Reading the last n lines of a file in Ruby?

Question

I need to read the last 25 lines from a file (for displaying the most recent log entries). Is there anyway in Ruby to start at the end of a file and read it backwards?

Answer 1

I just wrote a quick implemenation with #seek:

class File
  def tail(n)
    buffer = 1024
    idx = (size - buffer).abs
    chunks = []
    lines = 0

    begin
      seek(idx)
      chunk = read(buffer)
      lines += chunk.count("\n")
      chunks.unshift chunk
      idx -= buffer
    end while lines < n && pos != 0

    chunks.join.lines.reverse_each.take(n).reverse.join
  end
end

File.open('rpn-calculator.rb') do |f|
  p f.tail(10)
end

Answer 2

Here's a version of tail that doesn't store any buffers in memory while you go, but instead uses "pointers". Also does bound-checking so you don't end up seeking to a negative offset (if for example you have more to read but less than your chunk size left).

def tail(path, n)
  file = File.open(path, "r")
  buffer_s = 512
  line_count = 0
  file.seek(0, IO::SEEK_END)

  offset = file.pos # we start at the end

  while line_count <= n && offset > 0
    to_read = if (offset - buffer_s) < 0
                offset
              else
                buffer_s
              end

    file.seek(offset-to_read)
    data = file.read(to_read)

    data.reverse.each_char do |c|
      if line_count > n
        offset += 1
        break
      end
      offset -= 1
      if c == "\n"
        line_count += 1
      end
    end
  end

  file.seek(offset)
  data = file.read
end

test cases at https://gist.github.com/shaiguitar/6d926587e98fc8a5e301