Linux makefile assignment operators

This info copied from: https://vuanhduy.wordpress.com/2014/05/02/the-variants-of-gnu-makefile-assignment-operation/

The differences in =, :=, ?= and += when assigning a macro in Makefile.

• = operation

If the variable does not have a value, the operation assign the value to it
If the variable already had a value, it is replaced.
This value will be expanded when the variable is used.


1. HELLO = world
2. HELLO_WORLD = $(HELLO) world!
4. # This echoes "world world!"
5. echo $(HELLO_WORLD)
7. HELLO = hello
9. # This echoes "hello world!"
10. echo $(HELLO_WORLD)
• := operation

It is similar to the = operation but the value will be expanded during the assignment.


1. HELLO = world
2. HELLO_WORLD := $(HELLO) world!
4. # This echoes "world world!"
5. echo $(HELLO_WORLD)
7. HELLO = hello
9. # Still echoes "world world!"
10. echo $(HELLO_WORLD)
12. HELLO_WORLD := $(HELLO) world!
14. # This echoes "hello world!"
15. echo $(HELLO_WORLD)
• += operation

Similar to the = operation but instead of replacing the value, the value is appended to the current one, with a space in between.


1. HELLO_WORLD = hello
2. HELLO_WORLD += world!
4. # This echoes "hello world!"
5. echo $(HELLO_WORLD)
• ?= operation

It is similar to the = operation but only if the variable doesn’t have a value.


1. HELLO_WORLD ?= hello world!
3. # This echoes "hello world!"
4. echo $(HELLO_WORLD)
6. HELLO_WORLD ?= hi world!
8. # Still echoes "hello world!"