即席 tail

Windowsなマシンでちょっと必要になったので書いてみた。

tail.rb

#!ruby -Ku

class Tail
  def initialize(filename)
    @filename = filename
  end

  def run
    @interrupted = false
    size = File.size(@filename)
    size = print_head(size)
    until @interrupted
      size = print_diff(size)
      sleep 0.5
    end
  end

  def max(x, y)
    x > y ? x : y
  end

  def print_head(size)
    File.open(@filename, 'rb') do |f|
      f.seek(max(size - 1024, 0))
      print f.read
      return f.tell
    end
  end

  def print_diff(size)
    latest = File.size(@filename)
    return size if latest == size

    if latest < size
      puts
      puts "truncated..."
      puts
      return print_head(latest)
    end

    File.open(@filename, 'rb') do |f|
      f.seek(size)
      print f.read
      return f.tell
    end
  end

  def interrupt
    puts "^C"
    @interrupted = true;
  end
end

if $0 == __FILE__
  tail = Tail.new(File.expand_path(ARGV[0]))
  Signal.trap(:INT){ tail.interrupt }
  tail.run
end