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 Defining Canned Recipes), and also sections of makefile syntax to use with eval (see section The eval Function).

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 The Two Flavors of Variables). 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 subshell for each line. See section Recipe Execution.

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 The override Directive.

This document was generated by Davide Tacchella on November 3, 2010 using texi2html 1.82.