File: make.info,  Node: Environment,  Next: Target-specific,  Prev: Undefine Directive,  Up: Using Variables

6.10 Variables from the Environment
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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.
*Note Summary of Options: Options Summary.  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 recipe, some variables defined in the makefile are
placed into the environment of each command 'make' invokes.  By default,
only variables that came from the 'make''s environment or set on its
command line are placed into the environment of the commands.  You can
use the 'export' directive to pass other variables.  *Note Communicating
Variables to a Sub-'make': Variables/Recursion, 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 *note Choosing the Shell::.