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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 Summary of Options).
To pass down, or export, a variable, make
adds the
variable and its value to the environment for running each line of the
recipe. 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 if set on the command
line and its name consists only of 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 Communicating Options to a Sub-make
.
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.
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 add the special target .EXPORT_ALL_VARIABLES
to your makefile
instead of using the export
directive. This will be ignored by old
make
s, while the export
directive will cause a syntax
error.
When using export
by itself or .EXPORT_ALL_VARIABLES
to export
variables by default, only variables whose names consist solely of
alphanumerics and underscores will be exported. To export other variables you
must specifically mention them in an export
directive.
Adding a variable’s value to the environment requires it to be expanded. If
expanding a variable has side-effects (such as the info
or eval
or similar functions) then these side-effects will be seen every time a
command is invoked. You can avoid this by ensuring that such variables have
names which are not exportable by default. However, a better solution is to
not use this “export by default” facility at all, and instead
explicitly export
the relevant variables by name.
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 recipes 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 recipe.
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
.
Next: Communicating Options to a Sub-make
, Previous: How the MAKE
Variable Works, Up: Recursive Use of make
[Contents][Index]