documentation/poky-ref-manual: General edits up the Parsing section.
Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
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Scott Rifenbark
Saul Wold
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<title>Reference: BitBake</title>
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<para>
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BitBake is a program written in Python that interprets the metadata
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that makes up Poky. At some point, people wonder what actually happens
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when you type <command>bitbake poky-image-sato</command>. This section
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aims to give an overview of what happens behind the scenes from a
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BitBake perspective.
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BitBake is a program written in Python that interprets the metadata that makes up Poky.
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At some point, people wonder what actually happens when you enter:
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<literallayout class='monospaced'>
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$ bitbake poky-image-sato
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</literallayout>
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</para>
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<para>
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This appendix provides an overview of what happens behind the scenes from BitBake's perspective.
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</para>
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<para>
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It is worth noting that BitBake aims to be a generic "task" executor
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capable of handling complex dependency relationships. As such it has no
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real knowledge of what the tasks it is executing actually do. It just
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considers a list of tasks with dependencies and handles metadata
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consisting of variables in a certain format which get passed to the
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tasks.
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</para>
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<note><para>
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BitBake strives to be a generic "task" executor that is capable of handling complex dependency relationships.
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As such, it has no real knowledge of what the tasks being executed actually do.
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BitBake just considers a list of tasks with dependencies and handles metadata
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that consists of variables in a certain format that get passed to the tasks.
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</para></note>
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<section id='ref-bitbake-parsing'>
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<title>Parsing</title>
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<para>
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The first thing BitBake does is work out its configuration by
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looking for a file called <filename>bitbake.conf</filename>.
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BitBake searches through the <varname>BBPATH</varname> environment
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BitBake examines the <varname>BBPATH</varname> environment
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variable looking for a <filename class="directory">conf/</filename>
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directory containing a <filename>bitbake.conf</filename> file and
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adds the first <filename>bitbake.conf</filename> file found in
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directory that contains a <filename>bitbake.conf</filename> file.
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BitBake adds the first <filename>bitbake.conf</filename> file found in
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<varname>BBPATH</varname> (similar to the PATH environment variable).
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For Poky, <filename>bitbake.conf</filename> is found in <filename
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class="directory">meta/conf/</filename>.
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@ -42,32 +44,32 @@
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files to include from a <filename class="directory">conf/</filename>
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directory below the directories listed in <varname>BBPATH</varname>.
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In general the most important configuration file from a user's perspective
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is <filename>local.conf</filename>, which contains a users customized
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settings for Poky. Other notable configuration files are the distribution
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is <filename>local.conf</filename>, which contains a user's customized
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settings for Poky.
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Other notable configuration files are the distribution
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configuration file (set by the <glossterm><link linkend='var-DISTRO'>
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DISTRO</link></glossterm> variable) and the machine configuration file
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(set by the <glossterm><link linkend='var-MACHINE'>MACHINE</link>
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</glossterm> variable). The <glossterm><link linkend='var-DISTRO'>
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DISTRO</link></glossterm> and <glossterm><link linkend='var-MACHINE'>
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MACHINE</link></glossterm> environment variables are both usually set in
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the <filename>local.conf</filename> file. Valid distribution
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</glossterm> variable).
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The DISTRO and MACHINE environment variables are both usually set in
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the <filename>local.conf</filename> file.
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Valid distribution
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configuration files are available in the <filename class="directory">
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meta/conf/distro/</filename> directory and valid machine configuration
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files in the <filename class="directory">meta/conf/machine/</filename>
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directory. Within the <filename class="directory">
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meta/conf/machine/include/</filename> directory are various <filename>
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tune-*.inc</filename> configuration files which provide common
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"tuning" settings specific to and shared between particular
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architectures and machines.
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directory.
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Within the <filename class="directory">meta/conf/machine/include/</filename>
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directory are various <filename>tune-*.inc</filename> configuration files that provide common
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"tuning" settings specific to and shared between particular architectures and machines.
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</para>
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<para>
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After the parsing of the configuration files some standard classes
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are included. In particular, <filename>base.bbclass</filename> is
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always included, as will any other classes
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specified in the configuration using the <glossterm><link
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linkend='var-INHERIT'>INHERIT</link></glossterm>
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variable. Class files are searched for in a classes subdirectory
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After the parsing of the configuration files some standard classes are included.
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The <filename>base.bbclass</filename> file is always included.
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Other classes that are specified in the configuration using the
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<glossterm><link linkend='var-INHERIT'>INHERIT</link></glossterm>
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variable are also inculded.
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Class files are searched for in a classes subdirectory
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under the paths in <varname>BBPATH</varname> in the same way as
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configuration files.
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</para>
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@ -77,21 +79,19 @@
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variable <glossterm><link linkend='var-BBFILES'>BBFILES</link></glossterm>
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is set, usually in
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<filename>local.conf</filename>, and defines the list of places to search for
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<filename class="extension">.bb</filename> files. By
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default this specifies the <filename class="directory">meta/packages/
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<filename class="extension">.bb</filename> files.
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By default this specifies the <filename class="directory">meta/packages/
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</filename> directory within Poky, but other directories such as
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<filename class="directory">meta-extras/</filename> can be included
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too. Adding extra content to
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<glossterm><link linkend='var-BBFILES'>BBFILES</link></glossterm> is best
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acheived through the use of BitBake
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too.
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Adding extra content to BBFILES is best acheived through the use of BitBake
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<link linkend='usingpoky-changes-layers'>"layers"</link>.
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</para>
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<para>
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BitBake parses each <filename class="extension">.bb</filename> file in
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<glossterm><link linkend='var-BBFILES'>BBFILES</link></glossterm> and
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stores the values of various variables. In summary, for each
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<filename class="extension">.bb</filename>
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BitBake parses each <filename class="extension">.bb</filename> file in BBFILES and
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stores the values of various variables.
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In summary, for each <filename class="extension">.bb</filename>
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file the configuration + base class of variables are set, followed
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by the data in the <filename class="extension">.bb</filename> file
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itself, followed by any inherit commands that
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