to activate the package + dependency, it may cause a problem if that package has already been activated.

For these reasons, you must always explicitly list all dependencies. Although the documentation may list the package’s dependencies, often the developers assume people will use gem and won’t have to worry about it. Always check the *.gemspec file to find the true dependencies.

Check for the following clues in the *.gemspec file:

  • add_runtime_dependency

    These packages are required for installation.

  • add_dependency

    This is an alias for add_runtime_dependency

  • add_development_dependency

    These packages are optional dependencies used for development. They should not be added as dependencies of the package.

External documentation

For more information on Ruby packaging, see: