gistfile1.txt

Works with Ruby 1.8.7, but not Ruby 1.9.  This could have been a problem because I manually installed Ruby 1.9.
I would guess this is a bug in fileutils, a Windows-related problem, or some combination thereof.

#####

# test.rb
require 'fileutils'

filenames = ARGV

filenames.each do |filename|
  newfile = File.open("#{filename}new", "w")
  File.open(filename).each do |line|
    newfile.print line
  end
  newfile.close
  FileUtils.mv("#{filename}new", filename, :verbose => true)
end

#####

> ruby test.rb file.rb
mv file.rbnew file.rb

> ruby19 test.rb file.rb
mv file.rbnew file.rb
C:/cygwin/lib/ruby/1.9.1/fileutils.rb:1298:in `unlink': Permission denied - file.rb (Errno::EACCES)
        from C:/cygwin/lib/ruby/1.9.1/fileutils.rb:1298:in `block in remove_file'
        from C:/cygwin/lib/ruby/1.9.1/fileutils.rb:1306:in `platform_support'
        from C:/cygwin/lib/ruby/1.9.1/fileutils.rb:1297:in `remove_file'
        from C:/cygwin/lib/ruby/1.9.1/fileutils.rb:509:in `block in mv'
        from C:/cygwin/lib/ruby/1.9.1/fileutils.rb:1396:in `block in fu_each_src_dest'
        from C:/cygwin/lib/ruby/1.9.1/fileutils.rb:1412:in `fu_each_src_dest0'
        from C:/cygwin/lib/ruby/1.9.1/fileutils.rb:1394:in `fu_each_src_dest'
        from C:/cygwin/lib/ruby/1.9.1/fileutils.rb:502:in `mv'
        from test.rb:11:in `block in <main>'
        from test.rb:5:in `each'
        from test.rb:5:in `<main>'

#####

Workaround:

require 'fileutils'

filenames = ARGV

filenames.each do |filename|
  newfile = File.open("#{filename}new", "w")
  file = File.open(filename, "r")
  while (line = file.gets)
    newfile.print line
  end
  file.close
  newfile.close
  FileUtils.mv("#{filename}new", filename, :verbose => true)
end

#####

^^^^^ Works as expected for Ruby 1.8 and Ruby 1.9, but is more verbose.
Again, I would guess this is a bug in fileutils, a Windows-related problem, or some combination thereof.