Add Rake Only Lists Tasks With Descriptions as a ruby til.

README.md

@@ -120,6 +120,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Passing Arguments To A Rake Task](ruby/passing-arguments-to-a-rake-task.md)
 - [Percent Notation](ruby/percent-notation.md)
 - [Question Mark Operator](ruby/question-mark-operator.md)
+- [Rake Only Lists Tasks With Descriptions](ruby/rake-only-lists-tasks-with-descriptions.md)
 - [Squeeze Out The Extra Space](ruby/squeeze-out-the-extra-space.md)
 - [Summing Collections](ruby/summing-collections.md)
 - [`undef_method` And The Inheritance Hierarchy](ruby/undef-method-and-the-inheritance-hierarchy.md)

ruby/rake-only-lists-tasks-with-descriptions.md

@@ -0,0 +1,35 @@
+# Rake Only Lists Tasks With Descriptions
+
+Rake describes the `-T` flag as
+
+> Display the tasks (matching optional PATTERN) with descriptions, then exit.
+
+And `rake -T` does just exactly that. It lists all the tasks with
+descriptions. Any rake task that you define without a `desc` will not be
+included.
+
+Consider the following rake task definitions
+
+```ruby
+desc 'foobar does this and that'
+task :foobar do
+  puts 'this and that'
+end
+
+task :foobaz do
+  puts 'not so much'
+end
+```
+
+This is what I get when listing the rake tasks filtered by _foo_
+
+```bash
+$ rake -T foo
+rake foobar  # foobar does this and that
+```
+
+The `foobar` task (which has a description) is listed, but `foobaz` is not.
+
+A hack of sorts to get around this is to use the `-P` flag which will end up
+listing all tasks even if they do not have a description (`rake -P | grep
+'foo'`).