(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
$ cat Makefile
foo = foo
$ make foo=bar # GNU make 3.79.1
make: Entering directory `/home/adl'
make: Leaving directory `/home/adl'
$ pmake foo=bar # BSD make
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
$(MAKE) foo=$(foo) two
You need to foresee all macros that a user might want to override if
you do that.
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