.. SPDX-License-Identifier: CC-BY-2.5

================
Variable Context
================

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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``.