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 do not
override what is specified in the makefile used by the sub-make
makefile unless you use the ‘-e’ switch (see Summary of Options).
To pass down, or export, a variable, make adds the variable
and its value to the environment for running each command. The
sub-make, in turn, uses the environment to initialize its table
of variable values. See Variables from the Environment.
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 letters, 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 Choosing the Shell.
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 Options/Recursion.
Variables are not normally passed down if they were created by
default by make (see Variables Used by Implicit Rules). 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:
variable := value
export variable
Likewise,
export variable += value
is just like:
variable += value
export variable
See Appending More Text to Variables.
You may notice that the export and unexport directives
work in make in the same way they work in the shell, sh.
If you want all variables to be exported by default, you can use
export by itself:
export
This tells make that variables which are not explicitly mentioned
in an export or unexport directive should be exported.
Any variable given in an unexport directive will still not
be exported. If you use export by itself to export variables by
default, variables whose names contain characters other than
alphanumerics and underscores will not be exported unless specifically
mentioned in an export directive.
The behavior elicited by an export directive by itself was the
default in older versions of GNU make. If your makefiles depend
on this behavior and you want to be compatible with old versions of
make, you can write a rule for the special target
.EXPORT_ALL_VARIABLES instead of using the export directive.
This will be ignored by old makes, while the export
directive will cause a syntax error.
Likewise, you can use unexport by itself to tell make
not to export variables by default. Since this is the default
behavior, you would only need to do this if export had been used
by itself earlier (in an included makefile, perhaps). You
cannot use export and unexport by themselves to
have variables exported for some commands and not for others. The last
export or unexport directive that appears by itself
determines the behavior for the entire run of make.
As a special feature, the variable MAKELEVEL is changed when it
is passed down from level to level. This variable's value is a string
which is the depth of the level as a decimal number. The value is
‘0’ for the top-level make; ‘1’ for a sub-make,
‘2’ for a sub-sub-make, and so on. The incrementation
happens when make sets up the environment for a command.
The main use of MAKELEVEL is to test it in a conditional
directive (see Conditional Parts of Makefiles); this
way you can write a makefile that behaves one way if run recursively and
another way if run directly by you.
You can use the variable MAKEFILES to cause all sub-make
commands to use additional makefiles. The value of MAKEFILES is
a whitespace-separated list of file names. This variable, if defined in
the outer-level makefile, is passed down through the environment; then
it serves as a list of extra makefiles for the sub-make to read
before the usual or specified ones. See The Variable MAKEFILES.