Variables in make can come from the environment in which
make is run. Every environment variable that make sees
when it starts up is transformed into a make variable with the
same name and value. However, an explicit assignment in the makefile,
or with a command argument, overrides the environment. (If the
‘-e’ flag is specified, then values from the environment override
assignments in the makefile. See Summary of Options. But this is not recommended practice.)
Thus, by setting the variable CFLAGS in your environment, you can
cause all C compilations in most makefiles to use the compiler switches you
prefer. This is safe for variables with standard or conventional meanings
because you know that no makefile will use them for other things. (Note
this is not totally reliable; some makefiles set CFLAGS explicitly
and therefore are not affected by the value in the environment.)
When make runs a command script, variables defined in the
makefile are placed into the environment of that command. This allows
you to pass values to sub-make invocations (see Recursive Use of make). By default, only variables that came
from the environment or the command line are passed to recursive
invocations. You can use the export directive to pass other
variables. See Communicating Variables to a Sub-make, for full details.
Other use of variables from the environment is not recommended. It is not wise for makefiles to depend for their functioning on environment variables set up outside their control, since this would cause different users to get different results from the same makefile. This is against the whole purpose of most makefiles.
Such problems would be especially likely with the variable
SHELL, which is normally present in the environment to specify
the user's choice of interactive shell. It would be very undesirable
for this choice to affect make; so, make handles the
SHELL environment variable in a special way; see Choosing the Shell.