ruby中是否有关于如何使用“<<”的详细文档/信息(参见下面的示例)

    # use a YML file, which has key , value pairs
    yml_hosts = YAML::load(File.open('hosts.yml'))

      .....
     # for each pair
     yml_hosts.each_pair {|key_hosts , value_hosts|

       ......

     redirect to a String "value_hosts"

         value_hosts << "#{$.} #{line}" if line =~ /recoverable NFE/

if ! value_hosts.empty?
   to do an action, like sending email, etc
   but value_hosts is never Empty
    so i always get an email, even though, i ONLY want top get an email, if

line =~ /recoverable NFE/