Makefile: Default Value of Variable that is set but has null value

Question

I have a Makefile that has a variable that needs to have a default value in case when variable is unset or if set but has null value.

How can I achieve this?

I need this, as I invoke make inside a shell script and the value required by the makefile can be passed from the shell as $1. And to pass this to makefile I have to set it inside bash-script.

---

Ideas: (Not elegant) Inside the bash script the variable could be checked if its set but has null value, in that case it can be unset.

---

Snippets

Note: the following won't work if the variable are not defined in the terminal, as they are set in the bash script.

Makefile

dSourceP?=$(shell pwd)
Source?=$(notdir $(wildcard $(dSourceP)/*.md))

Bash Script

make all dSourceP="${1}" Source="${2}"

Terminal

bash ./MyScript.sh
bash ./MyScript.sh /home/nikhil/MyDocs
bash ./MyScript.sh /home/nikhil/MyDocs index.md

Answer 1

Since you’re using GNU make, you could use the ?= operator:

FOO ?= bar

but that doesn’t deal with pre-existing null (or rather, empty) values. The following deals with absent and empty values:

ifndef FOO
override FOO = bar
endif

test:
    echo "$(FOO)"

.PHONY: test

(Make sure line 6 starts with a real tab.)

You’d call this using

make FOO=blah

to set a value. make or make FOO= will end up setting FOO to bar; you need override to override variables set on the command-line.