How to conditionally set Makefile variable to something if it is empty?

Question

I want to set a variable if it is empty. I tried in this way:

.... TEST := $(something) ... TEST ?= $(something else) 

The first $(something) may return an empty string, however the conditional assignment ?= works only if the previous variable is not set, not if empty.

Any elegant solution to set the variable if empty?

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EDIT I found this solution:

.... TEST := $(something) ... TEST += $(something else) TEST := $(word 1, $(TEST)) 

but I think that there will be one more elegant.

Answer 1

Here's another alternative that I personally find quite elegant, because it's a one-liner and doesn't need the redundant else-branch:

TEST := $(or $(TEST),$(something else))

Answer 2

Any elegant solution to set the variable if empty?

GNU make is hardly known for elegant solutions. Unless you find trapdoors and minefields to be elegant. I know only of the two ways to accomplish what you want:

1. The standard ifeq/endif solution:

   ifeq ($(TEST),) TEST := $(something else) endif 

2. Use the $(if) function:

   TEST := $(if $(TEST),$(TEST),$(something else)) 

   One can try to package that construct into a function too, but that is inadvisable. The function would have the hidden pitfall of occasionally breaking the $(something else) if it contains the , (for which there are only wayward workarounds). (The built-in functions like $(if) are immune to the , bug.)

Elegance test is up to you.