File: make.info,  Node: Reference,  Next: Flavors,  Prev: Using_Variables.php">Using Variables,  Up: Using_Variables.php">Using Variables

6.1 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 recipe.

   Variable references can be used in any context: targets,
prerequisites, recipes, 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 '$x'.
However, this practice can lead to confusion (e.g., '$foo' refers to the
variable 'f' followed by the string 'oo') so we recommend using
parentheses or braces around all variables, even single-letter
variables, unless omitting them gives significant readability
improvements.  One place where readability is often improved is
automatic variables (*note Automatic Variables::).