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