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Diffstat (limited to 'doc/bitbake-user-manual/bitbake-user-manual-metadata.xml')
| -rw-r--r-- | doc/bitbake-user-manual/bitbake-user-manual-metadata.xml | 43 |
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 879e096b0..3a234e7b7 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> |