make macro=value
and SubmakesA 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.)
If you can foresee all macros that a user might want to override, then you can propagate them to submakes manually, from your makefile:
foo = foo one: @echo $(foo) $(MAKE) foo=$(foo) two two: @echo $(foo)
Another way to propagate a variable to submakes in a portable way is to expand an extra variable in every invocation of ‘$(MAKE)’ within your makefile:
foo = foo one: @echo $(foo) $(MAKE) $(SUBMAKEFLAGS) two two: @echo $(foo)
Users must be aware that this technique is in use to take advantage of
it, e.g. with make foo=bar SUBMAKEFLAGS='foo=bar'
, but it
allows any macro to be overridden. Makefiles generated by
automake use this technique, expanding $(AM_MAKEFLAGS)
on the command lines of submakes (see Automake).