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