spack/lib/spack/docs/environments.rst

.. Copyright 2013-2022 Lawrence Livermore National Security, LLC and other
   Spack Project Developers. See the top-level COPYRIGHT file for details.

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

.. _environments:

=========================
Environments (spack.yaml)
=========================

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:

#. 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!
#. Environments allow several specs to be built at once; a more robust
   solution than ad-hoc scripts making multiple calls to ``spack
   install``.
#. 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
<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:

#. 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.
#. 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 user input file is named ``spack.yaml`` and the lock
file is named ``spack.lock``

.. _environments-using:

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

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

^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Creating a named Environment
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

An environment is created by:

.. code-block:: console

   $ spack env create myenv

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

.. note::

   All named environments are stored in the ``var/spack/environments`` folder.

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:

.. code-block:: console

   $ spack env create myenv spack.yaml

To create an Environment from a ``spack.lock`` lockfile:

.. code-block:: console

   $ 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.

^^^^^^^^^^^^^^^^^^^^^^^^^
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] $ ...

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.

^^^^^^^^^^^^^^^^^^^^^^
Anonymous Environments
^^^^^^^^^^^^^^^^^^^^^^

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

.. code-block:: console

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

Anonymous specs can be created in place using the command:

.. code-block:: console

   $ spack env create -d .

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

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Environment Sensitive Commands
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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.

.. code-block:: console

  $ 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``, ``find``, ``extensions``, and more. In the
:ref:`environment-configuration` section we will discuss
Environment-sensitive commands further.

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

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.

.. code-block:: console

   $ spack env activate myenv
   $ spack add mpileaks

or

.. code-block:: console

   $ spack -e myenv add python

.. _environments_concretization:

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

Once some user specs have been added to an environment, they can be concretized.
There are at the moment three different modes of operation to concretize an environment,
which are explained in details in :ref:`environments_concretization_config`.
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:

.. 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 the specs can be done
instead with this command:

.. code-block:: console

   [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.

.. 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 installing individual specs into 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 first concretize
the Environment and then install 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:`env-generate-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 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.

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

The ``spack develop`` command allows one to develop Spack packages in
an environment. It requires a spec containing a concrete version, and
will configure Spack to install the package from local source. By
default, it will also clone the package to a subdirectory in the
environment. This package will have a special variant ``dev_path``
set, and 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 ensures that all instances of a
developed package in the environment are concretized to match the
version (and other constraints) passed as the spec argument to the
``spack develop`` command.

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 reinstalling the environment,
and eventually committed and pushed to the upstream git repo.

^^^^^^^
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-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 it's 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:
         compiler: [intel]
     ...

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

^^^^^^^^^^^^^^^^^^^^^^^
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:
     - relative/path/to/config.yaml
     - https://github.com/path/to/raw/config/compilers.yaml
     - /absolute/path/to/packages.yaml

Environments can include files or URLs. File paths can be relative or
absolute. URLs include the path to the text for individual files or
can be the path to a directory containing configuration files.

^^^^^^^^^^^^^^^^^^^^^^^^
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 independendent. 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

   When using *unified* concretization (when possible), the entire set of specs will be
   re-concretized after any addition of new user specs, to ensure that
   the environment remains consistent / minimal. When instead unified concretization is
   disabled, only the new specs will be concretized after any addition.

^^^^^^^^^^^^^
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

   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
^^^^^^^^^^^^^^^^^^^^

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

   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('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.

----------------
Filesystem Views
----------------

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.

.. _configuring_environment_views:

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Configuration in ``spack.yaml``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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.

.. code-block:: yaml

   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:

#. ``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.


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

.. code-block:: yaml

   spack:
     ...
     view: True

is equivalent to

.. code-block:: yaml

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

and

.. code-block:: yaml

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

is equivalent to

.. code-block:: yaml

   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.

.. _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}

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`` 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.

=================== =========
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 default view of
the environment from the user's path.


.. _env-generate-depfile:


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

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

   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:: 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-target-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-target-prefix`` flag and use the non-phony target
   ``<target-prefix>/env`` as prerequisite, instead of the phony target
   ``<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:: 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