(make) Variables/Recursion

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(make) MAKE Variable

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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 ( Summary of Options
 Options Summary.).
 
    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.   Variables from the Environment 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.
  Options/Recursion.
 
    Variables are _not_ normally passed down if they were created by
 default by `make' ( Variables Used by Implicit Rules Implicit
 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
 
 and
 
      export VARIABLE := value
 
 has the same result as:
 
      VARIABLE := value
      export VARIABLE
 
    Likewise,
 
      export VARIABLE += value
 
 is just like:
 
      VARIABLE += value
      export VARIABLE
 
  Appending More Text to Variables Appending.
 
    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
 ( Conditional Parts of Makefiles Conditionals.); 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.   The Variable `MAKEFILES'
 MAKEFILES Variable.
 

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