Full Stack Engineer

August 09, 2023

Fun with Make

Today I will be talking about Makefiles. Makefiles are really handy and as you'll see they have many, many uses. if you're using a mac or Linux machine then you should have Make installed already. If you're using windows you may need to install it.

Prerequisites

Make installed and running correctly
Basic knowledge of the terminal / commandline

The basics

So what is a Makefile anyway? A Makefile is a program building tool which runs on Unix, Linux, and their various builds and variations. In this guide, you will see that a Make can do a fair few things.

Variables

Variables inside a Makefile can be defined in the following ways:

name = value - recursively expanded variable
name := value - Simply expanded variable
name ::= value - Simply expanded variable

So whats the difference? Simply expanded variables are scanned only once where as, the recursively expanded variables are expanded whenever the variable is substitued for another string.


foo = $(bar)
bar = $(boo)
boo = wow!

all:;echo $(foo)

This will echo: wow! $(foo) expands to $(bar) which expands to $(boo) which finally expands to wow!.

You can read more on variables here

Ok. Just say I want to use the variable and I need to ensure it has a value. You could do something like:

  @$(if $(strip $(KMS_KEY)),,$(error KMS_KEY [$(KMS_KEY)] needs a value))

This will echo: Makefile:14: \*\*\* KMS_KEY [] needs a value. Stop.

Here we use a text function to remove the white space, with an if directive, more on these later.

You can read more on text functions here

You can also use conditional variable assignment like:

cond ?= another value
anotherCond ?= default value
cond = some value

conditionalVars:
	@echo $(cond)
	@echo $(anotherCond)

This will echo:

another value

default value

if I remove the default value then I will get the value i assigned:

cond ?=
anotherCond ?= default value
cond = some value

conditionalVars:
	@echo $(cond)
	@echo $(anotherCond)

This will echo:

some value

default value

Rules

Rules are commands that run targets. An example of a rule:

rule: [target]

A more concise example:

NAME="test me"

exportTest:
	@echo $(NAME)

This will echo: test me

You can read more about rules here

So what else can you do with Make? Well, lots.

How about your need to export a value and use it somewhere else?

export TEST_ME = Make me

exportTest:
	@echo ${TEST_ME}

This will echo: Make me

This configuration can be handy when you're building pipelines and your needing to export values dynamically.

Conditionals

Conditional directives can be handy for various tasks. The syntax of them is as follows.

conditional-directive

text-if-true

endif

The ifeq directive lines of the makefile following the ifeq are obeyed if the two arguments match; otherwise they are ignored.

directivesTest:
ifeq ($(empty_var), ok)
	echo "empty_var equals ok"
else
	echo "empty_var does not"
endif

This will echo empty_var does not

The lines of the makefile following the ifneq are obeyed if the two arguments do not match; otherwise they are ignored.

directivesTest:
ifneq ($(empty_var), ok)
	echo "empty_var equals ok"
else
	echo "empty_var does not"
endif

This will echo empty_var equals ok

Execution

The @ symbol. Adding before a command this will stop the command from being printed.

The $$ symbols. Adding this is how to use a variable in bash or sh

You can read more about it here

Functions

testTarget:
	@echo $(shell ls)

This will echo a list of files in the folder you're running the target in.

what about something more advanced like this?

testTarget:
	$(eval STACK_STATUS=$(shell aws cloudformation describe-stacks --stack-name my-cloudformation-stack --query 'Stacks[0].StackStatus' --output text))

This will store the status of a stack in a variable called STACK_STATUS which you can use in different parts of your Make target.