has_task.rb

#!/usr/bin/env ruby

$task_to_check = ARGV[0]

begin
  require 'rake'
rescue LoadError => e
  exit(4)
end

Rake.application.options.rakelib = ['rakelib']

begin
  Rake.application.raw_load_rakefile
rescue Exception => e
  if e.is_a?(RuntimeError) && e.message =~ /^No Rakefile found/
    exit(5)
  end
  exit(2)
end

unless Rake::Task.task_defined?($task_to_check)
  exit(3)
end

exit(0)

__END__

exit codes:

  0: task exists
  1: this script raised an uncaught exception (from line 11)
  2: exception while loading rakefile (app has a bug or depends on env)
  3: Task doesnt exist
  4: rake not present
  5: rakefile not present

problems:

  * The rakelib workaround might not work in all versions of rake
  * Rake.application.raw_load_rakefile is not a public API for rake
  * Brittle error message checking for exit code 5
  * $PWD probably matters; might need to chdir() to the right directory first