Only write the history back if it changed.

.irbrc

    @history = hist_file
    @src     = hist_src
    
    last_history = File.read @history rescue ""
    last_history.lines.each do |line|
      @src.push line.strip
    @last_history = File.read(@history).lines.map &:strip rescue []
    @last_history.each do |line|
      @src.push line
    end
    
    at_exit do
      puts "Writing history back to #{@history}"
      File.write @history, @src.to_a.uniq.reduce { |x,y| "#{x}\n#{y}" }
      unless @last_history == @src.to_a.uniq
        puts "Writing changed history back to #{@history}"
        File.write @history, @src.to_a.uniq.reduce { |x,y| "#{x}\n#{y}" }
      end
    end
  end
end