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<chapter id="user-manual-intro">
    <title>Overview</title>

    <section id="intro">
        <title>Introduction</title>

        <para>
            fundamentally, BitBake is a generic task execution
            engine that allows shell and Python tasks to be run
            efficiently and in parallel while working within
            complex inter-task dependency constraints.
            One of BitBake's main users, OpenEmbedded, takes this core
            and builds embedded Linux software stacks using
            a task-oriented approach.
        </para>

        <para>
            Conceptually, BitBake is similar to GNU Make in
            some regards but has significant differences:
            <itemizedlist>
                <listitem><para>
                    BitBake executes tasks according to provided
                    metadata that builds up the tasks.
                    Metadata is stored in recipe (<filename>.bb</filename>),
                    configuration (<filename>.conf</filename>), and class
                    (<filename>.bbclass</filename>) files and provides
                    BitBake with instructions on what tasks to run and
                    the dependencies between those tasks.
                    </para></listitem>
                <listitem><para>
                    BitBake includes a fetcher library for obtaining source
                    code from various places such as source control
                    systems or websites.
                    </para></listitem>
                <listitem><para>
                    The instructions for each unit to be built (e.g. a piece
                    of software) are known as recipe files and
                    contain all the information about the unit
                    (dependencies, source file locations, checksums, description
                    and so on).
                    </para></listitem>
                <listitem><para>
                    BitBake includes a client/server abstraction and can
                    be used from a command line or used as a service over XMLRPC and
                    has several different user interfaces.
                    </para></listitem>
            </itemizedlist>
        </para>
    </section>

    <section id="history-and-goals">
        <title>History and Goals</title>

        <para>
            BitBake was originally a part of the OpenEmbedded project.
            It was inspired by the Portage package management system
            used by the Gentoo Linux distribution.
            On December 7, 2004, OpenEmbedded project team member,
            Chris Larson split the project into two distinct pieces:
            <itemizedlist>
                <listitem><para>BitBake, a generic task executor</para></listitem>
                <listitem><para>OpenEmbedded, a metadata set utilized by
                    BitBake</para></listitem>
            </itemizedlist>
            Today, BitBake is the primary basis of the
            <ulink url="http://www.openembedded.org/">OpenEmbedded</ulink>
            project, which is being used to build and maintain a
            number of projects and embedded Linux distributions
            such as the Angstrom Distribution and the Yocto
            Project.
        </para>

        <para>
            Prior to BitBake, no other build tool adequately met the needs of
            an aspiring embedded Linux distribution.
            All of the build systems used by traditional desktop Linux
            distributions lacked important functionality, and none of the
            ad-hoc buildroot systems, prevalent in the
            embedded space, were scalable or maintainable.
        </para>

        <para>
            Some important original goals for BitBake were:
            <itemizedlist>
                <listitem><para>
                    Handle cross-compilation.
                    </para></listitem>
                <listitem><para>
                    Handle inter-package dependencies (build time on
                    target architecture, build time on native
                    architecture, and runtime).
                    </para></listitem>
                <listitem><para>
                    Support running any number of tasks within a given
                    package, including, but not limited to, fetching
                    upstream sources, unpacking them, patching them,
                    configuring them, and so forth.
                    </para></listitem>
                <listitem><para>
                    Be Linux distribution agnostic for both build and
                    target systems.
                    </para></listitem>
                <listitem><para>
                    Be architecture agnostic.
                    </para></listitem>
                <listitem><para>
                    Support multiple build and target operating systems
                    (e.g. Cygwin, the BSDs, and so forth).
                    </para></listitem>
                <listitem><para>
                    Be self contained, rather than tightly
                    integrated into the build machine's root
                    filesystem.
                    </para></listitem>
                <listitem><para>
                    Handle conditional metadata on the target architecture,
                    operating system, distribution, and machine.
                    </para></listitem>
                <listitem><para>
                    Be easy to use the tools to supply local metadata and packages
                    against which to operate.
                    </para></listitem>
                <listitem><para>
                    Be easy to use BitBake to collaborate between multiple
                    projects for their builds.
                    </para></listitem>
                <listitem><para>
                    Provide an inheritance mechanism that share
                    common metadata between many packages.
                    </para></listitem>
            </itemizedlist>
            Over time it became apparent that some further requirements
            were necessary:
            <itemizedlist>
                <listitem><para>
                    Handle variants of a base recipe (e.g. native, sdk,
                    and multilib).
                    </para></listitem>
                <listitem><para>
                    Split metadata into layers and allow layers
                    to override each other.
                    </para></listitem>
                <listitem><para>
                    Allow representation of a given set of input variables
                    to a task as a checksum.
                    Based on that checksum, allow acceleration of builds
                    with prebuilt components.
                    </para></listitem>
            </itemizedlist>
            BitBake satisfies all the original requirements and many more
            with extensions being made to the basic functionality to
            reflect the additional requirements.
            Flexibility and power have always been the priorities.
            BitBake is highly extensible and supports embedded Python code and
            execution of any arbitrary tasks.
        </para>
    </section>

    <section id="Concepts">
        <title>Concepts</title>

        <para>
            BitBake is a program written in the Python language.
            At the highest level, BitBake interprets metadata, decides
            what tasks are required to run, and executes those tasks.
            Similar to GNU Make, BitBake controls how software is
            built.
            GNU Make achieves its control through "makefiles".
            BitBake uses "recipes".
        </para>

        <para>
            BitBake extends the capabilities of a simple
            tool like GNU Make by allowing for much more complex tasks
            to be completed, such as assembling entire embedded Linux
            distributions.
        </para>

        <para>
            The remainder of this section introduces several concepts
            that should be understood in order to better leverage
            the power of BitBake.
        </para>

        <section id='recipes'>
            <title>Recipes</title>

            <para>
                BitBake Recipes, which are denoted by the file extension
                <filename>.bb</filename>, are the most basic metadata files.
                These recipe files provide BitBake the following:
                <itemizedlist>
                    <listitem><para>Descriptive information about the package</para></listitem>
                    <listitem><para>The version of the recipe</para></listitem>
                    <listitem><para>When dependencies exist</para></listitem>
                    <listitem><para>Where the source code resides</para></listitem>
                    <listitem><para>Whether the source code requires any patches</para></listitem>
                    <listitem><para>How to compile the source code</para></listitem>
                    <listitem><para>Where on the target machine to install the
                        package being compiled</para></listitem>
                </itemizedlist>
            </para>

            <para>
                Within the context of BitBake, or any project utilizing BitBake
                as it's build system, files with the <filename>.bb</filename>
                extension are referred to as recipes.
                <note>
                    The term "package" is also commonly used to describe recipes.
                    However, since the same word is used to describe packaged
                    output from a project, it is best to maintain a single
                    descriptive term, "recipes".
                </note>
            </para>
        </section>

        <section id='configuration-files'>
            <title>Configuration Files</title>

            <para>
                Configuration files, which are denoted by the
                <filename>.conf</filename> extension, define
                various configuration variables that govern the project's build
                process.
                These files fall into several areas that define
                machine configuration options, distribution configuration
                options, compiler tuning options, general common
                configuration options, and user configuration options.
                The main configuration file is the sample
                <filename>bitbake.conf</filename> file, which is
                located within the BitBake source tree
                <filename>conf</filename> directory.
            </para>
        </section>

        <section id='classes'>
            <title>Classes</title>

            <para>
                Class files, which are denoted by the
                <filename>.bbclass</filename> extension, contain
                information that is useful to share between metadata files.
                The BitBake source tree currently comes with one class metadata file
                called <filename>base.bbclass</filename>.
                You can find this file in the
                <filename>classes</filename> directory.
                The <filename>base.bbclass</filename> is special in that any
                new classes that a developer adds to a project are required to
                inherit <filename>base.bbclass</filename> automatically.
                This class contains definitions for standard basic tasks such
                as fetching, unpacking, configuring (empty by default),
                compiling (runs any Makefile present), installing (empty by
                default) and packaging (empty by default).
                These tasks are often overridden or extended by other classes
                added during the project development process.
            </para>
        </section>
    </section>

    <section id='obtaining-bitbake'>
        <title>Obtaining BitBake</title>

        <para>
            You can obtain BitBake several different ways:
            <itemizedlist>
                <listitem><para><emphasis>Installation using your Distribution
                    Package Management System:</emphasis>
                    This method is not
                    recommended because the BitBake version, in most
                    cases provided by your distribution, is several
                    releases behind a snapshot of the BitBake repository.
                    </para></listitem>
                <listitem><para><emphasis>Taking a snapshot of BitBake:</emphasis>
                    Downloading a snapshot of BitBake from the
                    source code repository is the recommended method
                    as you are assured of having the most recent stable
                    BitBake release.</para>
                    <para>The following example downloads a snapshot of
                    BitBake version 1.17.0:
                    <literallayout class='monospaced'>
     $ wget http://git.openembedded.org/bitbake/snapshot/bitbake-1.17.0.tar.gz
     $ tar zxpvf bitbake-1.17.0.tar.gz
                    </literallayout>
                    After extraction of the tarball using the tar utility,
                    you have a directory entitled
                    <filename>bitbake-1.17.0</filename>.
                    </para></listitem>
                <listitem><para><emphasis>Cloning BitBake:</emphasis>
                    Using Git to clone the BitBake source code repository
                    is also a recommended method when you need the absolute latest
                    BitBake source.
                    Realize that using this method could expose you to areas of
                    BitBake that are under development.</para>
                    <para>Here is an example:
                    <literallayout class='monospaced'>
     $ git clone git://git.openembedded.org/bitbake
                    </literallayout>
                    This command clones the BitBake Git repository into a
                    directory called <filename>bitbake</filename>.
                    Alternatively, you can
                    designate a directory after the
                    <filename>git clone</filename> command
                    if you want to call the new directory something
                    other than <filename>bitbake</filename>.
                    Here is an example that names the directory
                    <filename>bbdev</filename>:
                    <literallayout class='monospaced'>
     $ git clone git://git.openembedded.org/bitbake bbdev
                    </literallayout></para></listitem>
            </itemizedlist>
        </para>
    </section>
</chapter>
