example.conf

<match xxx.*>
  type in_exec
  command cat
  in_keys host,message
  out_keys host,message
  tag filtered.out
</match>

out_in_exec.rb

module Fluent


class InExecOutput < Output
  Plugin.register_output('in_exec', self)

  def initialize
    super
  end

  config_param :command, :string
  config_param :in_keys, :string
  config_param :out_keys, :string
  config_param :tag, :string
  #config_param :time_key, :string

  def configure
    super
    @in_keys = @in_keys.strip(',')
    @out_keys = @out_keys.strip(',')
  end

  def start
    @io = IO.popen(@command)
    @thread = Thread.new(&method(:run))
  end

  def run
    @io.each_line {|line|
      vals = line.strip("\t")
      record = Hash[@out_keys.zip(vals)]
      time = Engine.now  # TODO time_key & time_format
      Engine.emit(@tag, time, record)
    }
  end

  def emit(tag, es, chain)
    es.each {|time,record|
      vals = @in_keys.map {|k| record[k].to_s }
      @io.write vals.join("\t")
      @io.write "\n"
    }
    @io.flush
    chain.next
  end
end


end