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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 do not
override what is specified in the sub-`make''s makefile unless you use
the `-e' switch (Note: 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. Note: 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 special variables `SHELL' and `MAKEFLAGS' are always exported
(unless you unexport them). `MAKEFILES' is exported if you set it to
Variables are *not* normally passed down if they were created by
default by `make' (Note: Variables Used by Implicit Rules
Variables.). 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 ...
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 := value
has the same result as:
VARIABLE := value
export VARIABLE += value
is just like:
VARIABLE += value
Note: 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:
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 `make's, 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
(Note: 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. *Note The Variable `MAKEFILES':
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