Variables

Overview

Teaching: 15 min
Exercises: 5 min

Questions

• How can I eliminate redundancy in my Makefiles?

Objectives

• Use variables in a Makefile.

• Explain the benefits of decoupling configuration from computation.

Despite our efforts, our Makefile (available via git checkout 06-variables) still has repeated content, i.e. the name of our script – countwords.py, and the program we use to run it – python. If we renamed our script we’d have to update our Makefile in multiple places.

We can introduce a Make variable (called a macro in some versions of Make) to hold our script name:

COUNT_SRC=countwords.py

This is a variable assignment - COUNT_SRC is assigned the value countwords.py.

countwords.py is our script and it is invoked by passing it to python. We can introduce another couple of variables to represent this execution:

LANGUAGE=python
COUNT_EXE=$(LANGUAGE) $(COUNT_SRC)

$(...) tells Make to replace a variable with its value when Make is run. This is a variable reference. At any place where we want to use the value of a variable we have to write it, or reference it, in this way.

Here we reference the variables LANGUAGE and COUNT_SRC. This tells Make to replace the variable LANGUAGE with its value python, and to replace the variable COUNT_SRC with its value countwords.py. When Make is run it will assign to COUNT_EXE the value python countwords.py.

Defining the variable COUNT_EXE in this way avoids repeating python in our Makefile, and allows us to easily change how our script is run (e.g. we might want to use a different version of Python and need to change python to python2 – or we might want to rewrite the script using another language (e.g. switch from Python to R)).

Use Variables

Update Makefile so that the %.dat rule references the variables COUNT_SRC and COUNT_EXE. Then do the same for the zipf-test.py script and the results.txt rule, using ZIPF_SRC and ZIPF_EXE as variable names

Solution

This Makefile contains a solution to this challenge.

We place variables at the top of a Makefile so they are easy to find and modify. Alternatively, we can pull them out into a new file that just holds variable definitions (i.e. delete them from the original makefile). Let us create config.mk:

# Count words script.
LANGUAGE=python
COUNT_SRC=countwords.py
COUNT_EXE=$(LANGUAGE) $(COUNT_SRC)

# Test Zipf's rule
ZIPF_SRC=testzipf.py
ZIPF_EXE=$(LANGUAGE) $(ZIPF_SRC)

We can then import config.mk into Makefile using:

include config.mk

We can re-run Make to see that everything still works:

$ make clean
$ make dats
$ make results.txt

We have separated the configuration of our Makefile from its rules, the parts that do all the work. If we want to change our script name or how it is executed we just need to edit our configuration file, not our source code in Makefile. Decoupling code from configuration in this way is good programming practice, as it promotes more modular, flexible and reusable code.

Key Points

• Define variables by assigning values to names.

• Reference variables using $(...).