What is the difference between the GNU Makefile variable assignments =, ?=, := and +=?

Lazy Set

VARIABLE = value

Normal setting of a variable, but any other variables mentioned with the value field are recursively expanded with their value at the point at which the variable is used, not the one it had when it was declared

Immediate Set

VARIABLE := value

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

Lazy Set If Absent

VARIABLE ?= value

Setting of a variable only if it doesn't have a value. value is always evaluated when VARIABLE is accessed. It is equivalent to

ifeq ($(origin VARIABLE), undefined)
  VARIABLE = value
endif

See the documentation for more details.

Append

VARIABLE += value

Appending the supplied value to the existing value (or setting to that value if the variable didn't exist)

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Using = causes the variable to be assigned a value. If the variable already had a value, it is replaced. This value will be expanded when it is used. For example:

HELLO = world
HELLO_WORLD = $(HELLO) world!

# This echoes "world world!"
echo $(HELLO_WORLD)

HELLO = hello

# This echoes "hello world!"
echo $(HELLO_WORLD)

Using := is similar to using =. However, instead of the value being expanded when it is used, it is expanded during the assignment. For example:

HELLO = world
HELLO_WORLD := $(HELLO) world!

# This echoes "world world!"
echo $(HELLO_WORLD)

HELLO = hello

# Still echoes "world world!"
echo $(HELLO_WORLD)

HELLO_WORLD := $(HELLO) world!

# This echoes "hello world!"
echo $(HELLO_WORLD)

Using ?= assigns the variable a value iff the variable was not previously assigned. If the variable was previously assigned a blank value (VAR=), it is still considered set I think. Otherwise, functions exactly like =.

Using += is like using =, but instead of replacing the value, the value is appended to the current one, with a space in between. If the variable was previously set with :=, it is expanded I think. The resulting value is expanded when it is used I think. For example:

HELLO_WORLD = hello
HELLO_WORLD += world!

# This echoes "hello world!"
echo $(HELLO_WORLD)

If something like HELLO_WORLD = $(HELLO_WORLD) world! were used, recursion would result, which would most likely end the execution of your Makefile. If A := $(A) $(B) were used, the result would not be the exact same as using += because B is expanded with := whereas += would not cause B to be expanded.

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I suggest you do some experiments using "make". Here is a simple demo, showing the difference between = and :=.

/* Filename: Makefile*/
x := foo
y := $(x) bar
x := later

a = foo
b = $(a) bar
a = later

test:
    @echo x - $(x)
    @echo y - $(y)
    @echo a - $(a)
    @echo b - $(b)

make test prints:

x - later
y - foo bar
a - later
b - later bar

Check more elaborate explanation here

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When you use VARIABLE = value, if value is actually a reference to another variable, then the value is only determined when VARIABLE is used. This is best illustrated with an example:

VAL = foo
VARIABLE = $(VAL)
VAL = bar

# VARIABLE and VAL will both evaluate to "bar"

When you use VARIABLE := value, you get the value of value as it is now. For example:

VAL = foo
VARIABLE := $(VAL)
VAL = bar

# VAL will evaluate to "bar", but VARIABLE will evaluate to "foo"

Using VARIABLE ?= val means that you only set the value of VARIABLE if VARIABLE is not set already. If it's not set already, the setting of the value is deferred until VARIABLE is used (as in example 1).

VARIABLE += value just appends value to VARIABLE. The actual value of value is determined as it was when it was initially set, using either = or :=.

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In the above answers, it is important to understand what is meant by "values are expanded at declaration/use time". Giving a value like *.c does not entail any expansion. It is only when this string is used by a command that it will maybe trigger some globbing. Similarly, a value like $(wildcard *.c) or $(shell ls *.c) does not entail any expansion and is completely evaluated at definition time even if we used := in the variable definition.

Try the following Makefile in directory where you have some C files:

VAR1 = *.c
VAR2 := *.c
VAR3 = $(wildcard *.c)
VAR4 := $(wildcard *.c)
VAR5 = $(shell ls *.c)
VAR6 := $(shell ls *.c)

all :
    touch foo.c
    @echo "now VAR1 = \"$(VAR1)\"" ; ls $(VAR1)
    @echo "now VAR2 = \"$(VAR2)\"" ; ls $(VAR2)
    @echo "now VAR3 = \"$(VAR3)\"" ; ls $(VAR3)
    @echo "now VAR4 = \"$(VAR4)\"" ; ls $(VAR4)
    @echo "now VAR5 = \"$(VAR5)\"" ; ls $(VAR5)
    @echo "now VAR6 = \"$(VAR6)\"" ; ls $(VAR6)
    rm -v foo.c

Running make will trigger a rule that creates an extra (empty) C file, called foo.c but none of the 6 variables has foo.c in its value.

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The most upvoted answer can be improved.

Let me refer to GNU Make manual "Setting variables" and "Flavors", and add some comments.

Recursively expanded variables

The value you specify is installed verbatim; if it contains references to other variables, these references are expanded whenever this variable is substituted (in the course of expanding some other string). When this happens, it is called recursive expansion.

foo = $(bar)

The catch: foo will be expanded to the value of $(bar) each time foo is evaluated, possibly resulting in different values. Surely you cannot call it "lazy"! This can surprise you if executed on midnight:

# This variable is haunted!
WHEN = $(shell date -I)

something:
    touch $(WHEN).flag

# If this is executed on 00:00:00:000, $(WHEN) will have a different value!
something-else-later: something
    test -f $(WHEN).flag || echo "Boo!"

Simply expanded variable

VARIABLE := value
VARIABLE ::= value

Variables defined with ‘:=’ or ‘::=’ are simply expanded variables.

Simply expanded variables are defined by lines using ‘:=’ or ‘::=’ [...]. Both forms are equivalent in GNU make; however only the ‘::=’ form is described by the POSIX standard [...] 2012.

The value of a simply expanded variable is scanned once and for all, expanding any references to other variables and functions, when the variable is defined.

Not much to add. It's evaluated immediately, including recursive expansion of, well, recursively expanded variables.

The catch: If VARIABLE refers to ANOTHER_VARIABLE:

VARIABLE := $(ANOTHER_VARIABLE)-yohoho

and ANOTHER_VARIABLE is not defined before this assignment, ANOTHER_VARIABLE will expand to an empty value.

Assign if not set

FOO ?= bar

is equivalent to

ifeq ($(origin FOO), undefined)
FOO = bar
endif

where $(origin FOO) equals to undefined only if the variable was not set at all.

The catch: if FOO was set to an empty string, either in makefiles, shell environment, or command line overrides, it will not be assigned bar.

Appending

VAR += bar

Appending:

When the variable in question has not been defined before, ‘+=’ acts just like normal ‘=’: it defines a recursively-expanded variable. However, when there is a previous definition, exactly what ‘+=’ does depends on what flavor of variable you defined originally.

So, this will print foo bar:

VAR = foo
# ... a mile of code
VAR += $(BAR)
BAR = bar
$(info $(VAR))

but this will print foo:

VAR := foo
# ... a mile of code
VAR += $(BAR)
BAR = bar
$(info $(VAR))

The catch is that += behaves differently depending on what type of variable VAR was assigned before.

Multiline values

The syntax to assign multiline value to a variable is:

define VAR_NAME :=
line
line
endef

or

define VAR_NAME =
line
line
endef

Assignment operator can be omitted, then it creates a recursively-expanded variable.

define VAR_NAME
line
line
endef

The last newline before endef is removed.

Bonus: the shell assignment operator ‘!=’

 HASH != printf '\043'

is the same as

HASH := $(shell printf '\043')

Don't use it. $(shell) call is more readable, and the usage of both in a makefiles is highly discouraged. At least, $(shell) follows Joel's advice and makes wrong code look obviously wrong.

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