append.rb

action :append do
  execute "cat #{new_resource.line} >>#{new_resource.path}" do
    not_if "grep '#{new_resource.line}' #{new_resource.file}"
  end
end

append.rb

actions :append

attribute :path, :kind_of => String, :name_attribute => true
attribute :line, :kind_of => String, :required => true

default.rb

cookbookname_append "/path/to/file" do
  line "foo"
end