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52 GNU make

lines containing MAKE are executed normally despite the presence of a ﬂag that causes most

recipes not to be run. The usual MAKEFLAGS mechanism passes the ﬂags to the sub-make

(see Section 5.7.3 [Communicating Options to a Sub-make], page 54), so your request to

touch the ﬁles, or print the recipes, is propagated to the subsystem.

5.7.2 Communicating Variables to a Sub-make

Variable values of the top-level make can be passed to the sub-make through the environment

by explicit request. These variables are deﬁned in the sub-make as defaults, but they do

not override variables deﬁned in the makeﬁle used by the sub-make unless you use the ‘-e’

switch (see Section 9.7 [Summary of Options], page 104).

To pass down, or export, a variable, make adds the variable and its value to the envi-

ronment for running each line of the recipe. The sub-make, in turn, uses the environment

to initialize its table of variable values. See Section 6.10 [Variables from the Environment],

page 70.

Except by explicit request, make exports a variable only if it is either deﬁned in the

environment initially or set on the command line, and if its name consists only of let-

ters, numbers, and underscores. Some shells cannot cope with environment variable names

consisting of characters other than letters, numbers, and underscores.

The value of the make variable SHELL is not exported. Instead, the value of the SHELL

variable from the invoking environment is passed to the sub-make. You can force make to

export its value for SHELL by using the export directive, described below. See Section 5.3.2

[Choosing the Shell], page 45.

The special variable MAKEFLAGS is always exported (unless you unexport it). MAKEFILES

is exported if you set it to anything.

make automatically passes down variable values that were deﬁned on the command line,

by putting them in the MAKEFLAGS variable. See the next section.

Variables are not normally passed down if they were created by default by make (see

Section 10.3 [Variables Used by Implicit Rules], page 115). The sub-make will deﬁne these

for itself.

If you want to export speciﬁc variables to a sub-make, use the export directive, like this:

export variable ...

If you want to prevent a variable from being exported, use the unexport directive, like this:

unexport variable ...

In both of these forms, the arguments to export and unexport are expanded, and so could

be variables or functions which expand to a (list of) variable names to be (un)exported.

As a convenience, you can deﬁne a variable and export it at the same time by doing:

export variable = value

has the same result as:

variable = value

export variable

and

export variable := value

has the same result as:

1 2 ... 57 58 59 60 61 62 63 64 65 66 67 ... 211 212

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