Reference

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Basics of Variable References

To substitute a variable's value, write a dollar sign followed by the name of the variable in parentheses or braces: either $(foo) or ${foo} is a valid reference to the variable foo. This special significance of $ is why you must write $$ to have the effect of a single dollar sign in a file name or command.

Variable references can be used in any context: targets, prerequisites, commands, most directives, and new variable values. Here is an example of a common case, where a variable holds the names of all the object files in a program:

objects = program.o foo.o utils.o
program : $(objects)
        cc -o program $(objects)

$(objects) : defs.h

Variable references work by strict textual substitution. Thus, the rule

foo = c
prog.o : prog.$(foo)
        $(foo)$(foo) -$(foo) prog.$(foo)

could be used to compile a C program prog.c. Since spaces before the variable value are ignored in variable assignments, the value of foo is precisely c. (Don't actually write your makefiles this way!)

A dollar sign followed by a character other than a dollar sign, open-parenthesis or open-brace treats that single character as the variable name. Thus, you could reference the variable x with $x. However, this practice is strongly discouraged, except in the case of the automatic variables (Automatic Variables).