OverviewTeaching: 15 min
Exercises: 5 minQuestions
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 still has repeated content, i.e.
the name of our script –
countwords.py, and the program we use to run it –
If we renamed our script we’d have to update our Makefile in multiple places.
This is a variable assignment -
COUNT_SRC is assigned the value
countwords.py is our script and it is invoked by passing it to
python. We can introduce another couple of variables to represent this
LANGUAGE=python COUNT_EXE=$(LANGUAGE) $(COUNT_SRC)
$(...) tells Make to replace a variable with its value when Make
is run. This is a variable reference. At
any place where we want to use the value of a variable we have to
write it, or reference it, in this way.
Here we reference the variables
COUNT_SRC. This tells Make to
replace the variable
LANGUAGE with its value
and to replace the variable
COUNT_SRC with its value
Make is run it will assign to
COUNT_EXE the value
Defining the variable
COUNT_EXE in this way avoids repeating
python in our
Makefile, and allows us to easily
change how our script is run (e.g. we might want to use a different
version of Python and need to change
python2 – or we might want to
rewrite the script using another language (e.g. switch from Python to R)).
Makefileso that the
%.datrule references the variables
COUNT_EXE. Then do the same for the
zipf-test.pyscript and the
ZIPF_EXEas variable names
This Makefile contains a solution to this challenge.
We place variables at the top of a Makefile so they are easy to
find and modify. Alternatively, we can pull them out into a new
file that just holds variable definitions (i.e. delete them from
the original makefile). Let us create
# Count words script. LANGUAGE=python COUNT_SRC=countwords.py COUNT_EXE=$(LANGUAGE) $(COUNT_SRC) # Test Zipf's rule ZIPF_SRC=testzipf.py ZIPF_EXE=$(LANGUAGE) $(ZIPF_SRC)
We can then import
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.
Where We Are
Define variables by assigning values to names.
Reference variables using