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Chapter 6: How to Use Variables 69

6.8 Deﬁning Multi-Line Variables

Another way to set the value of a variable is to use the define directive. This directive

has an unusual syntax which allows newline characters to be included in the value, which

is convenient for deﬁning both canned sequences of commands (see Section 5.8 [Deﬁning

Canned Recipes], page 56), and also sections of makeﬁle syntax to use with eval (see

Section 8.9 [Eval Function], page 93).

The define directive is followed on the same line by the name of the variable being

deﬁned and an (optional) assignment operator, and nothing more. The value to give the

variable appears on the following lines. The end of the value is marked by a line containing

just the word endef. Aside from this diﬀerence in syntax, define works just like any other

variable deﬁnition. The variable name may contain function and variable references, which

are expanded when the directive is read to ﬁnd the actual variable name to use.

You may omit the variable assignment operator if you prefer. If omitted, make assumes

it to be ‘=’ and creates a recursively-expanded variable (see Section 6.2 [The Two Flavors

of Variables], page 60). When using a ‘+=’ operator, the value is appended to the previous

value as with any other append operation: with a single space separating the old and new

values.

You may nest define directives: make will keep track of nested directives and report

an error if they are not all properly closed with endef. Note that lines beginning with

the recipe preﬁx character are considered part of a recipe, so any define or endef strings

appearing on such a line will not be considered make directives.

define two-lines =

echo foo

echo $(bar)

endef

The value in an ordinary assignment cannot contain a newline; but the newlines that

separate the lines of the value in a define become part of the variable’s value (except for

the ﬁnal newline which precedes the endef and is not considered part of the value).

When used in a recipe, the previous example is functionally equivalent to this:

two-lines = echo foo; echo $(bar)

since two commands separated by semicolon behave much like two separate shell commands.

However, note that using two separate lines means make will invoke the shell twice, running

an independent sub-shell for each line. See Section 5.3 [Recipe Execution], page 44.

If you want variable deﬁnitions made with define to take precedence over command-line

variable deﬁnitions, you can use the override directive together with define:

override define two-lines =

foo

$(bar)

endef

See Section 6.7 [The override Directive], page 68.

6.9 Undeﬁning Variables

If you want to clear a variable, setting its value to empty is usually suﬃcient. Expanding

such a variable will yield the same result (empty string) regardless of whether it was set or

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