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62 GNU make

eﬀect (but be rather hard to read). If you put whitespace at the end of a variable value,

it is a good idea to put a comment like that at the end of the line to make your intent

clear. Conversely, if you do not want any whitespace characters at the end of your variable

value, you must remember not to put a random comment on the end of the line after some

whitespace, such as this:

dir := /foo/bar # directory to put the frobs in

Here the value of the variable dir is ‘/foo/bar ’ (with four trailing spaces), which was

probably not the intention. (Imagine something like ‘$(dir)/file’ with this deﬁnition!)

There is another assignment operator for variables, ‘?=’. This is called a conditional

variable assignment operator, because it only has an eﬀect if the variable is not yet deﬁned.

This statement:

FOO ?= bar

is exactly equivalent to this (see Section 8.10 [The origin Function], page 94):

ifeq ($(origin FOO), undefined)

FOO = bar

endif

Note that a variable set to an empty value is still deﬁned, so ‘?=’ will not set that

variable.

6.3 Advanced Features for Reference to Variables

This section describes some advanced features you can use to reference variables in more

ﬂexible ways.

6.3.1 Substitution References

A substitution reference substitutes the value of a variable with alterations that you specify.

It has the form ‘$(var:a=b)’ (or ‘${var:a=b}’) and its meaning is to take the value of the

variable var, replace every a at the end of a word with b in that value, and substitute the

resulting string.

When we say “at the end of a word”, we mean that a must appear either followed by

whitespace or at the end of the value in order to be replaced; other occurrences of a in the

value are unaltered. For example:

foo := a.o b.o c.o

bar := $(foo:.o=.c)

sets ‘bar’ to ‘a.c b.c c.c’. See Section 6.5 [Setting Variables], page 65.

A substitution reference is actually an abbreviation for use of the patsubst expansion

function (see Section 8.2 [Functions for String Substitution and Analysis], page 84). We

provide substitution references as well as patsubst for compatibility with other implemen-

tations of make.

Another type of substitution reference lets you use the full power of the patsubst func-

tion. It has the same form ‘$(var:a=b)’ described above, except that now a must contain a

single ‘%’ character. This case is equivalent to ‘$(patsubst a,b,$(var))’. See Section 8.2

[Functions for String Substitution and Analysis], page 84, for a description of the patsubst

function.

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