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

How about:

file = []
File.open("file.txt").each_line do |line|
  file << line
end

file.reverse.each_with_index do |line, index|
  puts line if index < 25
end

The performance would be awful over a big file as it iterates twice, the better approach would be the already mentioned read the file and store the last 25 lines in memory and display those. But this was just an alternative thought.

Answer 2

Is the file large enough that you need to avoid reading the whole thing? If not, you could just do

IO.readlines("file.log")[-25..-1]

If it is to big, you may need to use IO#seek to read from near the end of the file, and continue seeking toward the beginning until you've seen 25 lines.