6.2.3 Immediately Expanded Variable Assignment

Another form of assignment allows for immediate expansion, but unlike simple assignment the resulting variable is recursive: it will be re-expanded again on every use. In order to avoid unexpected results, after the value is immediately expanded it will automatically be quoted: all instances of $ in the value after expansion will be converted into $$. This type of assignment uses the ‘:::=’ operator. For example,

var = first
OUT :::= $(var)
var = second

results in the OUT variable containing the text ‘first’, while here:

var = one$$two
OUT :::= $(var)
var = three$$four

results in the OUT variable containing the text ‘one$$two’. The value is expanded when the variable is assigned, so the result is the expansion of the first value of var, ‘one$two’; then the value is re-escaped before the assignment is complete giving the final result of ‘one$$two’.

The variable OUT is thereafter considered a recursive variable, so it will be re-expanded when it is used.

This seems functionally equivalent to the ‘:=’ / ‘::=’ operators, but there are a few differences:

First, after assignment the variable is a normal recursive variable; when you append to it with ‘+=’ the value on the right-hand side is not expanded immediately. If you prefer the ‘+=’ operator to expand the right-hand side immediately you should use the ‘:=’ / ‘::=’ assignment instead.

Second, these variables are slightly less efficient than simply expanded variables since they do need to be re-expanded when they are used, rather than merely copied. However since all variable references are escaped this expansion simply un-escapes the value, it won’t expand any variables or run any functions.

Here is another example:

var = one$$two
OUT :::= $(var)
OUT += $(var)
var = three$$four

After this, the value of OUT is the text ‘one$$two $(var)’. When this variable is used it will be expanded and the result will be ‘one$two three$four’.

This style of assignment is equivalent to the traditional BSD make ‘:=’ operator; as you can see it works slightly differently than the GNU make ‘:=’ operator. The :::= operator is added to the POSIX specification in Issue 8 to provide portability.