Flexible handling of variables in a makefile

makefile_var_handling

# Example for the ultimate flexible variable handling in a makefile.
# Code adapted from "The GNU Make Book", pos. 300 (e-book).
# 
# Case 1: Default in makefile
# With no environment variable and no option set in command line,
# TEST contains foo:
# $ make
# foo file
#
# Case 2: Environment variable
# With environment variable set, TEST contains bar
# $ export TEST=bar
# $ make
# bar environment
#
# Case 3: Command line variable
# Setting the variable via command line beats all.
# $ make TEST=foobar
# foobar command line

# if TEST is not set, it will set to foo;
# otherwise, it is left unchanged.
TEST ?= foo

PHONY: all

all:
	$(info $(TEST) $(origin TEST))