Rake is a tool for controlling builds. In this part of the Rake tutorial, we see how to organize the Rake actions to apply to many similar tasks.

In the RakeTutorialIntroduction, we talked about the basics of specifying dependencies and associating actions to build the files. We ended up with a nice Rakefile that built our simple C program, but with some duplication in the build rules.

But First, Some Extra Rake Targets

But before we get into all that, lets add some convience targets to our Rakefile. First of all, it would be nice to have a default target that is invoked when we don’t give any explicit task names to rake. The default target looks like this:

   task :default => ["hello"]

Until now, the only kind of task we have seen in Rake are file tasks. File tasks are knowledgable about time stamps on files. A file task will not execute its action unless the file it represents doesn’t exist, or is older than any of its prerequisites.

A non-file task (or just plain “task”) does not represent the creation of a file. Since there is no timestamp for comparison, non-file tasks always execute their actions (if they have any). Since the default task does not represent a file named “default”, we use a regular non-file task for this purpose. Non-file tasks just use the task keyword (instead of the file keyword).

Here are a couple of other really useful tasks that I almost always include in a Rakefile.

Remove temporary files created during the build process.

Remove all files generated during the build process.

clean tidies up the directories and removes any files that generated as part of the build process, but are not the final goal of the build process. For example, the .o files used to link up the final executable hello program would fall in this category. After the executable program is built, the .o files are no longer needed and will be removed by saying “rake clean”.

clobber is like clean, but even more aggressive. “rake clobber” will remove all files that are not part of the original package. It should return a project to the “just checked out of CVS” state. So it removes the final executable program as well as the files removed by clean.

In fact, these tasks are so common, Rake comes with a predefined library that implements clean and clobber.

But every project is different, how do we specify which files are to be cleaned and clobbered on a per project basis?

The answer is File lists.

File Lists to the Rescue

A file list is simply a list of file names. Since a lot of what Rake does involves files and lists of those files, a file list has some special features to make manipulating file names rather easy.

Suppose you want a list of all the C files in your project. You could add this to your rake file:

  SRC = FileList['*.c']

This will collect all the files ending in ”.c” in the top level directory of your project. File lists understand glob patterns (i.e. things like "*.c") and will find all the matching files.

By the way, no matter where you invoke it, rake always executes in the directory where the Rakefile is found. This keeps your path names consistent without depending on the current directory the user interactive shell.

The clean and clobber tasks use file lists to manage the files to remove. So if we want to clean up all the .o files in a project we could try …

  CLEAN = FileList['*.o']

(CLEAN is the file list associated with the clean task. I’ll let you guess the name of the file list associated with clobber).

The Rakefile So Far …

With the addtion of a few extra tasks, our Rakefile now looks like this. Notice the require ‘rake/clean’ line used to enable the clean and clobber tasks.

  require 'rake/clean'

  CLEAN.include('*.o')
  CLOBBER.include('hello')

  task :default => ["hello"]

  file 'main.o' => ["main.c", "greet.h"] do
    sh "cc -c -o main.o main.c" 
  end

  file 'greet.o' => ['greet.c'] do
    sh "cc -c -o greet.o greet.c" 
  end

  file "hello" => ["main.o", "greet.o"] do
    sh "cc -o hello main.o greet.o" 
  end

Ok, now its time to address the redundant compile commands.

Dynamically Building Tasks

The command to compile the main.c and greet.c files is identical, except for the name of the files involved. The simpliest and most direct way to address the problem is to define the compile task in a loop. Perhaps something like this …

  SRC = FileList['*.c']
  SRC.each do |fn|
    obj = fn.sub(/\.[^.]*$/, '.o')
    file obj  do
      sh "cc -c -o #{obj} #{fn}" 
    end
  end

Just a couple things to note about the above code.

• The dependencies are not specified. This is a common where we specify the dependents at one place and the actions in another. Rake is smart enough to combine the dependencies with the actions.

• Although the task was named after the .o (which is, after all, what we want to generate), the file list is defined in terms of the .c files. Why?

The simple reason is that file lists search for file names that exist in the file system. We have no guarantee that the .o files even exist at this point (indeed, the will not after invoking the clean task). The .c are source and will always be there.

Rake Can Automatically Generate Tasks

Defining tasks in a loop is pretty cool, but is really not needed in a number of simple cases. Rake can automatically generate file based tasks according to some simple pattern matching rules.

For example, we can capture the above logic in a single rule … no need to find all the source files and iterate through them.

  rule '.o' => '.c' do |t|
    sh "cc -c -o #{t.name} #{t.source}" 
  end

The above rule says that if you want to generate a file ending in .o, then you if you have a file with the same base name, but ending in .c, then you can generate the .o from the .c.

t.name is the name of the task, and in file based tasks will be the name of the file we are trying to generate. t.source is the name of the source file, i.e. the one that matches the second have of the rule pattern. t.source is only valid in the body of a rule.

Rules are actually much more flexible than you are led to believe here. But that’s an advanced topic that we will save for another day.

Final Rakefile

Here is our final resule. Notice how we use the SRC and OBJ file lists to manage our lists of scource files and object files.

  require 'rake/clean'

  CLEAN.include('*.o')
  CLOBBER.include('hello')

  task :default => ["hello"]

  SRC = FileList['*.c']
  OBJ = SRC.ext('o')

  rule '.o' => '.c' do |t|
    sh "cc -c -o #{t.name} #{t.source}" 
  end

  file "hello" => OBJ do
    sh "cc -o hello #{OBJ}" 
  end

  # File dependencies go here ...
  file 'main.o' => ['main.c', 'greet.h']
  file 'greet.o' => ['greet.c']

Up Next

In our next tutorial, we will look at using Rake to handle some tasks other than compiling C code.

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