Question

Can I pass variables to a GNU Makefile as command line arguments? In other words, I want to pass some arguments which will eventually become variables in the Makefile.

Answer 1

You have several options to set up variables from outside your makefile:

• From environment – each environment variable is transformed into a makefile variable with the same name and value.

  You may also want to set -e option (aka --environments-override) on, and your environment variables will override assignments made into makefile (unless these assignments themselves use the override directive . However, it’s not recommended, and it’s much better and flexible to use ?= assignment (the conditional variable assignment operator, it only has an effect if the variable is not yet defined):

  FOO?=default_value_if_not_set_in_environment
  

  Note that certain variables are not inherited from environment:

  • MAKE is gotten from name of the script
  • SHELL is either set within a makefile, or defaults to /bin/sh (rationale: commands are specified within the makefile, and they’re shell-specific).

• From command line – make can take variable assignments as part of his command line, mingled with targets:

  make target FOO=bar
  

  But then all assignments to FOO variable within the makefile will be ignored unless you use the override directive in assignment. (The effect is the same as with -e option for environment variables).

• Exporting from the parent Make – if you call Make from a Makefile, you usually shouldn’t explicitly write variable assignments like this:

  # Don't do this!
  target:
          $(MAKE) -C target CC=$(CC) CFLAGS=$(CFLAGS)
  

  Instead, better solution might be to export these variables. Exporting a variable makes it into the environment of every shell invocation, and Make calls from these commands pick these environment variable as specified above.

  # Do like this
  CFLAGS=-g
  export CFLAGS
  target:
          $(MAKE) -C target
  

  You can also export all variables by using export without arguments.