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 can't vouch for Ruby but most of these languages follow the C idiom of file I/O. That means there's no way to do what you ask other than searching. This usually takes one of two approaches.

• Starting at the start of the file and scanning it all, remembering the most recent 25 lines. Then, when you hit end of file, print them out.
• A similar approach but attempting to seek to a best-guess location first. That means seeking to (for example) end of file minus 4000 characters, then doing exactly what you did in the first approach with the proviso that, if you didn't get 25 lines, you have to back up and try again (e.g., to end of file minus 5000 characters).

The second way is the one I prefer since, if you choose your first offset wisely, you'll almost certainly only need one shot at it. Log files still tend to have fixed maximum line lengths (I think coders still have a propensity for 80-column files long after their usefulness has degraded). I tend to choose number of lines desired multiplied by 132 as my offset.

And from a cursory glance of Ruby docs online, it looks like it does follow the C idiom. You would use "ios.seek(25*-132,IO::SEEK_END)" if you were to follow my advice, then read forward from there.