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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<chapter id="user-manual-intro">
<title>BitBake User Manual</title>
<section id="intro">
<title>Introduction</title>
<para>
BitBake is a tool for executing tasks commonly performed by software
developers when building systems on a daily basis.
BitBake can build Systems consisting of numerous individual pieces
of software, or can be used to build a single application.
Example tasks that BitBake can execute are fetching source code,
applying patches to source code, configuring, compiling, and
packaging applications into a complete system, and managing metadata.
BitBake abstracts the information for completing individual tasks
into files known as recipes.
Recipes contain all of the relevant information required by BitBake
to complete a given task including dependencies, source file
locations, etc.
BitBake is similar to
<ulink url='http://www.gnu.org/software/make/'>GNU Make</ulink>
and other build tools.
</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 <emphasis>buildroot</emphasis> 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>
</para>
<para>
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>
</para>
<para>
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>
</chapter>
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