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bitbake: bitbake-user-manual: Added section on modifying variables

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Fixes [YOCTO #12548]

I created a new section titled "Modifying Variable Values" that
provides instruction on how to use the "bitbake -e" command to
be sure your configuration and variable values are as expected.

(Bitbake rev: 5a697957d7687fe2c730896e178f7e1e054fe724)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
This commit is contained in:
Scott Rifenbark
2019-04-18 10:37:07 -07:00
committed by Richard Purdie
parent c23c8ebc7f
commit f0d4ef3cdc
1 changed files with 72 additions and 0 deletions
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bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml
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@@ -61,6 +61,78 @@
</para> </para>
</section> </section>
<section id='modifying-existing-variables'>
<title>Modifying Existing Variables</title>
<para>
Sometimes you need to modify existing variables.
Following are some cases where you might find you want to
modify an existing variable:
<itemizedlist>
<listitem><para>
Customize a recipe that uses the variable.
</para></listitem>
<listitem><para>
Change a variable's default value used in a
<filename>*.bbclass</filename> file.
</para></listitem>
<listitem><para>
Change the variable in a <filename>*.bbappend</filename>
file to override the variable in the original recipe.
</para></listitem>
<listitem><para>
Change the variable in a configuration file so that the
value overrides an existing configuration.
</para></listitem>
</itemizedlist>
</para>
<para>
Changing a variable value can sometimes depend on how the
value was originally assigned and also on the desired
intent of the change.
In particular, when you append a value to a variable that
has a default value, the resulting value might not be what
you expect.
In this case, the value you provide might replace the value
rather than append to the default value.
</para>
<para>
If after you have changed a variable's value and something
unexplained occurs, you can use BitBake to check the actual
value of the suspect variable.
You can make these checks for both configuration and recipe
level changes:
<itemizedlist>
<listitem><para>
For configuration changes, use the following:
<literallayout class='monospaced'>
$ bitbake -e
</literallayout>
This command displays variable values after the
configuration files (i.e. <filename>local.conf</filename>,
<filename>bblayers.conf</filename>,
<filename>bitbake.conf</filename> and so forth) have
been parsed.
<note>
Variables that are exported to the environment are
preceded by the string "export" in the command's
output.
</note>
</para></listitem>
<listitem><para>
For recipe changes, use the following:
<literallayout class='monospaced'>
$ bitbake <replaceable>recipe</replaceable> -e | grep VARIABLE="
</literallayout>
This command checks to see if the variable actually
makes it into a specific recipe.
</para></listitem>
</itemizedlist>
</para>
</section>
<section id='line-joining'> <section id='line-joining'>
<title>Line Joining</title> <title>Line Joining</title>