..
   Copyright Spack Project Developers. See COPYRIGHT file for details.

   SPDX-License-Identifier: (Apache-2.0 OR MIT)

.. meta::
   :description lang=en:
      Learn how to use Spack environments to manage reproducible software stacks, making it easy to share and recreate specific sets of packages and their dependencies.

.. _environments:

Environments (spack.yaml, spack.lock)
=====================================

An environment is used to group a set of specs intended for some purpose to be built, rebuilt, and deployed in a coherent fashion.
Environments define aspects of the installation of the software, such as:

#. *which* specs to install;
#. *how* those specs are configured; and
#. *where* the concretized software will be installed.

Aggregating this information into an environment for processing has advantages over the *à la carte* approach of building and loading individual Spack modules.

With environments, you concretize, install, or load (activate) all of the specs with a single command.
Concretization fully configures the specs and dependencies of the environment in preparation for installing the software.
This is a more robust solution than ad-hoc installation scripts.
And you can share an environment or even re-use it on a different computer.

Environment definitions, especially *how* specs are configured, allow the software to remain stable and repeatable even when Spack packages are upgraded.
Changes are only picked up when the environment is explicitly re-concretized.

Defining *where* specs are installed supports a filesystem view of the environment.
Yet Spack maintains a single installation of the software that can be re-used across multiple environments.

Activating an environment determines *when* all of the associated (and installed) specs are loaded so limits the software loaded to those specs actually needed by the environment.
Spack can even 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 <https://conda.io/docs/user-guide/tasks/manage-environments.html>`_ or `Python Virtual Environments <https://docs.python.org/3/tutorial/venv.html>`_.
Spack environments provide some distinctive features though:

#. A spec installed "in" an environment is no different from the same spec installed anywhere else in Spack.
#. Spack environments may contain more than one spec of the same package.

Spack uses a "manifest and lock" model similar to `Bundler gemfiles <https://bundler.io/man/gemfile.5.html>`_ and other package managers.
The environment's user input file (or manifest), is named ``spack.yaml``.
The lock file, which contains the fully configured and concretized specs, is named ``spack.lock``.

.. _environments-using:

Using Environments
------------------

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

.. _cmd-spack-env-create:

Creating a managed Environment
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

An environment is created by:

.. code-block:: console

   $ spack env create myenv

The directory ``$SPACK_ROOT/var/spack/environments/myenv`` is created to manage the environment.

.. note::

   By default, all managed environments are stored in the ``$SPACK_ROOT/var/spack/environments`` folder.
   This location can be changed by setting the ``environments_root`` variable in ``config.yaml``.

Spack creates the file ``spack.yaml``, hidden directory ``.spack-env``, and ``spack.lock`` file under ``$SPACK_ROOT/var/spack/environments/myenv``.
User interaction occurs through the ``spack.yaml`` file and the Spack commands that affect it.
Metadata and, by default, the view are stored in the ``.spack-env`` directory.
When the environment is concretized, Spack creates the ``spack.lock`` file with the fully configured specs and dependencies for the environment.

The ``.spack-env`` subdirectory also contains:

* ``repo/``: A subdirectory acting as the repo consisting of the Spack packages used in the environment.
  It allows the environment to build the same, in theory, even on different versions of Spack with different packages!
* ``logs/``: A subdirectory containing the build logs for the packages in this environment.

Spack Environments can also be created from another environment.
Environments can be created from the manifest file (the user input), the lockfile, or the entire environment at once.
Create an environment from a manifest using:

.. code-block:: console

   $ spack env create myenv spack.yaml

The resulting environment is guaranteed to have the same root specs as the original but may concretize differently in the presence of different explicit or default configuration settings (e.g., a different version of Spack or for a different user account).

Environments created from a manifest will copy any included configs from relative paths inside the environment.
Relative paths from outside the environment will cause errors, and absolute paths will be kept absolute.
For example, if ``spack.yaml`` includes:

.. code-block:: yaml

   spack:
     include: [./config.yaml]

then the created environment will have its own copy of the file ``config.yaml`` copied from the location in the original environment.

Create an environment from a ``spack.lock`` file using:

.. code-block:: console

   $ spack env create myenv spack.lock

The resulting environment, when on the same or a compatible machine, is guaranteed to initially have the same concrete specs as the original.

Create an environment from an entire environment using either the environment name or path:

.. code-block:: console

   $ spack env create myenv /path/to/env
   $ spack env create myenv2 myenv

The resulting environment will include the concrete specs from the original if the original is concretized (as when created from a lockfile) and all of the config options and abstract specs specified in the original (as when created from a manifest file).
It will also include any other files included in the environment directory, such as repos or source code, as they could be referenced in the environment by relative path.

.. note::

   Environment creation also accepts a full path to the file.

   If the path is not under the ``$SPACK_ROOT/var/spack/environments`` directory then the source is referred to as an :ref:`independent environment <independent_environments>`.

The name of an environment can be a nested path to help organize environments via subdirectories.

.. code-block:: console

   $ spack env create projectA/configA/myenv

This will create a managed environment under ``$environments_root/projectA/configA/myenv``.
Changing ``environment_root`` can therefore also be used to make a whole group of nested environments available.

.. _cmd-spack-env-activate:
.. _cmd-spack-env-deactivate:

Activating an Environment
^^^^^^^^^^^^^^^^^^^^^^^^^

To activate an environment, use the following command:

.. code-block:: console

   $ 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.

.. code-block:: console

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

The ``activate`` command can also be used to create a new environment, if it is not already defined, by adding the ``--create`` flag.
Managed and independent environments can both be created using the same flags that `spack env create` accepts.
If an environment already exists then Spack will simply activate it and ignore the create-specific flags.

.. code-block:: console

   $ spack env activate --create -p myenv
   # ...
   # [creates if myenv does not exist yet]
   # ...
   [myenv] $ ...

To deactivate an environment, use the command:

.. code-block:: console

   $ spack env deactivate

or the shortcut alias

.. code-block:: console

   $ despacktivate

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

.. _independent_environments:

Independent Environments
^^^^^^^^^^^^^^^^^^^^^^^^

Independent environments can be located in any directory outside of Spack.

.. note::

   When uninstalling packages, Spack asks the user to confirm the removal of packages that are still used in a managed environment.
   This is not the case for independent environments.

To create an independent environment, use one of the following commands:

.. code-block:: console

   $ spack env create --dir my_env
   $ spack env create ./my_env

As a shorthand, you can also create an independent environment upon activation if it does not already exist:

.. code-block:: console

   $ spack env activate --create ./my_env

For convenience, Spack can also place an independent environment in a temporary directory for you:

.. code-block:: console

   $ spack env activate --temp


Environment-Aware Commands
^^^^^^^^^^^^^^^^^^^^^^^^^^

Spack commands are environment-aware.
For example, the ``find`` command shows only the specs in the active environment if an environment has been activated.
Otherwise it shows all specs in the Spack instance.
The same rule applies to the ``install`` and ``uninstall`` commands.

.. code-block:: spec

  $ 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-aware in this way, including ``install``, ``uninstall``, ``find``, ``extensions``, etc.
In the :ref:`environment-configuration` section we will discuss environment-aware commands further.

.. _cmd-spack-add:

Adding Abstract Specs
^^^^^^^^^^^^^^^^^^^^^

An abstract spec is the user-specified spec before Spack applies 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 it as a root spec of the environment in the user input file (``spack.yaml``).
It does not affect the concrete specs in the lock file (``spack.lock``) and it does not install the spec.

The ``spack add`` command is environment-aware.
It adds the spec to the currently active environment.
An error is generated if there isn't an active environment.
All environment-aware commands can also be called using the ``spack -e`` flag to specify the environment.

.. code-block:: spec

   $ spack env activate myenv
   $ spack add mpileaks

or

.. code-block:: spec

   $ spack -e myenv add python

.. _cmd-spack-concretize:

Concretizing
^^^^^^^^^^^^

Once user specs have been added to an environment, they can be concretized.
There are three different modes of operation to concretize an environment, explained in detail in :ref:`environments_concretization_config`.
Regardless of which mode of operation is chosen, the following command will ensure all of the root specs are concretized according to the constraints that are prescribed in the configuration:

.. code-block:: console

   [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 of the specs can be done by adding the ``-f`` option:

.. code-block:: console

   [myenv]$ spack concretize -f

Without the option, 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.

.. code-block:: console

  [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


.. _installing-environment:

Installing an Environment
^^^^^^^^^^^^^^^^^^^^^^^^^

In addition to adding individual specs to an environment, one can install the entire environment at once using the command

.. code-block:: console

   [myenv]$ spack install

If the environment has been concretized, Spack will install the concretized specs.
Otherwise, ``spack install`` will concretize the environment before installing the concretized specs.

.. note::

   Every ``spack install`` process builds one package at a time with multiple build jobs, controlled by the ``-j`` flag and the ``config:build_jobs`` option (see :ref:`build-jobs`).
   To speed up environment builds further, independent packages can be installed in parallel by launching more Spack instances.
   For example, the following will build at most four packages in parallel using three background jobs:

   .. code-block:: console

      [myenv]$ spack install & spack install & spack install & spack install

   Another option is to generate a ``Makefile`` and run ``make -j<N>`` to control the number of parallel install processes.
   See :ref:`cmd-spack-env-depfile` for details.


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, 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.

.. _cmd-spack-develop:

Developing Packages in a Spack Environment
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``spack develop`` command allows one to develop Spack packages in an environment.
It will configure Spack to install the package from local source.
By default, ``spack develop`` will also clone the package to a subdirectory in the environment for the local source.
These choices can be overridden with the ``--path`` argument, and the ``--no-clone`` argument.
Relative paths provided to the ``--path`` argument will be resolved relative to the environment directory.
All of these options are recorded in the environment manifest, although default values may be left implied.

.. code-block:: console

   $ spack develop --path src/foo foo@develop
   $ cat `spack location -e`/spack.yaml
   spack:
     ...
     develop
       foo:
         spec: foo@develop
         path: src/foo

When ``spack develop`` is run in a concretized environment, Spack will modify the concrete specs in the environment to reflect the modified provenance.
Any package built from local source will have a ``dev_path`` variant, and the hash of any dependent of those packages will be modified to reflect the change.
The value of the ``dev_path`` variant will be the absolute path to the package source directory.
If the develop spec conflicts with the concrete specs in the environment, Spack will raise an exception and require the ``spack develop --no-modify-concrete-specs`` option, followed by a ``spack concretize --force`` to apply the ``dev_path`` variant and constraints from the develop spec.

When concretizing an environment with develop specs, the version, variants, and other attributes of the spec provided to the ``spack develop`` command will be treated as constraints by the concretizer (in addition to any constraints from the packages ``specs`` list).
If the ``develop`` configuration for the package does not include a spec version, Spack will choose the **highest** version of the package.
This means that any "infinity" versions (``develop``, ``main``, etc.) will be preferred for specs marked with the ``spack develop`` command, which is different from the standard Spack behavior to prefer the highest **numeric** version.
These packages will have an automatic ``dev_path`` variant added by the concretizer, with a value of the absolute path to the local source Spack is building from.

Spack will ensure the package and its dependents are rebuilt any time the environment is installed if the package's local source code has been modified.
Spack's native implementation is to check if ``mtime`` is newer than the installation.
A custom check can be created by overriding the ``detect_dev_src_change`` method in your package class.
This is particularly useful for projects using custom Spack repos to drive development and want to optimize performance.

When ``spack develop`` is run without any arguments, Spack will clone any develop specs in the environment for which the specified path does not exist.

When working deep in the graph it is often desirable to have multiple specs marked as ``develop`` so you don't have to restage and/or do full rebuilds each time you call ``spack install``.
The ``--recursive`` flag can be used in these scenarios to ensure that all the dependents of the initial spec you provide are also marked as develop specs.
The ``--recursive`` flag requires a pre-concretized environment so the graph can be traversed from the supplied spec all the way to the root specs.

For packages with ``git`` attributes, git branches, tags, and commits can also be used as valid concrete versions (see :ref:`version-specifier`).
This means that for a package ``foo``, ``spack develop foo@git.main`` will clone the ``main`` branch of the package, and ``spack install`` will install from that git clone if ``foo`` is in the environment.
Further development on ``foo`` can be tested by re-installing the environment, and eventually committed and pushed to the upstream git repo.

If the package being developed supports out-of-source builds then users can use the ``--build_directory`` flag to control the location and name of the build directory.
This is a shortcut to set the ``package_attributes:build_directory`` in the ``packages`` configuration (see :ref:`assigning-package-attributes`).
The supplied location will become the build-directory for that package in all future builds.

.. admonition:: Potential pitfalls of setting the build directory
   :class: warning

   Spack does not check for out-of-source build compatibility with the packages and so the onus of making sure the package supports out-of-source builds is on the user.
   For example, most ``autotool`` and ``makefile`` packages do not support out-of-source builds while all ``CMake`` packages do.
   Understanding these nuances is up to the software developers and we strongly encourage developers to only redirect the build directory if they understand their package's build-system.

Loading
^^^^^^^

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

.. code-block:: console

   $ 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.


.. _environment_include_concrete:

Included Concrete Environments
------------------------------

Spack environments can create an environment based off of information in already established environments.
You can think of it as a combination of existing environments.
It will gather information from the existing environment's ``spack.lock`` and use that during the creation of this included concrete environment.
When an included concrete environment is created it will generate a ``spack.lock`` file for the newly created environment.


Creating included environments
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
To create a combined concrete environment, you must have at least one existing concrete environment.
You will use the command ``spack env create`` with the argument ``--include-concrete`` followed by the name or path of the environment you'd like to include.
Here is an example of how to create a combined environment from the command line.

.. code-block:: spec

   $ spack env create myenv
   $ spack -e myenv add python
   $ spack -e myenv concretize
   $ spack env create --include-concrete myenv included_env


You can also include an environment directly in the ``spack.yaml`` file.
It involves adding the ``include_concrete`` heading in the yaml followed by the absolute path to the independent environments.
Note that you may use Spack config variables such as ``$spack`` or environment variables as long as the expression expands to an absolute path.

.. code-block:: yaml

   spack:
     specs: []
     concretizer:
       unify: true
     include_concrete:
     - /absolute/path/to/environment1
     - $spack/../path/to/environment2


Once the ``spack.yaml`` has been updated you must concretize the environment to get the concrete specs from the included environments.

Updating an included environment
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If changes were made to the base environment and you want that reflected in the included environment you will need to re-concretize both the base environment and the included environment for the change to be implemented.
For example:

.. code-block:: spec

   $ spack env create myenv
   $ spack -e myenv add python
   $ spack -e myenv concretize
   $ spack env create --include-concrete myenv included_env


   $ spack -e myenv find
   ==> In environment myenv
   ==> Root specs
   python

   ==> 0 installed packages


   $ spack -e included_env find
   ==> In environment included_env
   ==> No root specs
   ==> Included specs
   python

   ==> 0 installed packages

Here we see that ``included_env`` has access to the python package through the ``myenv`` environment.
But if we were to add another spec to ``myenv``, ``included_env`` will not be able to access the new information.

.. code-block:: spec

   $ spack -e myenv add perl
   $ spack -e myenv concretize
   $ spack -e myenv find
   ==> In environment myenv
   ==> Root specs
   perl  python

   ==> 0 installed packages


   $ spack -e included_env find
   ==> In environment included_env
   ==> No root specs
   ==> Included specs
   python

   ==> 0 installed packages

It isn't until you run the ``spack concretize`` command that the combined environment will get the updated information from the re-concretized base environment.

.. code-block:: console

   $ spack -e included_env concretize
   $ spack -e included_env find
   ==> In environment included_env
   ==> No root specs
   ==> Included specs
   perl  python

   ==> 0 installed packages

.. _environment-configuration:

Configuring Environments
------------------------

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:

#. Inline in the ``spack.yaml`` file

#. 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 configurations
^^^^^^^^^^^^^^^^^^^^^

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 its name.
For example, a ``spack.yaml`` manifest file containing some package preference configuration (as in a ``packages.yaml`` file) could contain:

.. code-block:: yaml

   spack:
     # ...
     packages:
       all:
         providers:
           mpi: [openmpi]
     # ...

This configuration sets the default mpi provider to be openmpi.

Included configurations
^^^^^^^^^^^^^^^^^^^^^^^

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

.. code-block:: yaml

   spack:
     include:
     - environment/relative/path/to/config.yaml
     - path: https://github.com/path/to/raw/config/compilers.yaml
       sha256: 26e871804a92cd07bb3d611b31b4156ae93d35b6a6d6e0ef3a67871fcb1d258b
     - /absolute/path/to/packages.yaml
     - path: /path/to/$os/$target/environment
       optional: true
     - path: /path/to/os-specific/config-dir
       when: os == "ventura"

Included configuration files are required *unless* they are explicitly optional or the entry's condition evaluates to ``false``.
Optional includes are specified with the ``optional`` clause and conditional with the ``when`` clause.
(See :ref:`include-yaml` for more information on optional and conditional entries.)

Files are listed using paths to individual files or directories containing them.
Path entries may be absolute or relative to the environment or specified as URLs.
URLs to individual files must link to the **raw** form of the file's contents (e.g., `GitHub <https://docs.github.com/en/repositories/working-with-files/using-files/viewing-and-understanding-files#viewing-or-copying-the-raw-file-content>`_ or `GitLab <https://docs.gitlab.com/ee/api/repository_files.html#get-raw-file-from-repository>`_) **and** include a valid sha256 for the file.
Only the ``file``, ``ftp``, ``http`` and ``https`` protocols (or schemes) are supported.
Spack-specific, environment and user path variables can be used.
(See :ref:`config-file-variables` for more information.)

.. warning::

   Recursive includes are not currently processed in a breadth-first manner so the value of a configuration option that is altered by multiple included files may not be what you expect.
   This will be addressed in a future update.


Configuration precedence
^^^^^^^^^^^^^^^^^^^^^^^^

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 configurations listed earlier will have higher precedence, as the included configs are applied in reverse order.

Manually Editing the Specs List
-------------------------------

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

.. code-block:: yaml

   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.

.. _environments_concretization_config:

Spec concretization
^^^^^^^^^^^^^^^^^^^
An environment can be concretized in three different modes and the behavior active under any environment is determined by the ``concretizer:unify`` configuration option.

The *default* mode is to unify all specs:

.. code-block:: yaml

   spack:
     specs:
     - hdf5+mpi
     - zlib@1.2.8
     concretizer:
       unify: true

This means that any package in the environment corresponds to a single concrete spec.
In the above example, when ``hdf5`` depends down the line of ``zlib``, it is required to take ``zlib@1.2.8`` instead of a newer version.
This mode of concretization is particularly useful when environment views are used: if every package occurs in only one flavor, it is usually possible to merge all install directories into a view.

A downside of unified concretization is that it can be overly strict.
For example, a concretization error would happen when both ``hdf5+mpi`` and ``hdf5~mpi`` are specified in an environment.

The second mode is to *unify when possible*: this makes concretization of root specs more independent.
Instead of requiring reuse of dependencies across different root specs, it is only maximized:

.. code-block:: yaml

   spack:
     specs:
     - hdf5~mpi
     - hdf5+mpi
     - zlib@1.2.8
     concretizer:
       unify: when_possible

This means that both ``hdf5`` installations will use ``zlib@1.2.8`` as a dependency even if newer versions of that library are available.

The third mode of operation is to concretize root specs entirely independently by disabling unified concretization:

.. code-block:: yaml

   spack:
     specs:
     - hdf5~mpi
     - hdf5+mpi
     - zlib@1.2.8
     concretizer:
       unify: false

In this example ``hdf5`` is concretized separately, and does not consider ``zlib@1.2.8`` as a constraint or preference.
Instead, it will take the latest possible version.

The last two concretization options are typically useful for system administrators and user support groups providing a large software stack for their HPC center.

.. note::

   The ``concretizer:unify`` config option was introduced in Spack 0.18 to replace the ``concretization`` property.
   For reference, ``concretization: together`` is replaced by ``concretizer:unify:true``, and ``concretization: separately`` is replaced by ``concretizer:unify:false``.

.. admonition:: Re-concretization of user specs

   The ``spack concretize`` command without additional arguments will *not* change any previously concretized specs.
   This may prevent it from finding a solution when using ``unify: true``, and it may prevent it from finding a minimal solution when using ``unify: when_possible``.
   You can force Spack to ignore the existing concrete environment with ``spack concretize -f``.

.. _environment-spec-matrices:

Spec Matrices
^^^^^^^^^^^^^

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:

.. code-block:: yaml

   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

.. code-block:: yaml

   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.

.. _spec-list-references:

Spec List References
^^^^^^^^^^^^^^^^^^^^

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.

.. code-block:: yaml

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

.. code-block:: yaml

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

.. note::

   Named spec lists in the definitions section may only refer to a named list defined above itself.
   Order matters.

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.

.. code-block:: yaml

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

.. note::

   Any definitions with the same named list with true ``when`` clauses (or absent ``when`` clauses) will be appended together

The valid variables for a ``when`` clause are:

#. ``platform``.
   The platform string of the default Spack architecture on the system.

#. ``os``.
   The os string of the default Spack architecture on the system.

#. ``target``.
   The target string of the default Spack architecture on the system.

#. ``architecture`` or ``arch``.
   A Spack spec satisfying the default Spack architecture on the system.
   This supports querying via the ``satisfies`` method, as shown above.

#. ``arch_str``.
   The architecture string of the default Spack architecture on the system.

#. ``re``.
   The standard regex module in Python.

#. ``env``.
   The user environment (usually ``os.environ`` in Python).

#. ``hostname``.
   The hostname of the system (if ``hostname`` is an executable in the user's PATH).

SpecLists as Constraints
^^^^^^^^^^^^^^^^^^^^^^^^

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``.

.. code-block:: yaml

   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.

Modifying Environment Variables
-------------------------------

Spack Environments can modify the active shell's environment variables when activated.
The environment can be configured to set, unset, prepend, or append using ``env_vars`` configuration in ``spack.yaml``:

.. code-block:: yaml

  spack:
    env_vars:
      set:
        ENVAR_TO_SET_IN_ENV_LOAD: "FOO"
      unset:
      - ENVAR_TO_UNSET_IN_ENV_LOAD
      prepend_path:
        PATH_LIST: "path/to/prepend"
      append_path:
        PATH_LIST: "path/to/append"
      remove_path:
        PATH_LIST: "path/to/remove"

Environment Views
-----------------

Spack Environments can have an associated filesystem view, which is a directory with a more traditional structure ``<view>/bin``, ``<view>/lib``, ``<view>/include`` in which all files of the installed packages are linked.

By default a view is created for each environment, thanks to the ``view: true`` option in the ``spack.yaml`` manifest file:

.. code-block:: yaml

   spack:
     specs: [perl, python]
     view: true

The view is created in a hidden directory ``.spack-env/view`` relative to the environment.
If you've used ``spack env activate``, you may have already interacted with this view.
Spack prepends its ``<view>/bin`` dir to ``PATH`` when the environment is activated, so that you can directly run executables from all installed packages in the environment.

Views are highly customizable: you can control where they are put, modify their structure, include and exclude specs, change how files are linked, and you can even generate multiple views for a single environment.

.. _configuring_environment_views:

Minimal view configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^

The minimal configuration

.. code-block:: yaml

   spack:
     # ...
     view: true

lets Spack generate a single view with default settings under the ``.spack-env/view`` directory of the environment.

Another short way to configure a view is to specify just where to put it:

.. code-block:: yaml

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

Views can also be disabled by setting ``view: false``.

.. _cmd-spack-env-view:

Advanced view configuration
^^^^^^^^^^^^^^^^^^^^^^^^^^^

One or more **view descriptors** can be defined under ``view``, keyed by a name.
The example from the previous section with ``view: /path/to/view`` is equivalent to defining a view descriptor named ``default`` with a ``root`` attribute:

.. code-block:: yaml

   spack:
     # ...
     view:
       default:  # name of the view
         root: /path/to/view  # view descriptor attribute

The ``default`` view descriptor name is special: when you ``spack env activate`` your environment, this view will be used to update (among other things) your ``PATH`` variable.

View descriptors must contain the root of the view, and optionally projections, ``select`` and ``exclude`` lists and link information via ``link`` and ``link_type``.

As a more advanced 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 GCC compiler at version 8.5.
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.

.. code-block:: yaml

   spack:
     # ...
     view:
       mpis:
         root: /path/to/view
         select: [^mpi]
         exclude: ["%gcc@8.5"]
         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:

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

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

.. tip::

   The option ``link: run`` can be used to create small environment views for Python packages.
   Python will be able to import packages *inside* of the view even when the environment is not activated, and linked libraries will be located *outside* of the view thanks to rpaths.

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 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.

.. _view_projections:

View Projections
""""""""""""""""
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:

.. code-block:: yaml

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

Projections also permit environment and Spack configuration variable expansions as shown below:

.. code-block:: yaml

   projections:
     all: "{name}-{version}/{compiler.name}-{compiler.version}/$date/$SYSTEM_ENV_VARIABLE"

where ``$date`` is the Spack configuration variable that will expand with the ``YYYY-MM-DD`` format and ``$SYSTEM_ENV_VARIABLE`` is an environment variable defined in the shell.

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.

Activating environment views
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``spack env activate <env>`` command has two effects:

1. It activates the environment so that further Spack commands such as ``spack install`` will run in the context of the environment.
2. It activates the view so that environment variables such as ``PATH`` are updated to include the view.

Without further arguments, the ``default`` view of the environment is activated.
If a view with a different name has to be activated, ``spack env activate --with-view <name> <env>`` can be used instead.
You can also activate the environment without modifying further environment variables using ``--without-view``.

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.

=================== =========
Variable            Paths
=================== =========
PATH                bin
MANPATH             man, share/man
ACLOCAL_PATH        share/aclocal
PKG_CONFIG_PATH     lib/pkgconfig, lib64/pkgconfig, share/pkgconfig
CMAKE_PREFIX_PATH   .
=================== =========

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 active view of the Spack environment from the user's environment variables.


.. _cmd-spack-env-depfile:


Generating Depfiles from Environments
------------------------------------------

Spack can generate ``Makefile``\s to make it easier to build multiple packages in an environment in parallel.

.. note::

   Since Spack v1.1, there is a new experimental installer that supports package-level parallelism out of the box with POSIX jobserver support.
   You can enable it with ``spack config add config:installer:new``.
   This new installer may provide a simpler alternative to the ``spack env depfile`` workflow described in this section for users primarily interested in speeding up environment installations.

Generated ``Makefile``\s expose targets that can be included in existing ``Makefile``\s, to allow other targets to depend on the environment installation.

A typical workflow is as follows:

.. code-block:: spec

   $ spack env create -d .
   $ spack -e . add perl
   $ spack -e . concretize
   $ spack -e . env depfile -o Makefile
   $ make -j64

This generates a ``Makefile`` from a concretized environment in the current working directory, and ``make -j64`` installs the environment, exploiting parallelism across packages as much as possible.
Spack respects the Make jobserver and forwards it to the build environment of packages, meaning that a single ``-j`` flag is enough to control the load, even when packages are built in parallel.

By default the following phony convenience targets are available:

- ``make all``: installs the environment (default target);
- ``make clean``: cleans files used by make, but does not uninstall packages.

.. tip::

   GNU Make version 4.3 and above have great support for output synchronization through the ``-O`` and ``--output-sync`` flags, which ensure that output is printed orderly per package install.
   To get synchronized output with colors, use ``make -j<N> SPACK_COLOR=always --output-sync=recurse``.

Specifying dependencies on generated ``make`` targets
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

An interesting question is how to include generated ``Makefile``\s in your own ``Makefile``\s.
This comes up when you want to install an environment that provides executables required in a command for a make target of your own.

The example below shows how to accomplish this: the ``env`` target specifies the generated ``spack/env`` target as a prerequisite, meaning that the environment gets installed and is available for use in the ``env`` target.

.. code-block:: Makefile

   SPACK ?= spack

   .PHONY: all clean env

   all: env

   spack.lock: spack.yaml
   	$(SPACK) -e . concretize -f

   env.mk: spack.lock
   	$(SPACK) -e . env depfile -o $@ --make-prefix spack

   env: spack/env
   	$(info environment installed!)

   clean:
   	rm -rf spack.lock env.mk spack/

   ifeq (,$(filter clean,$(MAKECMDGOALS)))
   include env.mk
   endif

This works as follows: when ``make`` is invoked, it first "remakes" the missing include ``env.mk`` as there is a target for it.
This triggers concretization of the environment and makes Spack output ``env.mk``.
At that point the generated target ``spack/env`` becomes available through ``include env.mk``.

As it is typically undesirable to remake ``env.mk`` as part of ``make clean``, the include is conditional.

.. note::

   When including generated ``Makefile``\s, it is important to use the ``--make-prefix`` flag and use the non-phony target ``<prefix>/env`` as prerequisite, instead of the phony target ``<prefix>/all``.

Building a subset of the environment
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The generated ``Makefile``\s contain install targets for each spec, identified by ``<name>-<version>-<hash>``.
This allows you to install only a subset of the packages in the environment.
When packages are unique in the environment, it's enough to know the name and let tab-completion fill out the version and hash.

The following phony targets are available: ``install/<spec>`` to install the spec with its dependencies, and ``install-deps/<spec>`` to *only* install its dependencies.
This can be useful when certain flags should only apply to dependencies.
Below we show a use case where a spec is installed with verbose output (``spack install --verbose``) while its dependencies are installed silently:

.. code-block:: console

   $ spack env depfile -o Makefile

   # Install dependencies in parallel, only show a log on error.
   $ make -j16 install-deps/python-3.11.0-<hash> SPACK_INSTALL_FLAGS=--show-log-on-error

   # Install the root spec with verbose output.
   $ make -j16 install/python-3.11.0-<hash> SPACK_INSTALL_FLAGS=--verbose

Adding post-install hooks
^^^^^^^^^^^^^^^^^^^^^^^^^

Another advanced use-case of generated ``Makefile``\s is running a post-install command for each package.
These "hooks" could be anything from printing a post-install message, running tests, or pushing just-built binaries to a buildcache.

This can be accomplished through the generated ``[<prefix>/]SPACK_PACKAGE_IDS`` variable.
Assuming we have an active and concrete environment, we generate the associated ``Makefile`` with a prefix ``example``:

.. code-block:: console

   $ spack env depfile -o env.mk --make-prefix example

And we now include it in a different ``Makefile``, in which we create a target ``example/push/%`` with ``%`` referring to a package identifier.
This target depends on the particular package installation.
In this target we automatically have the target-specific ``HASH`` and ``SPEC`` variables at our disposal.
They are respectively the spec hash (excluding leading ``/``), and a human-readable spec.
Finally, we have an entry point target ``push`` that will update the buildcache index once every package is pushed.
Note how this target uses the generated ``example/SPACK_PACKAGE_IDS`` variable to define its prerequisites.

.. code-block:: Makefile

   SPACK ?= spack
   BUILDCACHE_DIR = $(CURDIR)/tarballs

   .PHONY: all

   all: push

   include env.mk

   example/push/%: example/install/%
   	@mkdir -p $(dir $@)
   	$(info About to push $(SPEC) to a buildcache)
   	$(SPACK) -e . buildcache push --only=package $(BUILDCACHE_DIR) /$(HASH)
   	@touch $@

   push: $(addprefix example/push/,$(example/SPACK_PACKAGE_IDS))
   	$(info Updating the buildcache index)
   	$(SPACK) -e . buildcache update-index $(BUILDCACHE_DIR)
   	$(info Done!)
   	@touch $@
