Null check and empty string check in makefile - linux

Question

I have two variables in the makefile they can be empty or having empty string OR valid string
I need to error exit in case either of the variable is empty or empty string
Here is simple makefile I am using
ABC := ""
XYZ := hello
all:
ifeq ($(and $(ABC),$(XYZ)),)
$(error "Either of var is null")
endif
#echo "Done"
With this I get output as Done While I want it to fail.
If I change ifeq condition as follows,
ifeq ($(and $(ABC),$(XYZ)),"") then in following condition make is not error exiting
ABC :=
XYZ := hello
all:
ifeq ($(and $(ABC),$(XYZ)),"")
$(error "Either of var is null")
endif
#echo "Done"
One solution could be as follows,(?)
ABC := hello
XYZ := hello
all:
ifeq ($(and $(ABC),$(XYZ)),)
$(error "Var is null")
endif
ifeq ($(and $(ABC),$(XYZ)),"")
$(error "Var is null2")
endif
#echo "Done"
However I feel there could a better way of doing it, Any suggestions ?
EDIT
Just to explain what I want is,
if ABC is empty string(ABC := "") OR empty(ABC := ) OR
XYZ is empty string(XYZ := "") OR empty(XYZ := )
$(error "empty string or null")
endif

Answer 1

Just to be clear, make doesn't care about quotes in any way. When make talks about "empty" variables it means variable with no value. If you write:
ABC := ""
then that variable has a value, the literal characters "". To make, that's no different from assigning ab etc. (at least in how make interprets those values).
For your problem you can use something like:
ifeq (,$(subst ",,$(ABC)$(XYZ)))
$(error empty string or null)
endif
which will replace all quotes with nothing; if the result of that is an empty string then you know the variables were either empty or contained nothing but quotes.
Note that this will also cause variables that contain only one quote, or more than two quotes, to be considered empty; e.g.,
ABC := "
XYZ := """""""""""""
will also be considered empty. If you really want only to consider exactly two quotes to be empty then you need something more fancy.

Answer 2

Ack, I don't have enough reputation to make comments, but it appears as though none of the above actually answer your question. Concatenation is the functional equivalent of $(or, not $(and, so $(ABC)$(XYZ) is not equivalent to $(and $(ABC),$(XYZ)) (and $(subst ",,""xxx) will not be blank either).
Also, your second example in your question will not work, as if $(ABC) is "", then $(and $ABC,xxx) will be xxx, not "".
What you need is a macro, say unquote to remove the qoutes, and then do:
unquote=$(subst ",,$(1))
ifeq ($(and $(call unquote,$(ABC)),$(call unquote,$(XYZ))),)
Which is a bit ugly. You could of course change unquote to only simplify empty quoted strings.