Basic settings in config.yaml

Spack’s basic configuration options are set in config.yaml. You can see the default settings by looking at etc/spack/defaults/config.yaml:

# -------------------------------------------------------------------------
# This is the default spack configuration file.
# Settings here are versioned with Spack and are intended to provide
# sensible defaults out of the box. Spack maintainers should edit this
# file to keep it current.
# Users can override these settings by editing the following files.
# Per-spack-instance settings (overrides defaults):
#   $SPACK_ROOT/etc/spack/config.yaml
# Per-user settings (overrides default and site settings):
#   ~/.spack/config.yaml
# -------------------------------------------------------------------------
  # This is the path to the root of the Spack install tree.
  # You can use $spack here to refer to the root of the spack instance.
  install_tree: $spack/opt/spack

  # Locations where different types of modules should be installed.
    tcl:    $spack/share/spack/modules
    lmod:   $spack/share/spack/lmod
    dotkit: $spack/share/spack/dotkit

  # Temporary locations Spack can try to use for builds.
  # Spack will use the first one it finds that exists and is writable.
  # You can use $tempdir to refer to the system default temp directory
  # (as returned by tempfile.gettempdir()).
  # A value of $spack/var/spack/stage indicates that Spack should run
  # builds directly inside its install directory without staging them in
  # temporary space.
  # The build stage can be purged with `spack purge --stage`.
    - $tempdir
    - /nfs/tmp2/$user
    - $spack/var/spack/stage

  # Cache directory already downloaded source tarballs and archived
  # repositories. This can be purged with `spack purge --downloads`.
  source_cache: $spack/var/spack/cache

  # Cache directory for miscellaneous files, like the package index.
  # This can be purged with `spack purge --misc-cache`
  misc_cache: ~/.spack/cache

  # If this is false, tools like curl that use SSL will not verify
  # certifiates. (e.g., curl will use use the -k option)
  verify_ssl: true

  # If set to true, Spack will always check checksums after downloading
  # archives. If false, Spack skips the checksum step.
  checksum: true

  # If set to true, `spack install` and friends will NOT clean
  # potentially harmful variables from the build environment. Use wisely.
  dirty: false

  # The default number of jobs to use when running `make` in parallel.
  # If set to 4, for example, `spack install` will run `make -j4`.
  # If not set, all available cores are used by default.
  # build_jobs: 4

These settings can be overridden in etc/spack/config.yaml or ~/.spack/config.yaml. See Configuration Scopes for details.

Config file variables

You may notice some variables prefixed with $ in the settings above. Spack understands several variables that can be used in values of configuration parameters. They are:

  • $spack: path to the prefix of this spack installation
  • $tempdir: default system temporary directory (as specified in Python’s tempfile.tempdir variable.
  • $user: name of the current user

Note that, as with shell variables, you can write these as $varname or with braces to distinguish the variable from surrounding characters: ${varname}.


The location where Spack will install packages and their dependencies. Default is $spack/opt/spack.

install_hash_length and install_path_scheme

The default Spack installation path can be very long and can create problems for scripts with hardcoded shebangs. There are two parameters to help with that. Firstly, the install_hash_length parameter can set the length of the hash in the installation path from 1 to 32. The default path uses the full 32 characters.

Secondly, it is also possible to modify the entire installation scheme. By default Spack uses ${ARCHITECTURE}/${COMPILERNAME}-${COMPILERVER}/${PACKAGE}-${VERSION}-${HASH} where the tokens that are available for use in this directive are the same as those understood by the Spec.format method. Using this parameter it is possible to use a different package layout or reduce the depth of the installation paths. For example

  install_path_scheme: '${PACKAGE}/${VERSION}/${HASH:7}'

would install packages into sub-directories using only the package name, version and a hash length of 7 characters.

When using either parameter to set the hash length it only affects the representation of the hash in the installation directory. You should be aware that the smaller the hash length the more likely naming conflicts will occur. These parameters are independent of those used to configure module names.


Modifying the installation hash length or path scheme after packages have been installed will prevent Spack from being able to find the old installation directories.


Controls where Spack installs generated module files. You can customize the location for each type of module. e.g.:

  tcl:    $spack/share/spack/modules
  lmod:   $spack/share/spack/lmod
  dotkit: $spack/share/spack/dotkit

See Modules for details.


Spack is designed to run out of a user home directory, and on many systems the home directory is a (slow) network filesystem. On most systems, building in a temporary filesystem results in faster builds than building in the home directory. Usually, there is also more space available in the temporary location than in the home directory. So, Spack tries to create build stages in temporary space.

By default, Spack’s build_stage is configured like this:

 - $tempdir
 - /nfs/tmp2/$user
 - $spack/var/spack/stage

This is an ordered list of paths that Spack should search when trying to find a temporary directory for the build stage. The list is searched in order, and Spack will use the first directory to which it has write access. See Config file variables for more on $tempdir and $spack.

When Spack builds a package, it creates a temporary directory within the build_stage, and it creates a symbolic link to that directory in $spack/var/spack/stage. This is used to track the stage.

After a package is successfully installed, Spack deletes the temporary directory it used to build. Unsuccessful builds are not deleted, but you can manually purge them with spack purge –stage.


The last item in the list is $spack/var/spack/stage. If this is the only writable directory in the build_stage list, Spack will build directly in $spack/var/spack/stage and will not link to temporary space.


Location to cache downloaded tarballs and repositories. By default these are stored in $spack/var/spack/cache. These are stored indefinitely by default. Can be purged with spack purge –downloads.


Temporary directory to store long-lived cache files, such as indices of packages available in repositories. Defaults to ~/.spack/cache. Can be purged with spack purge –misc-cache.


When set to true (default) Spack will verify certificates of remote hosts when making ssl connections. Set to false to disable, and tools like curl will use their --insecure options. Disabling this can expose you to attacks. Use at your own risk.


When set to true, Spack verifies downloaded source code using a checksum, and will refuse to build packages that it cannot verify. Set to false to disable these checks. Disabling this can expose you to attacks. Use at your own risk.


By default, Spack unsets variables in your environment that can change the way packages build. This includes LD_LIBRARY_PATH, CPATH, LIBRARY_PATH, DYLD_LIBRARY_PATH, and others.

By default, builds are clean, but on some machines, compilers and other tools may need custom LD_LIBRARY_PATH settings to run. You can set dirty to true to skip the cleaning step and make all builds “dirty” by default. Be aware that this will reduce the reproducibility of builds.


Unless overridden in a package or on the command line, Spack builds all packages in parallel. For a build system that uses Makefiles, this means running make -j<build_jobs>, where build_jobs is the number of threads to use.

The default parallelism is equal to the number of cores on your machine. If you work on a shared login node or have a strict ulimit, it may be necessary to set the default to a lower value. By setting build_jobs to 4, for example, commands like spack install will run make -j4 instead of hogging every core.

To build all software in serial, set build_jobs to 1.