02-various-dependencies.md

Multiple Dependencies

Gaston Sanchez

Learning Objectives

• Rules with multiple dependencies
• Write a rule that takes various dependencies

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Dependencies

In the introduction, we talked about the concept of Make rules, and we described the conceptual structure of a rule:

target: dependencies ...
    commands
    ...

A rule tells Make how to execute a series of commands in order to build a target file from source files (i.e. the dependencies)

Our first example was very simple, formed with one rule consisting of a target file output.html and one dependency input.md

output.html: input.md
    pandoc input.md -s -o output.html

More than one dependency

Often, a target file will depend on several input files. For instance, consider the example 02-various-dependencies/ in the folder examples/

This example is about a banana bread recipe. There is one target output.html, but now there are three dependencies: first.md, second.md, and third.md.

We’ll use pandoc again to generate the html output file. One of the nice things about pandoc is that you can pass any number of input files and it will concatenate them to produce the output:

output.html: first.md second.md third.md
    pandoc first.md second.md third.md -s -o output.html

The pandoc command can actually be shorten using the asterisk wildcard to indicate all the input files with extension .md:

output.html: first.md second.md third.md
    pandoc *.md -s -o output.html

How does it work?

When you have multiple dependencies in a rule, everytime there is a change in any of the input files, running make will execute the commands.

If all the dependency files are up-to-date, running make will do nothing.

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