How do you set a node attribute to the value of stdout from a shell command?

output.txt

% chef-apply -l info recipe.rb
[2014-08-14T23:38:46-04:00] INFO: Run List is []
[2014-08-14T23:38:46-04:00] INFO: Run List expands to []
Recipe: (chef-apply cookbook)::(chef-apply recipe)
  * ruby_block[demo] action run[2014-08-14T23:38:46-04:00] INFO: Processing ruby_block[demo] action run ((chef-apply cookbook)::(chef-apply recipe) line 2)
[2014-08-14T23:38:46-04:00] INFO: ruby_block[demo] called

    - execute the ruby block demo
  * log[demo] action write[2014-08-14T23:38:46-04:00] INFO: Processing log[demo] action write ((chef-apply cookbook)::(chef-apply recipe) line 9)
[2014-08-14T23:38:46-04:00] INFO: Hello world

recipe.rb

Chef::Resource::RubyBlock.send(:include, Chef::Mixin::ShellOut)
ruby_block "demo" do
  block do
    s = shell_out("echo Hello world")
    node.default['foo'] = s.stdout
  end
end

log "demo" do
  message lazy { node['foo'] }
end