• 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.

foo:
	@if [ -z $(FOO) ]; then \
		echo "FOO must be set. Use `export FOO=<foo>`"; \
		exit 1; \
	fi

One line if

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 defined

ifndef FOO
	FOO = abc
endif

Sample from Git’s Makefile.

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)

Presumably these can be provided optionally by the user in the CLI.

$ make SHELL_PATH=/bin/bash install

Error on value not defined

My own code to abort if a value is not defined:

.check-env:
ifndef DEPLOYMENT_ENVIRONMENT
	echo "Must set DEPLOYMENT_ENVIRONMENT"
	exit 1
endif
	echo "DEPLOYMENT_ENVIRONMENT: $(DEPLOYMENT_ENVIRONMENT)"

deploy: .check-env
	# do stuff

Cause an error:

$ make .check-env

Print environment:

$ DEPLOYMENT_ENVIRONMENT=development make .check-env