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

check out file-tail gem

Answer 2

Working on idea of @Qianjigui but not using 100% CPU:

def watch_for(file, pattern)
  # Replace -n0 with -n+1 if you want to read from the beginning of file
  f = IO.popen(%W[tail -f -n0 #{file}])
  loop do
    select([f])
    while line = f.gets
      puts "Found it! #{line}" if line =~ pattern
    end
  end
end

watch_for('g.txt', /ERROR/)

Answer 3

def watch_for(file, pattern)
  f = File.open(file,"r")
  f.seek(0,IO::SEEK_END)
  while true do
    select([f])
    line = f.gets
    puts "Found it! #{line}" if line=~pattern
  end
end

watch_for("g.txt",/ERROR/)

Thanks for the ezpz's idea, using the select method you get get what you want. The select method is listening the IO's stream, read the bytes what comes 'late'.

Answer 4

You can use Kernel#select in the following way:

def watch_for(file,pattern)
   f = File.open(file,"r")

   # Since this file exists and is growing, seek to the end of the most recent entry
   f.seek(0,IO::SEEK_END)

   while true
      select([f])
      puts "Found it!" if f.gets =~ pattern
   end
end

Then call it like:

watch_for("some_file", /ERROR/)

I've elided all error checking and such - you will want to have that and probably some mechanism to break out of the loop. But the basic idea is there.

Answer 5

Poor man's approach for quick stuff:

1. a Ruby script that does

   ARGF.each do |line|
     ...
   

2. Running screen with:

   tail -f file | ruby script 
   

Answer 6

If you're on Linux...

tail -f log/development.log | grep "ERROR"

Unless you really wanted it to be a Ruby script for some reason.