Automation and Make

Makefiles

Learning Objectives

• Recognise the key parts of a Makefile, rules, targets, dependencies and actions.
• Write a simple Makefile.
• Run Make from the shell.
• Explain when and why to mark targets as .PHONY.
• Explain constraints on dependencies.

Create a file, called Makefile, with the following content:

# Count words.
isles.dat : books/isles.txt
        python wordcount.py books/isles.txt isles.dat

This is a simple build file, which for Make is called a Makefile - a file executed by Make. Let us go through each line in turn:

• # denotes a comment. Any text from # to the end of the line is ignored by Make.
• isles.dat is a target, a file to be created, or built.
• books/isles.txt is a dependency, a file that is needed to build or update the target. Targets can have zero or more dependencies.
• : separates targets from dependencies.
• python wordcount.py books/isles.txt isles.dat is an action, a command to run to build or update the target using the dependencies. Targets can have zero or more actions.
• Actions are indented using the TAB character, not 8 spaces. This is a legacy of Make’s 1970’s origins.
• Together, the target, dependencies, and actions form a rule.

Our rule above describes how to build the target isles.dat using the action python wordcount.py and the dependency books/isles.txt.

Let’s first sure we start from scratch and delete the .dat and .jpg files we created:

$ rm *.dat *.jpg

By default, Make looks for a Makefile, called Makefile, and we can run Make as follows:

$ make

Make prints out the actions it executes:

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

If we see,

Makefile:3: *** missing separator.  Stop.

then we have used a space instead of a TAB characters to indent one of our actions.

We don’t have to call our Makefile Makefile. However, if we call it something else we need to tell Make where to find it. This we can do using -f flag. For example:

$ make -f Makefile

As we have re-run our Makefile, Make now informs us that:

make: `isles.dat' is up to date.

This is because our target, isles.dat, has now been created, and Make will not create it again. To see how this works, let’s pretend to update one of the text files. Rather than opening the file in an editor, we can use the shell touch command to update its timestamp (which would happen if we did edit the file):

$ touch books/isles.txt

If we compare the timestamps of books/isles.txt and isles.dat,

$ ls -l books/isles.txt isles.dat

then we see that isles.dat, the target, is now older thanbooks/isles.txt, its dependency:

-rw-r--r--    1 mjj      Administ   323972 Jun 12 10:35 books/isles.txt
-rw-r--r--    1 mjj      Administ   182273 Jun 12 09:58 isles.dat

If we run Make again,

$ make

then it recreates isles.dat:

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

When it is asked to build a target, Make checks the ‘last modification time’ of both the target and its dependencies. If any dependency has been updated since the target, then the actions are re-run to update the target.

Let’s add another rule to the end of Makefile:

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

If we run Make,

$ make

then we get:

make: `isles.dat' is up to date.

Nothing happens because Make attempts to build the first target it finds in the Makefile, the default target, which is isles.dat which is already up-to-date. We need to explicitly tell Make we want to build abyss.dat:

$ make abyss.dat

Now, we get:

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

We may want to remove all our data files so we can explicitly recreate them all. We can introduce a new target, and associated rule, clean:

clean : 
        rm -f *.dat

This is an example of a rule that has no dependencies. clean has no dependencies on any .dat file as it makes no sense to create these just to remove them. We just want to remove the data files whether or not they exist. If we run Make and specify this target,

$ make clean

then we get:

rm -f *.dat

There is no actual thing built called clean. Rather, it is a short-hand that we can use to execute a useful sequence of actions. Such targets, though very useful, can lead to problems. For example, let us recreate our data files, create a directory called clean, then run Make:

$ make isles.dat abyss.dat
$ mkdir clean
$ make clean

We get:

make: `clean' is up to date.

Make finds a file (or directory) called clean and, as its clean rule has no dependencies, assumes that clean has been built and is up-to-date and so does not execute the rule’s actions. As we are using clean as a short-hand, we need to tell Make to always execute this rule if we run make clean, by telling Make that this is a phony target, that it does not build anything. This we do by marking the target as .PHONY:

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

If we run Make,

$ make clean

then we get:

rm -f *.dat

We can add a similar command to create all the data files:

.PHONY : dats
dats : isles.dat abyss.dat

This is an example of a rule that has dependencies that are targets of other rules. When Make runs, it will check to see if the dependencies exist and, if not, will see if rules are available that will create these. If such rules exist it will invoke these first, otherwise Make will raise an error.

This rule is also an example of a rule that has no actions. It is used purely to trigger the build of its dependencies, if needed.

If we run,

$ make dats

then Make creates the data files:

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

If we run dats again,

$ make dats

then Make sees that the data files exist:

make: Nothing to be done for `dats'.

Our Makefile now looks like this:

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

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

The following figure shows the dependencies embodied within our Makefile, involved in building the dats target:

Write two new rules

Write a new rule for last.dat, created from books/last.txt.

Update the dats rule with this target.

Write a new rule for analysis.tar.gz, which creates an archive of the data files. The rule needs to:

• Depend upon each of the three .dat files.
• Invoke the action tar -czf analysis.tar.gz isles.dat abyss.dat last.dat

Update clean to remove analysis.tar.gz.

The following figure shows the dependencies embodied within our Makefile, involved in building the analysis.tar.gz target: