Ruby 'script = $0'

I was looking at a Ruby script and I came across `script = $0`. I have done some Googling but I have not found a definite answer as to what this does. I believe that it protects you from reading a file bigger than memory, is that correct? Thanks, I have the full script below so you can see it in context: ``` # Takes the name of a file as an argument and assigns to filename filename = ARGV.first script = $0 puts "We're going to erase #{filename}." puts "If you don't want that, hit CTRL-C (^C)." puts "If you do want that, hit RETURN." print "? " STDIN.gets puts "Opening the file..." target = File.open(filename, 'w') puts "Truncating the file. Goodbye!" target.truncate(target.size) puts "Now I'm going to ask you for three lines." print "line 1: "; line1 = STDIN.gets.chomp() print "line 2: "; line2 = STDIN.gets.chomp() print "line 3: "; line3 = STDIN.gets.chomp() puts "I'm going to write these to the file." target.write(line1) target.write("\n") target.write(line2) target.write("\n") target.write(line3) target.write("\n") puts "And finally, we close it." target.close() ```