Replace attributes, run_list, and chef_environment for an existing node without losing automatic attributes.

node_update_from_file.rb

class NodeUpdateFromFile < ::Chef::Knife
  deps do
    require "chef/node"
    require "chef/json_compat"
    require "chef/knife/core/object_loader"
  end

  banner "knife node update from file FILE (options)"

  def loader
    @loader ||= ::Chef::Knife::Core::ObjectLoader.new(Chef::Node, ui)
  end

  def run
    update = loader.load_from("nodes", @name_args[0])
    node   = ::Chef::Node.load(update.name)

    # Replace attributes, run_list, and chef_environment from the new node
    # definition. 
    node.normal_attrs   = update.normal_attrs
    node.override_attrs = update.override_attrs
    node.default_attrs  = update.default_attrs

    node.run_list.reset!(update.run_list)
    node.chef_environment(update.chef_environment)

    # Expand the run_list in case it has changed
    node.expand!
    node.save

    output(format_for_display(node)) if config[:print_after]

    ui.info("update Node #{update.name}!")
  end
end