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06-variables.md

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Automation and Make
Variables
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Learning Objectives {.objectives}

  • Use variables in a Makefile.
  • Assign values to variables.
  • Reference variables.
  • Explain the benefits of decoupling configuration from computation.

Despite our efforts, our Makefile still has repeated content, namely the name of our script, wordcount.py. If we renamed our script we'd have to update our Makefile in multiple places.

We can introduce a Make variable (called a macro in some versions of Make) to hold our script name:

COUNT_SRC=wordcount.py

This is a variable assignment - COUNT_SRC is assigned the value wordcount.py.

wordcount.py is our script and it is invoked by passing it to python. We can introduce another variable to represent this execution:

COUNT_EXE=python $(COUNT_SRC)

$(...) tells Make to replace a variable with its value when Make is run. This is a variable reference. At any place where we want to use the value of a variable we have to write it, or reference it, in this way.

Here we reference the variable COUNT_SRC. This tells Make to replace the variable COUNT_SRC with its value wordcount.py. When Make is run it will assign to COUNT_EXE the value python wordcount.py.

Defining the variable COUNT_EXE in this way allows us to easily change how our script is run (if, for example, we changed the language used to implement our script from Python to R).

Use variables {.challenge}

Update Makefile so that the %.dat and analysis.tar.gz rules reference the variables COUNT_SRC and COUNT_EXE.

We place variables at the top of a Makefile means they are easy to find and modify. Alternatively, we can pull them out into a new Makefile that just holds variable definitions. Let us create config.mk:

# Count words script.
COUNT_SRC=wordcount.py
COUNT_EXE=python $(COUNT_SRC)

We can then import this Makefile into Makefile using:

include config.mk

We can re-run Make to see that everything still works:

$ make clean
$ make dats
$ make analysis.tar.gz

We have separated the configuration of our Makefile from its rules, the parts that do all the work. If we want to change our script name or how it is executed we just need to edit our configuration file, not our source code in Makefile. Decoupling code from configuration in this way is good programming practice, as it promotes more modular, flexible and reusable code.