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 (see Automatic Variables).