make sure file exists before writing to it

gistfile1.rb

if options.has_key? :file then
  file = options[:file]
  dir = File.dirname(file)
  if File.directory?(dir) && File.writable?(dir) then
    if File.exists?(file) && ! File.writable?(file) then
      abort "Can't write to file. No permission?"
    else
      $stdout = File.new file, 'w'
    end
  else
    abort "Can't write to directory. No permission?"
  end
end

Before refreshing the page and seeing your solution, I took your advice from twitter and did the following:

if options.has_key? :file then
  begin
    $stdout.reopen(options[:file], 'w')
  rescue Errno::ENOENT => e
    abort "Can't write into that directory. Does it exist?"
  rescue Errno::EACCES => e
    abort "Can't write to file or directory. Do you have permission?"
  rescue Errno::EISDIR => e
    abort "Please pass in a file name, not a directory name."
  rescue => e
    abort e.message
  end
end

But I like your solution even better. Thanks!