Makefile export and override

Makefile

foo ?= default
override foo += added
export foo

all::
	@echo outer: foo is "$(foo)"
	@$(MAKE) -s -f sub-make.mk

sub-make.mk

# Variable 'foo' was "exported" from the top-level Makefile.
all::
	@echo inner: foo is "$(foo)"

test.sh

set -x

# Q1
make

# Q2
foo=env make

# Q3
make foo=cmd