Git SHA1 of HEAD in your Rails App

deploy.rb

after "deploy:update_code", "deploy:write_sha1"
namespace :deploy do  
  desc "write sha1 to file"
  task :write_sha1 do
    run "cd #{latest_release} && git show-ref --heads --hash=7 > #{latest_release}/config/HEAD"
  end
end

production.rb

git_sha1_path = "#{RAILS_ROOT}/config/HEAD"
GIT_SHA1 = if File.file?(git_sha1_path)
             File.open(git_sha1_path).readlines[0]
           end