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- .. SPDX-License-Identifier: CC-BY-2.5
- ================
- Variable Context
- ================
- |
- Variables might only have an impact or can be used in certain contexts. Some
- should only be used in global files like ``.conf``, while others are intended only
- for local files like ``.bb``. This chapter aims to describe some important variable
- contexts.
- .. _ref-varcontext-configuration:
- BitBake's own configuration
- ===========================
- Variables starting with ``BB_`` usually configure the behaviour of BitBake itself.
- For example, one could configure:
- - System resources, like disk space to be used (:term:`BB_DISKMON_DIRS`),
- or the number of tasks to be run in parallel by BitBake (:term:`BB_NUMBER_THREADS`).
- - How the fetchers shall behave, e.g., :term:`BB_FETCH_PREMIRRORONLY` is used
- by BitBake to determine if BitBake's fetcher shall search only
- :term:`PREMIRRORS` for files.
- Those variables are usually configured globally.
- BitBake configuration
- =====================
- There are variables:
- - Like :term:`B` or :term:`T`, that are used to specify directories used by
- BitBake during the build of a particular recipe. Those variables are
- specified in ``bitbake.conf``. Some, like :term:`B`, are quite often
- overwritten in recipes.
- - Starting with ``FAKEROOT``, to configure how the ``fakeroot`` command is
- handled. Those are usually set by ``bitbake.conf`` and might get adapted in a
- ``bbclass``.
- - Detailing where BitBake will store and fetch information from, for
- data reuse between build runs like :term:`CACHE`, :term:`DL_DIR` or
- :term:`PERSISTENT_DIR`. Those are usually global.
- Layers and files
- ================
- Variables starting with ``LAYER`` configure how BitBake handles layers.
- Additionally, variables starting with ``BB`` configure how layers and files are
- handled. For example:
- - :term:`LAYERDEPENDS` is used to configure on which layers a given layer
- depends.
- - The configured layers are contained in :term:`BBLAYERS` and files in
- :term:`BBFILES`.
- Those variables are often used in the files ``layer.conf`` and ``bblayers.conf``.
- Recipes and packages
- ====================
- Variables handling recipes and packages can be split into:
- - :term:`PN`, :term:`PV` or :term:`PF` for example, contain information about
- the name or revision of a recipe or package. Usually, the default set in
- ``bitbake.conf`` is used, but those are from time to time overwritten in
- recipes.
- - :term:`SUMMARY`, :term:`DESCRIPTION`, :term:`LICENSE` or :term:`HOMEPAGE`
- contain the expected information and should be set specifically for every
- recipe.
- - In recipes, variables are also used to control build and runtime
- dependencies between recipes/packages with other recipes/packages. The
- most common should be: :term:`PROVIDES`, :term:`RPROVIDES`, :term:`DEPENDS`,
- and :term:`RDEPENDS`.
- - There are further variables starting with ``SRC`` that specify the sources in
- a recipe like :term:`SRC_URI` or :term:`SRCDATE`. Those are also usually set
- in recipes.
- - Which version or provider of a recipe should be given preference when
- multiple recipes would provide the same item, is controlled by variables
- starting with ``PREFERRED_``. Those are normally set in the configuration
- files of a ``MACHINE`` or ``DISTRO``.
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