Rake: how to output a list of tasks from inside a task?

I'd like my :default task in a Rakefile to be a helpful message that also includes the list of available tasks (the output of rake -T) for people who are not as familiar with rake.

How do you invoke rake -T from inside a task?

Solution

Invoking rake -T from within tasks is a bit more complicated in newer versions of rake. The options that need to be set may be derived from rake/lib/application.rb in method standard_rake_options. Basically this boils down to

Rake::TaskManager.record_task_metadata = true

task :default do
  Rake::application.options.show_tasks = :tasks  # this solves sidewaysmilk problem
  Rake::application.options.show_task_pattern = //
  Rake::application.display_tasks_and_comments
end

Note that record_task_metadata cannot be set from within the default task, as it will already be too late when the task is executed (descriptions won’t have been collected, thus those are nil and therefore no task matches the pattern). Trying to reload the Rakefile from within a task will lead to a closed loop. I assume there are performance trade ofs when always collecting metadata. If that’s an issue

task :default do
  system("rake -sT")  # s for silent
end

might be more suitable.

Both work for me using rake 0.9.2.2.