Robert, I don't think that the YP manuals do. That's more Bitbake. However, here is some copied out of my docs. (List excuse the formatting. It's copied from an HTML page.)
- *Variable Setting (=)* Set a variable by assigning it a value using the *=* sign. Values must be surrounded by double quotes. * VAR = "value" * In this example, *VAR* is set to *value*. - *Early Default Value Assignment (?=)* A variable can be early-assigned a default value using the *?=* assignment operator: * A ?= "1" B ?= "2" B ?= "3" * In this example *A* will contain *1* if it was not previously set. *B* will contain *2* because the *?=* assignment operator is immediate meaning that if there are multiple *?=* assignments to a single variable, the first one will be used. - *Late Default Value Assignment (??=)* A variable can be late-assigned a default value using the *??=* assignment operator: * A ??= "1" B ??= "2" B ??= "3" * In this example *A* will contain *1* if it was not previously set. *B* will contain *3* because the *??=* assignment operator is a late or lazy assignment operator and assignment will not take place until the end of the parsing process. If there are multiple *??=* assignments to a single variable, the last one will be used. - *Variable Expansion* Bitbake supports referencing a variables content similar to shell scripting. * A = "jumps over" B = "The quick brown fox ${A} the lazy dog." echo $B * This results in the the text *The quick brown fox jumps over the lazy dog.* printed to the console. - *Immediate Variable Expansion (:=)* Typically, Bitbake expands a variable when it is used. The *:=* assignment operator however expands the variable immediately when it is assigned. * A = "11" B = "B:${A}" A = "22" C := "C:${A}" echo $C $B * This results in *C:22 B:22* printed to the console since the content of * A* is expanded immediately on assignment to variable *C*. Variable *B* did not get expanded until it was actually used in the *echo*statement. - *Appending (+=) and Prepending (=+)* The *+=* and *=+* append and prepend variables respectively while adding a single space between the values. * A = "12" A += "34" B = "89" B =+ "67" * This results in variable *A* containing *12 34* and variable *B* containing *67 89*. - *Appending (.=) and Prepending (=.) without Spaces* The *.=* and *=.* work like the above appending and prepending operators however without placing an additional space between the values. * A = "12" A .= "34" B = "89" B =. "67" * This results in variable *A* containing *1234* and variable *B* containing *6789*. Rudi
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