Passing additional variables from command line to make

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.

Solution

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.