What's the difference between := and = in Makefile?

Question

For variable assignment in Make, I see := and = operator. What\'s the difference between them?

Answer 1

For me, the best way to see it in practice is during this Makefile snippet:

Simple assignment

XX := $(shell date) // date will be executed once
tt:
    @echo $(XX)
    $(shell sleep 2)
    @echo $(XX)

Running

make tt

Will produce:

sex 22 jan 2021 14:56:08 -03
sex 22 jan 2021 14:56:08 -03

( Same value )

Expanded assignment

XX = $(shell date) // date will be executed every time you use XX
tt:
    @echo $(XX)
    $(shell sleep 2)
    @echo $(XX)

Running

make tt

Will produce:

sex 22 jan 2021 14:56:08 -03
sex 22 jan 2021 14:58:08 -03

Different values