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<!DOCTYPE appendix PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<appendix id='ref-bitbake'>

    <title>Reference: BitBake</title>

    <para>
        BitBake is a program written in Python that interprets the metadata that makes up Poky. 
        At some point, people wonder what actually happens when you enter:
    <literallayout class='monospaced'>
     $ bitbake poky-image-sato
    </literallayout>
    </para>
   
    <para>
        This appendix provides an overview of what happens behind the scenes from BitBake's perspective.
    </para>

    <note><para>
        BitBake strives to be a generic "task" executor that is capable of handling complex dependency relationships. 
        As such, it has no real knowledge of what the tasks being executed actually do. 
        BitBake just considers a list of tasks with dependencies and handles metadata 
        that consists of variables in a certain format that get passed to the tasks.
    </para></note>

    <section id='ref-bitbake-parsing'>
        <title>Parsing</title>

        <para>
            BitBake parses configuration files, classes, and <filename>.bb</filename> files. 
        </para>

        <para>
            The first thing BitBake does is look for the <filename>bitbake.conf</filename> file.
            Poky keeps this file in <filename>meta/conf/</filename>.
            BitBake finds it by examining the <filename>BBPATH</filename> environment 
            variable and looking for the <filename>meta/conf/</filename> 
            directory.
        </para>

        <para>
            In Poky, <filename>bitbake.conf</filename> lists other configuration 
            files to include from a <filename>conf/</filename> 
            directory below the directories listed in <filename>BBPATH</filename>. 
            In general the most important configuration file from a user's perspective 
            is <filename>local.conf</filename>, which contains a user's customized 
            settings for Poky. 
            Other notable configuration files are the distribution 
            configuration file (set by the <glossterm><link linkend='var-DISTRO'>
            DISTRO</link></glossterm> variable) and the machine configuration file 
            (set by the <glossterm><link linkend='var-MACHINE'>MACHINE</link>
            </glossterm> variable).  
            The DISTRO and MACHINE environment variables are both usually set in 
            the <filename>local.conf</filename> file. 
            Valid distribution 
            configuration files are available in the <filename>
            meta/conf/distro/</filename> directory and valid machine configuration 
            files in the <filename>meta/conf/machine/</filename> 
            directory. 
            Within the <filename>meta/conf/machine/include/</filename> 
            directory are various <filename>tune-*.inc</filename> configuration files that provide common 
            "tuning" settings specific to and shared between particular architectures and machines.
        </para>

        <para>
            After the parsing of the configuration files some standard classes are included. 
            The <filename>base.bbclass</filename> file is always included.
            Other classes that are specified in the configuration using the 
            <glossterm><link linkend='var-INHERIT'>INHERIT</link></glossterm>
            variable are also inculded. 
            Class files are searched for in a classes subdirectory 
            under the paths in <filename>BBPATH</filename> in the same way as
            configuration files.
        </para>

        <para>
            After classes are included, the 
            variable <glossterm><link linkend='var-BBFILES'>BBFILES</link></glossterm> 
            is set, usually in
            <filename>local.conf</filename>, and defines the list of places to search for 
            <filename>.bb</filename> files. 
            By default, the BBFILES variable specifies the <filename>meta/recipes-*/
            </filename> directory within Poky. 
            Adding extra content to BBFILES is best achieved through the use of BitBake
            <link linkend='usingpoky-changes-layers'>"layers"</link>.