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When inserting a shell script inside a Makefile we have (?) to use a double dollar sign ($$) to make reference to variables. Why is that so?

for number in 1 2 3 4 ; do \
    echo $$number ; \
done

Answers

As per gnu make official doc:

Variable and function references in recipes have identical syntax and semantics to references elsewhere in the makefile. They also have the same quoting rules: if you want a dollar sign to appear in your recipe, you must double it (‘$$’). For shells like the default shell, that use dollar signs to introduce variables, it’s important to keep clear in your mind whether the variable you want to reference is a make variable (use a single dollar sign) or a shell variable (use two dollar signs).

So in short:

• makefile variable => use a single dollar sign
• shell variable => use two dollar signs