what ruby features are used in chef recipes?

Question

I just started using chef and don't know much about ruby.

I have problems understanding the language-syntax used in recipes.

Say, I create a directory in a cookbook in recipes/default.rb like:

directory "/home/test/mydir" do
  owner "test"
  mode "0755"
  action :create
  recursive true
end

I assume this is part of a valid ruby script. What do lines like owner "test" mean? Is this a function call, a variable assignment or something else entirely?

Answer 1

Chef is written in Ruby and makes an extensive use of Ruby ability to design custom DSL. Almost every chef configuration file is written with a Ruby-based DSL.

This means that in order to use chef effectively you should be familiar with the basic of Ruby syntax including

• Grammar
• Data types (the main difference compared to other languages are Symbols)
• Blocks

You don't need to know a lot about metaprogramming in Ruby.

The case of the code you posted is an excellent example of a Ruby based DSL. Let me explain it a little bit.

# Call the method directory passing the path and a block
# containing some code to be evaluated
directory "/home/test/mydir" do

  # chown the directory to the test user
  owner "test"

  # set the permissions to 0555
  mode "0755"

  # create the directory if it does not exists
  action :create

  # equivalent of -p flag in the mkdir
  recursive true

end

Blocks are a convenient way to specify a group of operations (in this case create, set permissions, etc) to be evaluated in a single context (in this case in the context of that path).

Answer 2

Let's break it down.

directory "/home/test/mydir" do
  ...
end

You are just calling a global method defined by Chef called directory, passing one argument "/home/test/mydir", and a block (everything between the do and end).

This block is probably excecuted in a special scope created by Chef in which all of the options (owner, mode, action, etc.) are method.