(autoconf.info)Macros and Submakes


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11.7 `make macro=value' and Submakes
====================================

A command-line variable definition such as `foo=bar' overrides any
definition of `foo' in a makefile.  Some `make' implementations (such
as GNU `make') propagate this override to subsidiary invocations of
`make'.  Some other implementations do not pass the substitution along
to submakes.

     $ cat Makefile
     foo = foo
     one:
             @echo $(foo)
             $(MAKE) two
     two:
             @echo $(foo)
     $ make foo=bar            # GNU make 3.79.1
     bar
     make two
     make[1]: Entering directory `/home/adl'
     bar
     make[1]: Leaving directory `/home/adl'
     $ pmake foo=bar           # BSD make
     bar
     pmake two
     foo

   You have a few possibilities if you do want the `foo=bar' override
to propagate to submakes.  One is to use the `-e' option, which causes
all environment variables to have precedence over the makefile macro
definitions, and declare foo as an environment variable:

     $ env foo=bar make -e

   The `-e' option is propagated to submakes automatically, and since
the environment is inherited between `make' invocations, the `foo'
macro is overridden in submakes as expected.

   This syntax (`foo=bar make -e') is portable only when used outside
of a makefile, for instance from a script or from the command line.
When run inside a `make' rule, GNU `make' 3.80 and prior versions
forget to propagate the `-e' option to submakes.

   Moreover, using `-e' could have unexpected side effects if your
environment contains some other macros usually defined by the makefile.
(See also the note about `make -e' and `SHELL' below.)

   Another way to propagate overrides to submakes is to do it manually,
from your makefile:

     foo = foo
     one:
             @echo $(foo)
             $(MAKE) foo=$(foo) two
     two:
             @echo $(foo)

   You need to foresee all macros that a user might want to override if
you do that.


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