Spack has many configuration files. Here is a quick list of them, in case you want to skip directly to specific docs:

• :ref:`compilers.yaml <compiler-config>`
• :ref:`concretizer.yaml <concretizer-options>`
• :ref:`config.yaml <config-yaml>`
• :ref:`mirrors.yaml <mirrors>`
• :ref:`modules.yaml <modules>`
• :ref:`packages.yaml <build-settings>`
• :ref:`repos.yaml <repositories>`

You can also add any of these as inline configuration in spack.yaml in an :ref:`environment <environment-configuration>`.

Spack configuration files are written in YAML. We chose YAML because it's human readable, but also versatile in that it supports dictionaries, lists, and nested sections. For more details on the format, see yaml.org and libyaml. Here is an example config.yaml file:

config:
  install_tree: $spack/opt/spack
  build_stage:
    - $tempdir/$user/spack-stage
    - ~/.spack/stage

Each Spack configuration file is nested under a top-level section corresponding to its name. So, config.yaml starts with config:, mirrors.yaml starts with mirrors:, etc.

Spack pulls configuration data from files in several directories. There are six configuration scopes. From lowest to highest:

1. defaults: Stored in $(prefix)/etc/spack/defaults/. These are the "factory" settings. Users should generally not modify the settings here, but should override them in other configuration scopes. The defaults here will change from version to version of Spack.
2. system: Stored in /etc/spack/. These are settings for this machine, or for all machines on which this file system is mounted. The site scope can be used for settings idiosyncratic to a particular machine, such as the locations of compilers or external packages. These settings are presumably controlled by someone with root access on the machine. They override the defaults scope.
3. site: Stored in $(prefix)/etc/spack/. Settings here affect only this instance of Spack, and they override the defaults and system scopes. The site scope can can be used for per-project settings (one Spack instance per project) or for site-wide settings on a multi-user machine (e.g., for a common Spack instance).
4. user: Stored in the home directory: ~/.spack/. These settings affect all instances of Spack and take higher precedence than site, system, or defaults scopes.
5. custom: Stored in a custom directory specified by --config-scope. If multiple scopes are listed on the command line, they are ordered from lowest to highest precedence.
6. environment: When using Spack :ref:`environments`, Spack reads additional configuration from the environment file. See :ref:`environment-configuration` for further details on these scopes. Environment scopes can be referenced from the command line as env:name (to reference environment foo, use env:foo).
7. command line: Build settings specified on the command line take precedence over all other scopes.

Each configuration directory may contain several configuration files, such as config.yaml, compilers.yaml, or mirrors.yaml. When configurations conflict, settings from higher-precedence scopes override lower-precedence settings.

Commands that modify scopes (e.g., spack compilers, spack repo, etc.) take a --scope=<name> parameter that you can use to control which scope is modified. By default, they modify the highest-precedence scope.

In addition to the defaults, system, site, and user scopes, you may add configuration scopes directly on the command line with the --config-scope argument, or -C for short.

For example, the following adds two configuration scopes, named scopea and scopeb, to a spack spec command:

$ spack -C ~/myscopes/scopea -C ~/myscopes/scopeb spec ncurses

Custom scopes come after the spack command and before the subcommand, and they specify a single path to a directory full of configuration files. You can add the same configuration files to that directory that you can add to any other scope (config.yaml, packages.yaml, etc.).

If multiple scopes are provided:

1. Each must be preceded with the --config-scope or -C flag.
2. They must be ordered from lowest to highest precedence.

Suppose that you need to support simultaneous building of release and development versions of mypackage, where mypackage -> A -> B. You could create The following files:

packages:
    mypackage:
        version: [1.7]
    A:
        version: [2.3]
    B:
        version: [0.8]

packages:
    mypackage:
        version: [develop]
    A:
        version: [develop]
    B:
        version: [develop]

You can switch between release and develop configurations using configuration arguments. You would type spack -C ~/myscopes/release when you want to build the designated release versions of mypackage, A, and B, and you would type spack -C ~/myscopes/develop when you want to build all of these packages at the develop version.

Suppose that you need to build two software packages, packagea and packageb. packagea is Python 2-based and packageb is Python 3-based. packagea only builds with OpenMPI and packageb only builds with MPICH. You can create different configuration scopes for use with packagea and packageb:

packages:
    python:
        version: [2.7.11]
    all:
        providers:
            mpi: [openmpi]

packages:
    python:
        version: [3.5.2]
    all:
        providers:
            mpi: [mpich]

For each scope above (excluding environment scopes), there can also be platform-specific settings. For example, on most platforms, GCC is the preferred compiler. However, on macOS (darwin), Clang often works for more packages, and is set as the default compiler. This configuration is set in $(prefix)/etc/spack/defaults/darwin/packages.yaml. It will take precedence over settings in the defaults scope, but can still be overridden by settings in system, system/darwin, site, site/darwin, user, user/darwin, custom, or custom/darwin. So, the full scope precedence is:

1. defaults
2. defaults/<platform>
3. system
4. system/<platform>
5. site
6. site/<platform>
7. user
8. user/<platform>
9. custom
10. custom/<platform>

You can get the name to use for <platform> by running spack arch --platform. The system config scope has a <platform> section for sites at which /etc is mounted on multiple heterogeneous machines.

When spack queries for configuration parameters, it searches in higher-precedence scopes first. So, settings in a higher-precedence file can override those with the same key in a lower-precedence one. For list-valued settings, Spack prepends higher-precedence settings to lower-precedence settings. Completely ignoring higher-level configuration options is supported with the :: notation for keys (see :ref:`config-overrides` below).

Let's look at an example of overriding a single key in a Spack file. If your configurations look like this:

config:
  install_tree: $spack/opt/spack
  build_stage:
    - $tempdir/$user/spack-stage
    - ~/.spack/stage

config:
  install_tree: /some/other/directory

Spack will only override install_tree in the config section, and will take the site preferences for other settings. You can see the final, combined configuration with the spack config get <configtype> command:

$ spack config get config
config:
  install_tree: /some/other/directory
  build_stage:
    - $tempdir/$user/spack-stage
    - ~/.spack/stage

Above, the user config.yaml only overrides specific settings in the default config.yaml. Sometimes, it is useful to completely override lower-precedence settings. To do this, you can use two colons at the end of a key in a configuration file. For example:

config::
  install_tree: /some/other/directory

Spack will ignore all lower-precedence configuration under the config:: section:

$ spack config get config
config:
  install_tree: /some/other/directory

Let's revisit the config.yaml example one more time. The build_stage setting's value is an ordered list of directories:

build_stage:
  - $tempdir/$user/spack-stage
  - ~/.spack/stage

Suppose the user configuration adds its own list of build_stage paths:

build_stage:
  - /lustre-scratch/$user/spack
  - ~/mystage

Spack will first look at the paths in the defaults config.yaml, then the paths in the user's ~/.spack/config.yaml. The list in the higher-precedence scope is prepended to the defaults. spack config get config shows the result:

$ spack config get config
config:
  install_tree: /some/other/directory
  build_stage:
    - /lustre-scratch/$user/spack
    - ~/mystage
    - $tempdir/$user/spack-stage
    - ~/.spack/stage

As in :ref:`config-overrides`, the higher-precedence scope can completely override the lower-precedence scope using ::. So if the user config looked like this:

build_stage::
  - /lustre-scratch/$user/spack
  - ~/mystage

The merged configuration would look like this:

$ spack config get config
config:
  install_tree: /some/other/directory
  build_stage:
    - /lustre-scratch/$user/spack
    - ~/mystage

Spack understands several variables which can be used in config file paths wherever they appear. There are three sets of these variables: Spack-specific variables, environment variables, and user path variables. Spack-specific variables and environment variables are both indicated by prefixing the variable name with $. User path variables are indicated at the start of the path with ~ or ~user.

Spack understands several special variables. These are:

• $env: name of the currently active :ref:`environment <environments>`
• $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
• $user_cache_path: user cache directory (~/.spack unless :ref:`overridden <local-config-overrides>`)

Note that, as with shell variables, you can write these as $varname or with braces to distinguish the variable from surrounding characters: ${varname}. Their names are also case insensitive, meaning that $SPACK works just as well as $spack. These special variables are substituted first, so any environment variables with the same name will not be used.

After Spack-specific variables are evaluated, environment variables are expanded. These are formatted like Spack-specific variables, e.g., ${varname}. You can use this to insert environment variables in your Spack configuration.

Spack performs Unix-style tilde expansion on paths in configuration files. This means that tilde (~) will expand to the current user's home directory, and ~user will expand to a specified user's home directory. The ~ must appear at the beginning of the path, or Spack will not expand it.

Spack allows to prescribe custom environment modifications in a few places within its configuration files. Every time these modifications are allowed they are specified as a dictionary, like in the following example:

environment:
  set:
    LICENSE_FILE: '/path/to/license'
  unset:
  - CPATH
  - LIBRARY_PATH
  append_path:
    PATH: '/new/bin/dir'

The possible actions that are permitted are set, unset, append_path, prepend_path and finally remove_path. They all require a dictionary of variable names mapped to the values used for the modification. The only exception is unset that requires just a list of variable names. No particular order is ensured on the execution of each of these modifications.

With so many scopes overriding each other, it can sometimes be difficult to understand what Spack's final configuration looks like.

Spack provides two useful ways to view the final "merged" version of any configuration file: spack config get and spack config blame.

spack config get shows a fully merged configuration file, taking into account all scopes. For example, to see the fully merged config.yaml, you can type:

$ spack config get config
config:
  debug: false
  checksum: true
  verify_ssl: true
  dirty: false
  build_jobs: 8
  install_tree: $spack/opt/spack
  template_dirs:
  - $spack/templates
  directory_layout: {architecture}/{compiler.name}-{compiler.version}/{name}-{version}-{hash}
  build_stage:
  - $tempdir/$user/spack-stage
  - ~/.spack/stage
  - $spack/var/spack/stage
  source_cache: $spack/var/spack/cache
  misc_cache: ~/.spack/cache
  locks: true

Likewise, this will show the fully merged packages.yaml:

$ spack config get packages

You can use this in conjunction with the -C / --config-scope argument to see how your scope will affect Spack's configuration:

$ spack -C /path/to/my/scope config get packages

spack config blame functions much like spack config get, but it shows exactly which configuration file each preference came from. If you do not know why Spack is behaving a certain way, this can help you track down the problem:

$ spack --insecure -C ./my-scope -C ./my-scope-2 config blame config
==> Warning: You asked for --insecure. Will NOT check SSL certificates.
---                                                   config:
_builtin                                                debug: False
/home/myuser/spack/etc/spack/defaults/config.yaml:72    checksum: True
command_line                                            verify_ssl: False
./my-scope-2/config.yaml:2                              dirty: False
_builtin                                                build_jobs: 8
./my-scope/config.yaml:2                                install_tree: /path/to/some/tree
/home/myuser/spack/etc/spack/defaults/config.yaml:23    template_dirs:
/home/myuser/spack/etc/spack/defaults/config.yaml:24    - $spack/templates
/home/myuser/spack/etc/spack/defaults/config.yaml:28    directory_layout: {architecture}/{compiler.name}-{compiler.version}/{name}-{version}-{hash}
/home/myuser/spack/etc/spack/defaults/config.yaml:49    build_stage:
/home/myuser/spack/etc/spack/defaults/config.yaml:50    - $tempdir/$user/spack-stage
/home/myuser/spack/etc/spack/defaults/config.yaml:51    - ~/.spack/stage
/home/myuser/spack/etc/spack/defaults/config.yaml:52    - $spack/var/spack/stage
/home/myuser/spack/etc/spack/defaults/config.yaml:57    source_cache: $spack/var/spack/cache
/home/myuser/spack/etc/spack/defaults/config.yaml:62    misc_cache: ~/.spack/cache
/home/myuser/spack/etc/spack/defaults/config.yaml:86    locks: True

You can see above that the build_jobs and debug settings are built in and are not overridden by a configuration file. The verify_ssl setting comes from the --insceure option on the command line. dirty and install_tree come from the custom scopes ./my-scope and ./my-scope-2, and all other configuration options come from the default configuration files that ship with Spack.

Spack's system and user scopes provide ways for administrators and users to set global defaults for all Spack instances, but for use cases where one wants a clean Spack installation, these scopes can be undesirable. For example, users may want to opt out of global system configuration, or they may want to ignore their own home directory settings when running in a continuous integration environment.

Spack also, by default, keeps various caches and user data in ~/.spack, but users may want to override these locations.

Spack provides three environment variables that allow you to override or opt out of configuration locations:

• SPACK_USER_CONFIG_PATH: Override the path to use for the user scope (~/.spack by default).
• SPACK_SYSTEM_CONFIG_PATH: Override the path to use for the system scope (/etc/spack by default).
• SPACK_DISABLE_LOCAL_CONFIG: set this environment variable to completely disable both the system and user configuration directories. Spack will only consider its own defaults and site configuration locations.

And one that allows you to move the default cache location:

• SPACK_USER_CACHE_PATH: Override the default path to use for user data (misc_cache, tests, reports, etc.)

With these settings, if you want to isolate Spack in a CI environment, you can do this:

export SPACK_DISABLE_LOCAL_CONFIG=true
export SPACK_USER_CACHE_PATH=/tmp/spack