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Chapter 6: How to Use Variables 61

Simply expanded variables are deﬁned by lines using ‘:=’ or ‘::=’ (see Section 6.5 [Setting

Variables], page 65). Both forms are equivalent in GNU make; however only the ‘::=’ form

is described by the POSIX standard (support for ‘::=’ was added to the POSIX standard

in 2012, so older versions of make won’t accept this form either).

The value of a simply expanded variable is scanned once and for all, expanding any

references to other variables and functions, when the variable is deﬁned. The actual value

of the simply expanded variable is the result of expanding the text that you write. It does

not contain any references to other variables; it contains their values as of the time this

variable was deﬁned. Therefore,

x := foo

y := $(x) bar

x := later

is equivalent to

y := foo bar

x := later

When a simply expanded variable is referenced, its value is substituted verbatim.

Here is a somewhat more complicated example, illustrating the use of ‘:=’ in conjunction

with the shell function. (See Section 8.13 [The shell Function], page 97.) This example

also shows use of the variable MAKELEVEL, which is changed when it is passed down from

level to level. (See Section 5.7.2 [Communicating Variables to a Sub-make], page 52, for

information about MAKELEVEL.)

ifeq (0,${MAKELEVEL})

whoami := $(shell whoami)

host-type := $(shell arch)

MAKE := ${MAKE} host-type=${host-type} whoami=${whoami}

endif

An advantage of this use of ‘:=’ is that a typical ‘descend into a directory’ recipe then looks

like this:

${subdirs}:

${MAKE} -C $@ all

Simply expanded variables generally make complicated makeﬁle programming more pre-

dictable because they work like variables in most programming languages. They allow you

to redeﬁne a variable using its own value (or its value processed in some way by one of

the expansion functions) and to use the expansion functions much more eﬃciently (see

Chapter 8 [Functions for Transforming Text], page 83).

You can also use them to introduce controlled leading whitespace into variable values.

Leading whitespace characters are discarded from your input before substitution of variable

references and function calls; this means you can include leading spaces in a variable value

by protecting them with variable references, like this:

nullstring :=

space := $(nullstring) # end of the line

Here the value of the variable space is precisely one space. The comment

‘# end of the line’ is included here just for clarity. Since trailing space characters are not

stripped from variable values, just a space at the end of the line would have the same

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