OverviewTeaching: 10 min
Exercises: 5 minQuestions
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 testzipf.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 countwords.py books/isles.txt isles.dat abyss.dat : books/abyss.txt python countwords.py books/abyss.txt abyss.dat last.dat : books/last.txt python countwords.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.
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 testzipf.py abyss.dat isles.dat last.dat > results.txt
Looking at the results file name first, we can replace it in the action
results.txt : isles.dat abyss.dat last.dat python testzipf.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 testzipf.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
python countwords.py books/isles.txt isles.dat python countwords.py books/abyss.txt abyss.dat python countwords.py books/last.txt last.dat python testzipf.py isles.dat abyss.dat last.dat > results.txt
What will happen if you now execute:
$ touch *.dat $ make results.txt
- all files recreated
The rules for
*.datare not executed because their corresponding
.txtfiles haven’t been modified.
If you run:
$ touch books/*.txt $ make results.txt
you will find that the
.datfiles as well as
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
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
we add additional dependencies (as we will soon do) then we don’t want
these being passed as input files to
countwords.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’.
.datRules to Use Automatic Variables
.datrule 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.
See this file for a solution to this challenge.
$@to refer to the target of the current rule.
$^to refer to the dependencies of the current rule.
$<to refer to the first dependency of the current rule.