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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 makefile used by the sub-`make` makefile unless you use the ``-e'` switch (see section 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 section 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 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 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 ...

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 section 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 `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 (see section 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 section The Variable `MAKEFILES`.

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