An environment is used to group together a set of specs for the purpose of building, rebuilding and deploying in a coherent fashion. Environments provide a number of advantages over the à la carte approach of building and loading individual Spack modules:

1. Environments separate the steps of (a) choosing what to install, (b) concretizing, and (c) installing. This allows Environments to remain stable and repeatable, even if Spack packages are upgraded: specs are only re-concretized when the user explicitly asks for it. It is even possible to reliably transport environments between different computers running different versions of Spack!
2. Environments allow several specs to be built at once; a more robust solution than ad-hoc scripts making multiple calls to spack install.
3. An Environment that is built as a whole can be loaded as a whole into the user environment. An Environment can be built to maintain a filesystem view of its packages, and the environment can load that view into the user environment at activation time. Spack can also generate a script to load all modules related to an environment.

Other packaging systems also provide environments that are similar in some ways to Spack environments; for example, Conda environments or Python Virtual Environments. Spack environments provide some distinctive features:

1. A spec installed "in" an environment is no different from the same spec installed anywhere else in Spack. Environments are assembled simply by collecting together a set of specs.
2. Spack Environments may contain more than one spec of the same package.

Spack uses a "manifest and lock" model similar to Bundler gemfiles and other package managers. The user input file is named spack.yaml and the lock file is named spack.lock

Here we follow a typical use case of creating, concretizing, installing and loading an environment.

An environment is created by:

$ spack env create myenv

Spack then creates the directory var/spack/environments/myenv.

In the var/spack/environments/myenv directory, Spack creates the file spack.yaml and the hidden directory .spack-env.

Spack stores metadata in the .spack-env directory. User interaction will occur through the spack.yaml file and the Spack commands that affect it. When the environment is concretized, Spack will create a file spack.lock with the concrete information for the environment.

In addition to being the default location for the view associated with an Environment, the .spack-env directory also contains:

• repo/: A repo consisting of the Spack packages used in this environment. This allows the environment to build the same, in theory, even on different versions of Spack with different packages!
• logs/: A directory containing the build logs for the packages in this Environment.

Spack Environments can also be created from either a spack.yaml manifest or a spack.lock lockfile. To create an Environment from a spack.yaml manifest:

$ spack env create myenv spack.yaml

To create an Environment from a spack.lock lockfile:

$ spack env create myenv spack.lock

Either of these commands can also take a full path to the initialization file.

A Spack Environment created from a spack.yaml manifest is guaranteed to have the same root specs as the original Environment, but may concretize differently. A Spack Environment created from a spack.lock lockfile is guaranteed to have the same concrete specs as the original Environment. Either may obviously then differ as the user modifies it.

To activate an environment, use the following command:

$ spack env activate myenv

By default, the spack env activate will load the view associated with the Environment into the user environment. The -v, --with-view argument ensures this behavior, and the -V, --without-view argument activates the environment without changing the user environment variables.

The -p option to the spack env activate command modifies the user's prompt to begin with the environment name in brackets.

$ spack env activate -p myenv
[myenv] $ ...

To deactivate an environment, use the command:

$ spack env deactivate

or the shortcut alias

$ despacktivate

If the environment was activated with its view, deactivating the environment will remove the view from the user environment.

Any directory can be treated as an environment if it contains a file spack.yaml. To load an anonymous environment, use:

$ spack env activate -d /path/to/directory

Anonymous specs can be created in place using the command:

$ spack env create -d .

In this case Spack simply creates a spack.yaml file in the requested directory.

Spack commands are environment sensitive. For example, the find command shows only the specs in the active Environment if an Environment has been activated. Similarly, the install and uninstall commands act on the active environment.

$ spack find
==> 0 installed packages

$ spack install zlib@1.2.11
==> Installing zlib-1.2.11-q6cqrdto4iktfg6qyqcc5u4vmfmwb7iv
==> No binary for zlib-1.2.11-q6cqrdto4iktfg6qyqcc5u4vmfmwb7iv found: installing from source
==> zlib: Executing phase: 'install'
[+] ~/spack/opt/spack/linux-rhel7-broadwell/gcc-8.1.0/zlib-1.2.11-q6cqrdto4iktfg6qyqcc5u4vmfmwb7iv

$ spack env activate myenv

$ spack find
==> In environment myenv
==> No root specs
==> 0 installed packages

$ spack install zlib@1.2.8
==> Installing zlib-1.2.8-yfc7epf57nsfn2gn4notccaiyxha6z7x
==> No binary for zlib-1.2.8-yfc7epf57nsfn2gn4notccaiyxha6z7x found: installing from source
==> zlib: Executing phase: 'install'
[+] ~/spack/opt/spack/linux-rhel7-broadwell/gcc-8.1.0/zlib-1.2.8-yfc7epf57nsfn2gn4notccaiyxha6z7x
==> Updating view at ~/spack/var/spack/environments/myenv/.spack-env/view

$ spack find
==> In environment myenv
==> Root specs
zlib@1.2.8

==> 1 installed package
-- linux-rhel7-broadwell / gcc@8.1.0 ----------------------------
zlib@1.2.8

$ despacktivate

$ spack find
==> 2 installed packages
-- linux-rhel7-broadwell / gcc@8.1.0 ----------------------------
zlib@1.2.8  zlib@1.2.11

Note that when we installed the abstract spec zlib@1.2.8, it was presented as a root of the Environment. All explicitly installed packages will be listed as roots of the Environment.

All of the Spack commands that act on the list of installed specs are Environment-sensitive in this way, including install, uninstall, activate, deactivate, find, extensions, and more. In the :ref:`environment-configuration` section we will discuss Environment-sensitive commands further.

An abstract spec is the user-specified spec before Spack has applied any defaults or dependency information.

Users can add abstract specs to an Environment using the spack add command. The most important component of an Environment is a list of abstract specs.

Adding a spec adds to the manifest (the spack.yaml file), which is used to define the roots of the Environment, but does not affect the concrete specs in the lockfile, nor does it install the spec.

The spack add command is environment aware. It adds to the currently active environment. All environment aware commands can also be called using the spack -e flag to specify the environment.

$ spack env activate myenv
$ spack add mpileaks

or

$ spack -e myenv add python

Once some user specs have been added to an environment, they can be concretized. By default specs are concretized separately, one after the other. This mode of operation permits to deploy a full software stack where multiple configurations of the same package need to be installed alongside each other. Central installations done at HPC centers by system administrators or user support groups are a common case that fits in this behavior. Environments can also be configured to concretize all the root specs in a self-consistent way to ensure that each package in the environment comes with a single configuration. This mode of operation is usually what is required by software developers that want to deploy their development environment.

Regardless of which mode of operation has been chosen, the following command will ensure all the root specs are concretized according to the constraints that are prescribed in the configuration:

[myenv]$ spack concretize

In the case of specs that are not concretized together, the command above will concretize only the specs that were added and not yet concretized. Forcing a re-concretization of all the specs can be done instead with this command:

[myenv]$ spack concretize -f

When the -f flag is not used to reconcretize all specs, Spack guarantees that already concretized specs are unchanged in the environment.

The concretize command does not install any packages. For packages that have already been installed outside of the environment, the process of adding the spec and concretizing is identical to installing the spec assuming it concretizes to the exact spec that was installed outside of the environment.

The spack find command can show concretized specs separately from installed specs using the -c (--concretized) flag.

[myenv]$ spack add zlib
[myenv]$ spack concretize
[myenv]$ spack find -c
==> In environment myenv
==> Root specs
zlib

==> Concretized roots
-- linux-rhel7-x86_64 / gcc@4.9.3 -------------------------------
zlib@1.2.11

==> 0 installed packages

In addition to installing individual specs into an Environment, one can install the entire Environment at once using the command

[myenv]$ spack install

If the Environment has been concretized, Spack will install the concretized specs. Otherwise, spack install will first concretize the Environment and then install the concretized specs.

As it installs, spack install creates symbolic links in the logs/ directory in the Environment, allowing for easy inspection of build logs related to that environment. The spack install command also stores a Spack repo containing the package.py file used at install time for each package in the repos/ directory in the Environment.

The --no-add option can be used in a concrete environment to tell spack to install specs already present in the environment but not to add any new root specs to the environment. For root specs provided to spack install on the command line, --no-add is the default, while for dependency specs on the other hand, it is optional. In other words, if there is an unambiguous match in the active concrete environment for a root spec provided to spack install on the command line, spack does not require you to specify the --no-add option to prevent the spec from being added again. At the same time, a spec that already exists in the environment, but only as a dependency, will be added to the environment as a root spec without the --no-add option.

Once an environment has been installed, the following creates a load script for it:

$ spack env loads -r

This creates a file called loads in the environment directory. Sourcing that file in Bash will make the environment available to the user; and can be included in .bashrc files, etc. The loads file may also be copied out of the environment, renamed, etc.

A variety of Spack behaviors are changed through Spack configuration files, covered in more detail in the :ref:`configuration` section.

Spack Environments provide an additional level of configuration scope between the custom scope and the user scope discussed in the configuration documentation.

There are two ways to include configuration information in a Spack Environment:

1. Inline in the spack.yaml file
2. Included in the spack.yaml file from another file.

Many Spack commands also affect configuration information in files automatically. Those commands take a --scope argument, and the environment can be specified by env:NAME (to affect environment foo, set --scope env:foo). These commands will automatically manipulate configuration inline in the spack.yaml file.

Inline Environment-scope configuration is done using the same yaml format as standard Spack configuration scopes, covered in the :ref:`configuration` section. Each section is contained under a top-level yaml object with it's name. For example, a spack.yaml manifest file containing some package preference configuration (as in a packages.yaml file) could contain:

spack:
  ...
  packages:
    all:
      compiler: [intel]
  ...

This configuration sets the default compiler for all packages to intel.

Spack environments allow an include heading in their yaml schema. This heading pulls in external configuration files and applies them to the Environment.

spack:
  include:
  - relative/path/to/config.yaml
  - /absolute/path/to/packages.yaml

Environments can include files with either relative or absolute paths. Inline configurations take precedence over included configurations, so you don't have to change shared configuration files to make small changes to an individual Environment. Included configs listed earlier will have higher precedence, as the included configs are applied in reverse order.

The list of abstract/root specs in the Environment is maintained in the spack.yaml manifest under the heading specs.

spack:
    specs:
      - ncview
      - netcdf
      - nco
      - py-sphinx

Appending to this list in the yaml is identical to using the spack add command from the command line. However, there is more power available from the yaml file.

Specs can be concretized separately or together, as already explained in :ref:`environments_concretization`. The behavior active under any environment is determined by the concretization property:

spack:
    specs:
      - ncview
      - netcdf
      - nco
      - py-sphinx
    concretization: together

which can currently take either one of the two allowed values together or separately (the default).

Entries in the specs list can be individual abstract specs or a spec matrix.

A spec matrix is a yaml object containing multiple lists of specs, and evaluates to the cross-product of those specs. Spec matrices also contain an excludes directive, which eliminates certain combinations from the evaluated result.

The following two Environment manifests are identical:

spack:
  specs:
    - zlib %gcc@7.1.0
    - zlib %gcc@4.9.3
    - libelf %gcc@7.1.0
    - libelf %gcc@4.9.3
    - libdwarf %gcc@7.1.0
    - cmake

spack:
  specs:
    - matrix:
        - [zlib, libelf, libdwarf]
        - ['%gcc@7.1.0', '%gcc@4.9.3']
      exclude:
        - libdwarf%gcc@4.9.3
    - cmake

Spec matrices can be used to install swaths of software across various toolchains.

The concretization logic for spec matrices differs slightly from the rest of Spack. If a variant or dependency constraint from a matrix is invalid, Spack will reject the constraint and try again without it. For example, the following two Environment manifests will produce the same specs:

spack:
  specs:
    - matrix:
        - [zlib, libelf, hdf5+mpi]
        - [^mvapich2@2.2, ^openmpi@3.1.0]

spack:
  specs:
    - zlib
    - libelf
    - hdf5+mpi ^mvapich2@2.2
    - hdf5+mpi ^openmpi@3.1.0

This allows one to create toolchains out of combinations of constraints and apply them somewhat indiscriminately to packages, without regard for the applicability of the constraint.

The last type of possible entry in the specs list is a reference.

The Spack Environment manifest yaml schema contains an additional heading definitions. Under definitions is an array of yaml objects. Each object has one or two fields. The one required field is a name, and the optional field is a when clause.

The named field is a spec list. The spec list uses the same syntax as the specs entry. Each entry in the spec list can be a spec, a spec matrix, or a reference to an earlier named list. References are specified using the $ sigil, and are "splatted" into place (i.e. the elements of the referent are at the same level as the elements listed separately). As an example, the following two manifest files are identical.

spack:
  definitions:
    - first: [libelf, libdwarf]
    - compilers: ['%gcc', '%intel']
    - second:
        - $first
        - matrix:
            - [zlib]
            - [$compilers]
  specs:
    - $second
    - cmake

spack:
  specs:
    - libelf
    - libdwarf
    - zlib%gcc
    - zlib%intel
    - cmake

In short files like the example, it may be easier to simply list the included specs. However for more complicated examples involving many packages across many toolchains, separately factored lists make Environments substantially more manageable.

Additionally, the -l option to the spack add command allows one to add to named lists in the definitions section of the manifest file directly from the command line.

The when directive can be used to conditionally add specs to a named list. The when directive takes a string of Python code referring to a restricted set of variables, and evaluates to a boolean. The specs listed are appended to the named list if the when string evaluates to True. In the following snippet, the named list compilers is ['%gcc', '%clang', '%intel'] on x86_64 systems and ['%gcc', '%clang'] on all other systems.

spack:
  definitions:
    - compilers: ['%gcc', '%clang']
    - when: arch.satisfies('x86_64:')
      compilers: ['%intel']

The valid variables for a when clause are:

1. platform. The platform string of the default Spack architecture on the system.
2. os. The os string of the default Spack architecture on the system.
3. target. The target string of the default Spack architecture on the system.
4. architecture or arch. A Spack spec satisfying the default Spack architecture on the system. This supports querying via the satisfies method, as shown above.
5. arch_str. The architecture string of the default Spack architecture on the system.
6. re. The standard regex module in Python.
7. env. The user environment (usually os.environ in Python).
8. hostname. The hostname of the system (if hostname is an executable in the user's PATH).

Dependencies and compilers in Spack can be both packages in an environment and constraints on other packages. References to SpecLists allow a shorthand to treat packages in a list as either a compiler or a dependency using the $% or $^ syntax respectively.

For example, the following environment has three root packages: gcc@8.1.0, mvapich2@2.3.1 %gcc@8.1.0, and hdf5+mpi %gcc@8.1.0 ^mvapich2@2.3.1.

spack:
  definitions:
  - compilers: [gcc@8.1.0]
  - mpis: [mvapich2@2.3.1]
  - packages: [hdf5+mpi]

  specs:
  - $compilers
  - matrix:
    - [$mpis]
    - [$%compilers]
  - matrix:
    - [$packages]
    - [$^mpis]
    - [$%compilers]

This allows for a much-needed reduction in redundancy between packages and constraints.

Spack Environments can define filesystem views, which provide a direct access point for software similar to the directory hierarchy that might exist under /usr/local. Filesystem views are updated every time the environment is written out to the lock file spack.lock, so the concrete environment and the view are always compatible. The files of the view's installed packages are brought into the view by symbolic or hard links, referencing the original Spack installation, or by copy.

The Spack Environment manifest file has a top-level keyword view. Each entry under that heading is a view descriptor, headed by a name. Any number of views may be defined under the view heading. The view descriptor contains the root of the view, and optionally the projections for the view, select and exclude lists for the view and link information via link and link_type.

For example, in the following manifest file snippet we define a view named mpis, rooted at /path/to/view in which all projections use the package name, version, and compiler name to determine the path for a given package. This view selects all packages that depend on MPI, and excludes those built with the PGI compiler at version 18.5. The root specs with their (transitive) link and run type dependencies will be put in the view due to the link: all option, and the files in the view will be symlinks to the spack install directories.

spack:
  ...
  view:
    mpis:
      root: /path/to/view
      select: [^mpi]
      exclude: ['%pgi@18.5']
      projections:
        all: {name}/{version}-{compiler.name}
      link: all
      link_type: symlink

The default for the select and exclude values is to select everything and exclude nothing. The default projection is the default view projection ({}). The link attribute allows the following values:

1. link: all include root specs with their transitive run and link type dependencies (default);
2. link: run include root specs with their transitive run type dependencies;
3. link: roots include root specs without their dependencies.

The link_type defaults to symlink but can also take the value of hardlink or copy.

There are two shorthands for environments with a single view. If the environment at /path/to/env has a single view, with a root at /path/to/env/.spack-env/view, with default selection and exclusion and the default projection, we can put view: True in the environment manifest. Similarly, if the environment has a view with a different root, but default selection, exclusion, and projections, the manifest can say view: /path/to/view. These views are automatically named default, so that

spack:
  ...
  view: True

is equivalent to

spack:
  ...
  view:
    default:
      root: .spack-env/view

and

spack:
  ...
  view: /path/to/view

is equivalent to

spack:
  ...
  view:
    default:
      root: /path/to/view

By default, Spack environments are configured with view: True in the manifest. Environments can be configured without views using view: False. For backwards compatibility reasons, environments with no view key are treated the same as view: True.

From the command line, the spack env create command takes an argument --with-view [PATH] that sets the path for a single, default view. If no path is specified, the default path is used (view: True). The argument --without-view can be used to create an environment without any view configured.

The spack env view command can be used to change the manage views of an Environment. The subcommand spack env view enable will add a view named default to an environment. It takes an optional argument to specify the path for the new default view. The subcommand spack env view disable will remove the view named default from an environment if one exists. The subcommand spack env view regenerate will regenerate the views for the environment. This will apply any updates in the environment configuration that have not yet been applied.

The default projection into a view is to link every package into the root of the view. The projections attribute is a mapping of partial specs to spec format strings, defined by the :meth:`~spack.spec.Spec.format` function, as shown in the example below:

projections:
  zlib: {name}-{version}
  ^mpi: {name}-{version}/{^mpi.name}-{^mpi.version}-{compiler.name}-{compiler.version}
  all: {name}-{version}/{compiler.name}-{compiler.version}

The entries in the projections configuration file must all be either specs or the keyword all. For each spec, the projection used will be the first non-all entry that the spec satisfies, or all if there is an entry for all and no other entry is satisfied by the spec. Where the keyword all appears in the file does not matter.

Given the example above, the spec zlib@1.2.8 will be linked into /my/view/zlib-1.2.8/, the spec hdf5@1.8.10+mpi %gcc@4.9.3 ^mvapich2@2.2 will be linked into /my/view/hdf5-1.8.10/mvapich2-2.2-gcc-4.9.3, and the spec hdf5@1.8.10~mpi %gcc@4.9.3 will be linked into /my/view/hdf5-1.8.10/gcc-4.9.3.

If the keyword all does not appear in the projections configuration file, any spec that does not satisfy any entry in the file will be linked into the root of the view as in a single-prefix view. Any entries that appear below the keyword all in the projections configuration file will not be used, as all specs will use the projection under all before reaching those entries.

The spack env activate command will put the default view for the environment into the user's path, in addition to activating the environment for Spack commands. The arguments -v,--with-view and -V,--without-view can be used to tune this behavior. The default behavior is to activate with the environment view if there is one.

The environment variables affected by the spack env activate command and the paths that are used to update them are determined by the :ref:`prefix inspections <customize-env-modifications>` defined in your modules configuration; the defaults are summarized in the following table.

Each of these paths are appended to the view root, and added to the relevant variable if the path exists. For this reason, it is not recommended to use non-default projections with the default view of an environment.

The spack env deactivate command will remove the default view of the environment from the user's path.