Automation and Make

Automatic variables

After the exercise at the end of the previous part, our Makefile look like this:

# 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

# Generate archive file.
analysis.tar.gz : isles.dat abyss.dat last.dat
        tar -czf analysis.tar.gz isles.dat abyss.dat last.dat

.PHONY : clean
clean :
        rm -f *.dat
        rm -f analysis.tar.gz

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 the rename it elsewhere. Let us set about removing some of this repetition.

In our analysis.tar.gz rule we duplicate the data file names and the archive name:

analysis.tar.gz : isles.dat abyss.dat last.dat
        tar -czf analysis.tar.gz isles.dat abyss.dat last.dat

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

analysis.tar.gz : isles.dat abyss.dat last.dat
        tar -czf $@ isles.dat abyss.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 $^:

analysis.tar.gz : isles.dat abyss.dat last.dat
        tar -czf $@ $^

$^ 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 analysis.tar.gz

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
tar -czf analysis.tar.gz isles.dat abyss.dat last.dat

We can use the bash wild-card in our dependency list:

analysis.tar.gz : *.dat
        tar -czf $@ $^

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

$ touch books/*.txt
$ make analysis.tar.gz

We get the same as above.

Now let’s delete the data files and re-run our rule:

$ make clean
$ make analysis.tar.gz

We get:

make: *** No rule to make target `*.dat', needed by `analysis.tar.gz'.  Stop.

As there are no files that match the pattern *.dat the name *.dat is used as a file name itself, and there is no file matching that, nor any rule so we get an error. We need to explicitly rebuild the .dat files first:

$ make dats
$ make analysis.tar.gz

$ touch *.dat
$ make analysis.tar.gz

As we saw, $^ means ‘all the dependencies of the current rule’. This works well for analysis.tar.gz as its action treats all the dependencies the same - as the contents of the archive.

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’.