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Chapter 6: How to Use Variables 59

6 How to Use Variables

A variable is a name deﬁned in a makeﬁle to represent a string of text, called the variable’s

value. These values are substituted by explicit request into targets, prerequisites, recipes,

and other parts of the makeﬁle. (In some other versions of make, variables are called macros.)

Variables and functions in all parts of a makeﬁle are expanded when read, except for

in recipes, the right-hand sides of variable deﬁnitions using ‘=’, and the bodies of variable

deﬁnitions using the define directive.

Variables can represent lists of ﬁle names, options to pass to compilers, programs to run,

directories to look in for source ﬁles, directories to write output in, or anything else you can

imagine.

A variable name may be any sequence of characters not containing ‘:’, ‘#’, ‘=’, or white-

space. However, variable names containing characters other than letters, numbers, and

underscores should be considered carefully, as in some shells they cannot be passed through

the environment to a sub-make (see Section 5.7.2 [Communicating Variables to a Sub-make],

page 52). Variable names beginning with ‘.’ and an uppercase letter may be given special

meaning in future versions of make.

Variable names are case-sensitive. The names ‘foo’, ‘FOO’, and ‘Foo’ all refer to diﬀerent

variables.

It is traditional to use upper case letters in variable names, but we recommend using lower

case letters for variable names that serve internal purposes in the makeﬁle, and reserving

upper case for parameters that control implicit rules or for parameters that the user should

override with command options (see Section 9.5 [Overriding Variables], page 103).

A few variables have names that are a single punctuation character or just a few char-

acters. These are the automatic variables, and they have particular specialized uses. See

Section 10.5.3 [Automatic Variables], page 120.

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

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)

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Roland Ver. 4.5 Technical Information Seite 69