Makefile: Exploring different ways to set value of a variable

makefile: variable assignment

# Ref: https://www.gnu.org/software/make/manual/html_node/Setting.html#Setting

########
# Setting variables:
# ?= Set variable if it is empty
# := Expand value variable before assignment, 
#	NOTE: (1) The behaviour of ::= is same as :=
#         (2) Expansion of value variable will be only once at the of value assignment.
# = Simple variable assignment, value will evaluated/expanded on its usage
# += Append value to variable
# != Value variable will be evaluated before assignment. 
# override directive

########
# UnComment following line NOT_SET_VAR will have VAL_0
# otherwise VAL_1
# NOT_SET_VAR = VAL_0

# Only set value to VAL_1 if NOT_SET_VAR hasn't got any value
NOT_SET_VAR ?= VAL_1

########

########
# EXPAND_VAR_ASSIGNMENT should be set to ZERO

VAR_0 := 10
VAR_1 := $(VAR_0)
ifeq ($(VAR_1),10)
	EXPAND_VAR_ASSIGNMENT := ZERO
else
	EXPAND_VAR_ASSIGNMENT := ONE
endif

########
# Simple assignment

VAR_X = 100
VAR_Y = 200
VAR_Z = $(VAR_X),$(VAR_Y),300		# On usage of VAR_Z, it will be expanded
									# and it will end up with 100,200,300

# UnComment following line, the VAR_Z will show 200,200,300
# VAR_X = 200

########
# Appending
VAR_APPEND := 1
VAR_APPEND += 2		# VAR_APPEND will have 1 2

########
# Override: If a var has been set with command argument then
# simple assignment will not change its value. 
# make -f variables.mk VAR_OVERRIDE_0=300
override VAR_OVERRIDE_0 = 200
override VAR_OVERRIDE_1 := 200
override VAR_OVERRIDE_APPEND += 200

all:
	@echo "NOT_SET_VAR is having $(NOT_SET_VAR)"
	@echo "EXPAND_VAR_ASSIGNMENT is having $(EXPAND_VAR_ASSIGNMENT)"
	@echo "VAR_Z is having $(VAR_Z)"
	@echo "VAR_APPEND is having $(VAR_APPEND)"
	@echo "VAR_OVERRIDE_0 is having $(VAR_OVERRIDE_0)"
	@echo "VAR_OVERRIDE_1 is having $(VAR_OVERRIDE_1)"
	@echo "VAR_OVERRIDE_APPEND is having $(VAR_OVERRIDE_APPEND)"