Ruby 'script = $0'

Question

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()

Answer 1

$0 is one of Ruby's global variables. From here:

$0 -- Contains the name of the script being executed. May be assignable.

Answer 2

Oh the classic Zed Shaw books! lol The $0 gets the input on the command line before the first argument. So say you were to run this through the command line using the ruby interpreter you could put "ruby (fileName) test.txt" and $0 will pick up the fileName and save it to the variable 'script'. I'm not really sure why your doing it here because you do use it later in the program, but that's it. The way you could have tested this would be to print it on the screen using puts, perhaps add this bit of code to see it yourself somewhere in the code:

puts "The $0 has saved #{script} to it, I wonder where it got that."

and see it will name your file.