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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>
<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.
It tells 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 .bb 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 bitbake.conf file,
located within the bitbake source tree /conf 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 base.bbclass and it is found in the /classes
directory.
The base.bbclass 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>
</chapter>
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