Pattern Rules

Overview

Teaching: 10 min
Exercises: 0 min

Questions

• How can I define rules to operate on similar files?

Objectives

• Write Make pattern rules.

Our Makefile still has repeated content. The rules for each .dat file are identical apart from the text and data file names. We can replace these rules with a single pattern rule which can be used to build any .dat file from a .txt file in books/:

%.dat : books/%.txt countwords.py
	python countwords.py $< $*.dat

% is a Make wildcard. $* is a special variable which gets replaced by the stem with which the rule matched.

This rule can be interpreted as: “In order to build a file named [something].dat (the target) find a file named books/[that same something].txt (the dependency) and run countwords.py [the dependency] [the target].”

If we re-run Make,

$ make clean
$ make dats

then we get:

python countwords.py books/isles.txt isles.dat
python countwords.py books/abyss.txt abyss.dat
python countwords.py books/last.txt last.dat

Note that we can still use Make to build individual .dat targets as before, and that our new rule will work no matter what stem is being matched.

$ make sierra.dat

which gives the output below:

python countwords.py books/sierra.txt sierra.dat

Using Make Wildcards

The Make % wildcard can only be used in a target and in its dependencies. It cannot be used in actions. In actions, you may however use $*, which will be replaced by the stem with which the rule matched.

Our Makefile is now much shorter and cleaner:

# Generate summary table.
results.txt : testzipf.py isles.dat abyss.dat last.dat
	python $< *.dat > $@

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

%.dat : books/%.txt countwords.py
	python countwords.py $< $*.dat

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

Where We Are

This Makefile contains all of our work so far.

This episode has introduced pattern rules, and used the $* variable in the dat rule in order to explain how to use it. Arguably, a neater solution would have been to use $@ to refer to the target of the current rule (see below), but then we wouldn’t have learned about $*.

%.dat : books/%.txt countwords.py
	python countwords.py $< $@

Key Points

• Use the wildcard % as a placeholder in targets and dependencies.

• Use the special variable $* to refer to matching sets of files in actions.