Automatic Variables

Overview

Teaching: 15 min
Exercises: 15 min
Questions
  • How can I abbreviate the rules in my Makefiles?

Objectives
  • Use Make automatic variables to remove duplication in a Makefile.

  • Explain why shell wildcards in dependencies can cause problems.

After the exercise at the end of the previous episode, our Makefile looked like this:

# Generate summary table.
results.txt : isles.dat abyss.dat last.dat
        python zipf_test.py abyss.dat isles.dat last.dat > results.txt

# Count words.
.PHONY : dats
dats : isles.dat abyss.dat last.dat

isles.dat : books/isles.txt
	python wordcount.py books/isles.txt isles.dat

abyss.dat : books/abyss.txt
	python wordcount.py books/abyss.txt abyss.dat

last.dat : books/last.txt
	python wordcount.py books/last.txt last.dat

.PHONY : clean
clean :
	rm -f *.dat
	rm -f results.txt

Our Makefile has a lot of duplication. For example, the names of text files and data files are repeated in many places throughout the Makefile. Makefiles are a form of code and, in any code, repeated code can lead to problems e.g. we rename a data file in one part of the Makefile but forget to rename it elsewhere.

D.R.Y. (Don’t Repeat Yourself)

In many programming languages, the bulk of the language features are there to allow the programmer to describe long-winded computational routines as short, expressive, beautiful code. Features in Python or R or Java, such as user-defined variables and functions are useful in part because they mean we don’t have to write out (or think about) all of the details over and over again. This good habit of writing things out only once is known as the “Don’t Repeat Yourself” principle or D.R.Y.

Let us set about removing some of the repetition from our Makefile.

In our results.txt rule we duplicate the data file names and the name of the results file name:

results.txt : isles.dat abyss.dat last.dat
	python zipf_test.py abyss.dat isles.dat last.dat > results.txt

Looking at the results file name first, we can replace it in the action with $@:

results.txt : isles.dat abyss.dat last.dat
	python zipf_test.py abyss.dat isles.dat last.dat > $@

$@ is a Make automatic variable which means ‘the target of the current rule’. When Make is run it will replace this variable with the target name.

We can replace the dependencies in the action with $^:

results.txt : isles.dat abyss.dat last.dat
	python zipf_test.py $^ > $@

$^ is another automatic variable which means ‘all the dependencies of the current rule’. Again, when Make is run it will replace this variable with the dependencies.

Let’s update our text files and re-run our rule:

$ touch books/*.txt
$ make results.txt

We get:

python wordcount.py books/isles.txt isles.dat
python wordcount.py books/abyss.txt abyss.dat
python wordcount.py books/last.txt last.dat
python zipf_test.py isles.dat abyss.dat last.dat > results.txt

Update Dependencies

What will happen if you now execute:

$ touch *.dat
$ make results.txt
  1. nothing
  2. all files recreated
  3. only .dat files recreated
  4. only results.txt recreated

Solution

4. Only results.txt recreated.

The rules for *.dat are not executed because their corresponding .txt files haven’t been modified.

If you run:

$ touch books/*.txt
$ make results.txt

you will find that the .dat files as well as results.txt are recreated.

As we saw, $^ means ‘all the dependencies of the current rule’. This works well for results.txt as its action treats all the dependencies the same - as the input for the zipf_test.py script.

However, for some rules, we may want to treat the first dependency differently. For example, our rules for .dat use their first (and only) dependency specifically as the input file to wordcount.py. If we add additional dependencies (as we will soon do) then we don’t want these being passed as input files to wordcount.py as it expects only one input file to be named when it is invoked.

Make provides an automatic variable for this, $< which means ‘the first dependency of the current rule’.

Rewrite .dat Rules to Use Automatic Variables

Rewrite each .dat rule to use the automatic variables $@ (‘the target of the current rule’) and $< (‘the first dependency of the current rule’). This file contains the Makefile immediately before the challenge.

Solution

See this file for a solution to this challenge.

Key Points

  • Use $@ to refer to the target of the current rule.

  • Use $^ to refer to the dependencies of the current rule.

  • Use $< to refer to the first dependency of the current rule.