From Setting Variables in the docs:

  • Use as =, :=, or ::= and a value.
  • White space is ignored.

Plain definition

FOO =
CXX = g++
objects = main.o foo.o bar.o utils.o
SHELL = /bin/bash
JS_DIR = assets/js

You don’t need quotes around multiple words.

You can define a value referencing another variable, including user input.

FOO = abc
BAR = $(FOO)

Evaluating works above without colon unlike next section.

Evaluation

Note colon and equals sign here.

shell

Using := $(shell ...) syntax:

hash := $(shell printf '\043')
var := $(shell find . -name "*.c")

if

Using a Make if statement.

BUZZ := $(if $(FOO), 'abc', $(FIZZ))

not equal

Using != ... syntax:

If the result of the execution could produce a $, and you don’t intend what follows that to be interpreted as a make variable or function reference, then you must replace every $ with $$ as part of the execution.

hash != printf '\043'
file_list != find . -name '*.c'

Default value

Here are two equivalent ways to set a variable if it is not defined.

FOO ?= bar

Or

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

User input with a default value

Use the ?= syntax to set a Make variable with a default value which can be overridden by the user.

Code:

  • Makefile 
      name ?= World

  greet:
      @echo "Hello, $(name)!"

Usage:

$ make greet
Hello, World!

$ make greet name=dev
Hello, dev!

Note that variable names are case-sensitive - so make sure what you pass in on the shell matches what is in the Makefile.

See Parameters section on passing variables.

Export variables

How to let Make variables be accessible to subprocesses.

The Make variables are always accessible as Make variables.

ENV = dev

foo:
    echo $(ENV)

But variables are not accessible in Shell commands, unless you use Make’s export prefix.

export ENV = dev

foo:
    echo $$ENV

Using export within a target does not work, as variables are not persisted between commands in a target.

But you can use like this. Here we simulate a command using ENV by doing an echo inside a subshell.

test:
	ENV=dev bash -c 'echo $$ENV'

    # More verbose.
	@export ENV=dev && bash -c 'echo $$ENV'