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66 GNU make

deﬁnes a variable named objects. Whitespace around the variable name and immediately

after the ‘=’ is ignored.

Variables deﬁned with ‘=’ are recursively expanded variables. Variables deﬁned with

‘:=’ or ‘::=’ are simply expanded variables; these deﬁnitions can contain variable references

which will be expanded before the deﬁnition is made. See Section 6.2 [The Two Flavors of

Variables], page 60.

The variable name may contain function and variable references, which are expanded

when the line is read to ﬁnd the actual variable name to use.

There is no limit on the length of the value of a variable except the amount of memory

on the computer. You can split the value of a variable into multiple physical lines for

readability (see Section 3.1.1 [Splitting Long Lines], page 12).

Most variable names are considered to have the empty string as a value if you have never

set them. Several variables have built-in initial values that are not empty, but you can set

them in the usual ways (see Section 10.3 [Variables Used by Implicit Rules], page 115).

Several special variables are set automatically to a new value for each rule; these are called

the automatic variables (see Section 10.5.3 [Automatic Variables], page 120).

If you’d like a variable to be set to a value only if it’s not already set, then you can

use the shorthand operator ‘?=’ instead of ‘=’. These two settings of the variable ‘FOO’ are

identical (see Section 8.10 [The origin Function], page 94):

FOO ?= bar

and

ifeq ($(origin FOO), undefined)

FOO = bar

endif

The shell assignment operator ‘!=’ can be used to execute a program and set a variable

to its output. This operator ﬁrst evaluates the right-hand side, then passes that result to

the shell for execution. If the result of the execution ends in a newline, that one newline is

removed; all other newlines are replaced by spaces. The resulting string is then placed into

the named recursively-expanded variable. For example:

hash != printf ’\043’

file_list != find . -name ’*.c’

If the result of the execution could produce a $, and you don’t intend what follows that

to be interpreted as a make variable or function reference, then you must replace every $

with $$ as part of the execution. Alternatively, you can set a simply expanded variable to

the result of running a program using the shell function call. See Section 8.13 [The shell

Function], page 97. For example:

hash := $(shell printf ’\043’)

var := $(shell find . -name "*.c")

6.6 Appending More Text to Variables

Often it is useful to add more text to the value of a variable already deﬁned. You do this

with a line containing ‘+=’, like this:

objects += another.o

1 2 ... 71 72 73 74 75 76 77 78 79 80 81 ... 211 212

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