path: root/doc/user-manual/user-manual-intro.xml

<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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<chapter id="user-manual-intro">
    <title>BitBake User Manual</title>

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

        <para>
            At it's core, BitBake is a generic task execution
            engine allowing shell and python tasks to be run
            efficiently and in parallel while working within
            complex inter-task dependency constraints.
            One of it's main users, OpenEmbedded, takes this core
            and builds embedded Linux software stacks using
            a task oriented approach.
            Conceptually, BitBake is similar to GNU Make in
            some regards but has significant differences.
            BitBake executes tasks based upon metadata provided
            to it that build up tasks.
            It includes a fetcher library for obtaining source
            code from various places such as source control
            systems or websites.
            Metadata is stored in recipe (<filename>.bb</filename>),
            configuration (<filename>.conf</filename>), and class
            (<filename>.bbclass</filename>) files and provide to
            BitBake the instructions about which tasks to run and
            the dependencies between them.
            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).
            BitBake includes a client/server abstraction and can
            be used from a command line or as a service over XMLRPC and
            has several different user interfaces.
        </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, etc.
                    </para></listitem>
                <listitem><para>
                    Must be Linux distribution agnostic (both build and
                    target).
                    </para></listitem>
                <listitem><para>
                    Must be architecture agnostic
                    </para></listitem>
                <listitem><para>
                    Must support multiple build and target operating systems
                    (including Cygwin, the BSDs, etc).
                    </para></listitem>
                <listitem><para>
                    Must be able to be self contained, rather than tightly
                    integrated into the build machine's root
                    filesystem.
                    </para></listitem>
                <listitem><para>
                    There must be a way to handle conditional metadata
                    (on target architecture, operating system,
                    distribution, machine).
                    </para></listitem>
                <listitem><para>
                    It must be easy for the person using the tools to
                    supply their own local metadata and packages
                    to operate against.
                    </para></listitem>
                <listitem><para>
                    Must make it easy to collaborate between multiple
                    projects using BitBake for their builds.
                    </para></listitem>
                <listitem><para>
                    Should provide an inheritance mechanism to share
                    common metadata between many packages.
                    </para></listitem>
            </itemizedlist>
            Over time it has become apparent that some further requirements
            were necessary:
            <itemizedlist>
                <listitem><para>
                    Handle variants of a base recipe (native, sdk, multilib).
                    </para></listitem>
                <listitem><para>
                    Able to 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.
            It is highly extensible, supporting 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 does this using "makefiles". BitBake uses
            "recipes".
            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. Several concepts must be understood to be able
            to leverage the power of the tool.
        </para>

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

            <para>
                A BitBake Recipe, denoted by the file extension
                <filename>.bb</filename> is the most
                basic metadata file.
                These recipe files tell 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.
                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".
            </para>
        </section>

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

            <para>
                Configuration files, denoted by the
                <filename>.conf</filename> extension define
                various configuration variables that govern the project 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,
                located within the bitbake source tree
                <filename>/conf</filename> directory.
            </para>
        </section>

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

            <para>
                Class files, denoted by the
                <filename>.bbclass</filename> extension contain
                information that is useful to share between metadata files.
                The BitBake source tree comes with one class metadata file
                currently, called <filename>base.bbclass</filename> and it is
                found 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 is required to
                inherit it 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 classes 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>
            There are several ways to obtain BitBake.
            These include installing using your Linux distribution's
            package management system (not recommended), downloading
            a snapshot from the BitBake source
            code repository, or using Git to clone the BitBake source
            code repository.
            The recommended method for daily BitBake use is to download
            a stable release from the BitBake source code repository.
            Using your distribution's version as provided in the package
            management system is generally not
            recommended as in most cases, such as with the Ubuntu and
            Fedora distributions, the version provided is several releases
            behind the repository snapshot version and is missing
            important bug fixes and enhancements. Similarly, daily use
            of the latest clone of the Git repository is not recommended
            as it can be unstable.
            However, the Git repository clone will provide the User
            with the absolute latest version of BitBake.
        </para>
    </section>

    <section id='downloading-a-snapshot-from-the-bitbake-source-tree'>
        <title>Downloading a Snapshot from the BitBake Source Tree</title>

        <para>
            The recommended method for obtaining and using BitBake on
            a daily basis is to download the most
            recent stable snapshot from the Git source code repository
            as follows:
            <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 will have a directory entitled
            <filename>bitbake-1.17.0</filename>.
        </para>
    </section>

    <section id='cloning-the-bitbake-git-repository'>
        <title>Cloning the BitBake Git Repository</title>

        <para>
            To obtain the latest BitBake source code from the
            BitBake Git repository:
            <literallayout class='monospaced'>
     $ git clone git://git.openembedded.org/bitbake
            </literallayout>
            This will clone the BitBake Git repository into a
            directory called <filename>bitbake</filename>.
            Alternatively, you can
            designate a directory after the git clone command
            if you'd prefer to call the new directory something
            other than <filename>bitbake</filename>.
            For example:
            <literallayout class='monospaced'>
     $ git clone git://git.openembedded.org/bitbake bbdev
            </literallayout>
            This would clone the Git repository into a local directory
            called <filename>bbdev</filename>.
            Please note that although this method of obtaining the
            source code will provide the absolute latest version,
            it is under active development and may not be as stable
            as a released snapshot.
        </para>
    </section>

    <section id='summary'>
        <title>Summary</title>

        <para>
            At this point you should have a general idea of the concepts
            that BitBake was built on and how the
            source code is organized.
            You should have a working version of BitBake installed
            and understand how to setup your environment.
        </para>
    </section>
</chapter>