knife exec script for setting the value of a particular node attribute (i.e. core.revision or haystack.revision)

setrevision.rb

# knife-exec script to set git revision to be deployed for specified application
# Example usage: knife exec /path/to/this/script/setrevision haystack i-fb106a9f v69.10

require 'chef/knife'
require 'chef/shef/ext'

# - ARGV[2] expected to be the app name (e.g. haystack, core)
# - ARGV[3] expected to be the node name
# - ARGV[4] expected to be the revision id

abort("usage: knife exec setrevision APP NODE REVISION") unless ARGV[2]

begin
  nodes.find(:name => ARGV[3]).each do |n|
    case ARGV[2]
    when 'haystack'
      n['haystack']['revision'] = ARGV[4]
      begin
        if n.save
          puts "Set haystack.revision to #{ARGV[4]} on node #{n['name']}."
        end
      rescue => e
        puts "Failed to modify node #{n['name']}: #{e.inspect}"
      end
    when 'core'
      n['core']['revision'] = ARGV[4]
      begin
        if n.save
          puts "Set core.revision to #{ARGV[4]} on node #{n['name']}."
        end
      rescue => e
        puts "Failed to modify node #{n['name']}: #{e.inspect}"
      end
    else
      puts "Did not recognize #{ARGV[2]} as a valid application."
      exit 1
    end
  end
rescue => e
  puts "Error encountered when searching for node #{ARGV[3]}: #{e.inspect}"
end

exit 0