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

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

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+# 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'`).