• Definition page, including default values for variables.

Shell if

Using if, identical to the Bash if except you have add backslash escapes across multiple lines.

This can be used as a check used in other commands.

Optionally use @ to make it quiet for Make output.

foo:
	@if [ -z $(FOO) ]; then \
		echo 'Must set FOO'; \
		exit 1; \
	fi

Note that this is used within a target while using Make directives like ifdef are evaluated always even if a target is not triggered.

Note using $(error ...) does not work within Bash if statement since it will always execute, so rather keep it all Bash or all Make.

One line if

Set a value based on whether another variable is set.

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

Equal and not equal

Use an ifeq expression. Or ifneq to negate.

It looks like the conditions have to be something like $(MY_VALUE), $(MY_OTHER_VALUE). I found that this never matched so was not helpful - $(MY_VALUE), MY_TEXT.

TARGET:
ifeq (CONDITION1, CONDITION2)
  EXPRESSION
endif

TARGET:
ifeq (CONDITION1, CONDITION2)
  EXPRESSION
else
  EXPESSION
endif

An example:

DEV = development

deploy:
ifeq ($(DEPLOYMENT_ENVIRONMENT), $(DEV))
	cd widget && \
		npm run build:dev
else
	cd widget && \
		npm run build:prod
endif

If an expression match is false, evaluate the block.

foo:
ifneq ($(wildcard ./fizz/$(BUZZ)),)
	$(eval NAME=$(shell sh -c "grep ..."))
endif
	# ...

If value not defined

FOO ?= abc

This is equivalent to this:

ifndef FOO
	FOO = abc
endif

Sample from Git’s Makefile using ifndef, though perhaps using ?= approach is equivalent but lighter to read.

ifndef SHELL_PATH
	SHELL_PATH = /bin/sh
endif
ifndef PERL_PATH
	PERL_PATH = /usr/bin/perl
endif
ifndef PYTHON_PATH
	PYTHON_PATH = /usr/bin/python
endif

export PERL_PATH
export PYTHON_PATH

TEST_SHELL_PATH = $(SHELL_PATH)

Error on values not set

Abort if a value is not defined using indef.

ifndef DEPLOYMENT_ENVIRONMENT
    @echo "DEPLOYMENT_ENVIRONMENT is set to: $(DEPLOYMENT_ENVIRONMENT)"
else
    $(error DEPLOYMENT_ENVIRONMENT must be set)
endif

deploy:
	echo 'Deploying'

Make sure the value is not empty, using ifneq.

# Make sure it is always defined but default to empty value.
DEPLOYMENT_ENVIRONMENT ?=

ifneq ($(DEPLOYMENT_ENVIRONMENT),)
    @echo "DEPLOYMENT_ENVIRONMENT is set to: $(DEPLOYMENT_ENVIRONMENT)"
else
    $(error DEPLOYMENT_ENVIRONMENT must be set)
endif

deploy:
	echo 'Deploying'

Test it out:

  1. Cause an error: 
     $ make deploy
  2. Print environment: 
     $ DEPLOYMENT_ENVIRONMENT=development make deploy

     $ make deploy DEPLOYMENT_ENVIRONMENT=development 

     $ export DEPLOYMENT_ENVIRONMENT=development
 $ make deploy