Make variable assignment difference

Ask:

What is the difference:

 VARIABLE = value
 VARIABLE ?= value
 VARIABLE := value
 VARIABLE += value

I have read the section in GNU make's manual, but it still doesn' t make sense to me.

 

Explanation:

 

Variable = Value

Normal setting of a variable-values within it are recursively expanded when the variable is used, not when it's declared

Variable: = Value

Setting of a variable with simple expansion of the values inside-values within it are expanded at declaration time.

Variable? = Value

Setting of a variable only if it doesn't have a value

Variable + = Value

Appending the supplied value to the existing value

========================================================== ====================================

########################## Makefile ############# ######
A = orginal_value
B = $ ()
A = later_value
ALL:
@ Echo $ (B)

######################################## ##########
# Make
Later_value

########################## Makefile ############# ######
A = orginal_value
B: = $ ()
A = later_value
ALL:
@ Echo $ (B)

######################################## ###########
# Make
Original_value

######################################## ###########
The difference is obvious: = the defined variable does not extend if the value content is itself a variable. If it is =, it will extend. Therefore, when using it, you must add the following when you do not need to extend it to prevent uncontrollable situations.