Overview
Teaching: 15 min Exercises: 15 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, namely
the name of our script, wordcount.py
. 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=wordcount.py
This is a variable assignment -
COUNT_SRC
is assigned the value wordcount.py
.
wordcount.py
is our script and it is invoked by passing it to
python
. We can introduce another variable to represent this
execution:
COUNT_EXE=python $(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 variable COUNT_SRC
. This tells Make to
replace the variable COUNT_SRC
with its value wordcount.py
. When
Make is run it will assign to COUNT_EXE
the value python
wordcount.py
.
Defining the variable COUNT_EXE
in this way allows us to easily
change how our script is run (if, for example, we changed the language
used to implement our script from Python to R).
Use Variables
Update
Makefile
so that the%.dat
rule references the variablesCOUNT_SRC
andCOUNT_EXE
. Then do the same for thezipf-test.py
script and theresults.txt
rule, usingZIPF_SRC
andZIPF_EXE
as variable namesSolution
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.
COUNT_SRC=wordcount.py
COUNT_EXE=python $(COUNT_SRC)
# Test Zipf's rule
ZIPF_SRC=zipf_test.py
ZIPF_EXE=python $(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.
Where We Are
This Makefile and its accompanying
config.mk
contain all of our work so far.
Key Points
Define variables by assigning values to names.
Reference variables using
$(...)
.