This is a high-level overview of features that make Spack different from other package managers and port systems.

Installing the default version of a package is simple. This will install the latest version of the mpileaks package and all of its dependencies:

$ spack install mpileaks

Spack allows installation to be customized. Users can specify the version, build compiler, compile-time options, and cross-compile platform, all on the command line.

# Install a particular version by appending @
$ spack install mpileaks@1.1.2

# Specify a compiler (and its version), with %
$ spack install mpileaks@1.1.2 %gcc@4.7.3

# Add special compile-time options by name
$ spack install mpileaks@1.1.2 %gcc@4.7.3 debug=True

# Add special boolean compile-time options with +
$ spack install mpileaks@1.1.2 %gcc@4.7.3 +debug

# Add compiler flags using the conventional names
$ spack install mpileaks@1.1.2 %gcc@4.7.3 cppflags="-O3 -floop-block"

# Cross-compile for a different micro-architecture with target=
$ spack install mpileaks@1.1.2 target=icelake

Users can specify as many or few options as they care about. Spack will fill in the unspecified values with sensible defaults. The two listed syntaxes for variants are identical when the value is boolean.

Spack allows dependencies of a particular installation to be customized extensively. Suppose that hdf5 depends on openmpi and indirectly on hwloc. Using ^, users can add custom configurations for the dependencies:

# Install hdf5 and link it with specific versions of openmpi and hwloc
$ spack install hdf5@1.10.1 %gcc@4.7.3 +debug ^openmpi+cuda fabrics=auto ^hwloc+gl

Spack installs every unique package/dependency configuration into its own prefix, so new installs will not break existing ones.

Spack avoids library misconfiguration by using RPATH to link dependencies. When a user links a library or runs a program, it is tied to the dependencies it was built with, so there is no need to manipulate LD_LIBRARY_PATH at runtime.

To create a new packages, all Spack needs is a URL for the source archive. The spack create command will create a boilerplate package file, and the package authors can fill in specific build steps in pure Python.

For example, this command:

$ spack create http://www.mr511.de/software/libelf-0.8.13.tar.gz

creates a simple python file:

from spack import *


class Libelf(Package):
    """FIXME: Put a proper description of your package here."""

    # FIXME: Add a proper url for your package's homepage here.
    homepage = "http://www.example.com"
    url      = "http://www.mr511.de/software/libelf-0.8.13.tar.gz"

    version('0.8.13', '4136d7b4c04df68b686570afa26988ac')

    # FIXME: Add dependencies if required.
    # depends_on('foo')

    def install(self, spec, prefix):
        # FIXME: Modify the configure line to suit your build system here.
        configure('--prefix={0}'.format(prefix))

        # FIXME: Add logic to build and install here.
        make()
        make('install')

It doesn't take much python coding to get from there to a working package:

.. literalinclude:: _spack_root/var/spack/repos/builtin/packages/libelf/package.py
   :lines: 6-

Spack also provides wrapper functions around common commands like configure, make, and cmake to make writing packages simple.