Watch/read a growing log file

Question

I have a log file that is constantly growing. How can I watch and parse it via a Ruby script?

The script will parse each new line as it is written to the file and ou

Answer 1

There are two approach:

• poll the file in an infinite loop (like in Qianjigui's answer, but it is good to put some sleep inside the infinite loop)
• use OS event subsystem: kqueue on BSD, inotify on Linux

Here is an article I wrote about this: Ruby for Admins: Reading Growing Files. So the program combining both event subsystem and polling looks like this:

def tail_dash_f(filename)
  open(filename) do |file|
    file.read          
    case RUBY_PLATFORM   # string with OS name, like "amd64-freebsd8"
    when /bsd/, /darwin/
      require 'rb-kqueue'
      queue = KQueue::Queue.new     
      queue.watch_file(filename, :extend) do
        yield file.read             
      end
      queue.run                     
    when /linux/
      require 'rb-inotify'
      queue = INotify::Notifier.new  
      queue.watch(filename, :modify) do
        yield file.read             
      end
      queue.run                      
    else
      loop do           
        changes = file.read
        unless changes.empty?  
          yield changes
        end
        sleep 1.0       
      end
    end
  end
end

tail_dash_f ARGV.first do |data|
  print data
  if data =~ /error/i
    # do something else, for example send an email to administrator
  end
end