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GNU Info (make.info)ReferenceBasics 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 (Note: Automatic Variables.). |