[Ruby] Problem mit Stringvergleich oder stdin auslesen, nicht ganz sicher

answer = $stdin.gets

#!/usr/bin/env ruby

require 'fileutils'

=begin
  This simple programm converts all leading tabulators of each line by the
  passed file (first argument) to  whitespaces. The file itself isn't changed
  but the changes are  written to a new file "<file>_converted". A second
  argument is optional and  determines with how many whitespaces each tab is
  replaced. (default is 2 to fit with ruby conventions)
  
  Feel free to use or modify this program, but usage is at your own risk and
  without any warranty.
  
  @author  :  bender
  @version :  0.5
=end

# check if file exists, is no directory and readable

if ARGV.size < 1 || ARGV.size > 2
  $stderr.print "ERROR: bad argument count, expected: <file> [whitespaces]"
  return -1
end

if not File.exist? ARGV[0]
  $stderr.print "ERROR: could not find file \"", ARGV[0], "\"\n"
  return -1
end

if File.directory? ARGV[0]
  $stderr.print "ERROR: \"", ARGV[0], "\" is a directory\n"
  return -1
end

if not File.readable? ARGV[0]
  $stderr.print "ERROR: \"", ARGV[0], "\" is not readable\n"
  return -1
end

# check whether <file>_converted exists, if it does ask user if it shall be
# overwritten
if File.exist?(ARGV[0] + "_converted")
  $stderr.print "> there is already a file \"", ARGV[0], "_converted\"",
                " delete old one? (y)es or (n)o\n(if answer is no, the",
                " program will leave the file untouched and quit)\n"
  
  answer = $stdin.gets
  
  if answer == "y" || answer == "yes" || answer == "+" || answer == "1"
    File.delete(ARGV[0] + "_converted")
    print "> deleted file, continue with conversion"
  elsif answer == "n" || answer == "no" || answer == "-" || answer == "0"
    return 0
  else
    $stderr.print "ERROR, could not evaluate answer, assuming \"no\" was",
                      " meant and quitting the program"
  end
  
end

# open file to read and create new file for conversion
orig_file = File.open(ARGV[0], "r")
conv_file = File.new(ARGV[0] + "_converted", "a+")

# default whitespaces are 2 if the user didn't pass a second arg
space_count = 2
if ARGV[1]
  space_count = ARGV[1].to_i == 0 ? space_count : ARGV[1].to_i
end
space_str = " " * space_count

while line = orig_file.gets

  i = 0

  while line[i, 1] == "\t"
    line[i, 1] = space_str
    i += space_count
  end
  
  conv_file.puts line
end

print "> converted each tab to ", space_count.to_s, " whitespaces, check ",
      "result in \"", ARGV[0], "_converted\"\n"