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authorScott Rifenbark <srifenbark@gmail.com>2016-07-21 09:49:08 -0700
committerRichard Purdie <richard.purdie@linuxfoundation.org>2016-08-02 15:19:17 +0100
commit480096ca93c0a649ebfff68dfc7d9bbe8eb2ea2d (patch)
treefc31865638d385364862389861b2880a3ba6ac37 /doc/bitbake-user-manual/bitbake-user-manual-metadata.xml
parent3f871865eced7dd27a33f26e19847a9863a036d4 (diff)
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bitbake-user-manual: Updated the variable expansion section.
Fixes [YOCTO #9984] Added more detail to the examples that show the effects of variable expanison. Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Diffstat (limited to 'doc/bitbake-user-manual/bitbake-user-manual-metadata.xml')
-rw-r--r--doc/bitbake-user-manual/bitbake-user-manual-metadata.xml43
1 files changed, 31 insertions, 12 deletions
diff --git a/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml b/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml
index 879e096b..3a234e7b 100644
--- a/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml
+++ b/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml
@@ -50,22 +50,41 @@
<title>Variable Expansion</title>
<para>
- BitBake supports variables referencing one another's
- contents using a syntax that is similar to shell scripting.
- Following is an example that results in <filename>A</filename>
- containing "aval" and <filename>B</filename> evaluating to
- "preavalpost" based on that current value of
- <filename>A</filename>.
+ Variables can reference the contents of other variables
+ using a syntax that is similar to variable expansion in
+ Bourne shells.
+ The following assignments
+ result in A containing "aval" and B evaluating to "preavalpost".
<literallayout class='monospaced'>
A = "aval"
B = "pre${A}post"
</literallayout>
- You should realize that whenever <filename>B</filename> is
- referenced, its evaluation will depend on the state of
- <filename>A</filename> at that time.
- Thus, later evaluations of <filename>B</filename> in the
- previous example could result in different values
- depending on the value of <filename>A</filename>.
+ <note>
+ Unlike in Bourne shells, the curly braces are mandatory:
+ Only <filename>${FOO}</filename> and not
+ <filename>$FOO</filename> is recognized as an expansion of
+ <filename>FOO</filename>.
+ </note>
+ The "=" operator does not immediately expand variable
+ references in the right-hand side.
+ Instead, expansion is deferred until the variable assigned to
+ is actually used.
+ The result depends on the current values of the referenced
+ variables.
+ The following example should clarify this behavior:
+ <literallayout class='monospaced'>
+ A = "${B} baz"
+ B = "${C} bar"
+ C = "foo"
+ *At this point, ${A} equals "foo bar baz"*
+ C = "qux"
+ *At this point, ${A} equals "qux bar baz"*
+ B = "norf"
+ *At this point, ${A} equals "norf baz"*
+ </literallayout>
+ Contrast this behavior with the
+ <link linkend='immediate-variable-expansion'>immediate variable expansion</link>
+ operator (i.e. ":=").
</para>
</section>