class File

def self.atomic_write(file_name, temp_dir = Dir.tmpdir)

end
file.write('hello')
File.atomic_write('/data/something.important', '/data/tmp') do |file|

trying to write, you can provide a different temporary directory.
If your temp directory is not on the same filesystem as the file you're

end
file.write('hello')
File.atomic_write('important.file') do |file|

want other processes or threads to see half-written files.
Write to a file atomically. Useful for situations where you don't 
def self.atomic_write(file_name, temp_dir = Dir.tmpdir)
  require 'tempfile' unless defined?(Tempfile)
  require 'fileutils' unless defined?(FileUtils)
  temp_file = Tempfile.new(basename(file_name), temp_dir)
  temp_file.binmode
  yield temp_file
  temp_file.close
  if File.exist?(file_name)
    # Get original file permissions
    old_stat = stat(file_name)
  else
    # If not possible, probe which are the default permissions in the
    # destination directory.
    old_stat = probe_stat_in(dirname(file_name))
  end
  # Overwrite original file with temp file
  FileUtils.mv(temp_file.path, file_name)
  # Set correct permissions on new file
  begin
    chown(old_stat.uid, old_stat.gid, file_name)
    # This operation will affect filesystem ACL's
    chmod(old_stat.mode, file_name)
  rescue Errno::EPERM, Errno::EACCES
    # Changing file ownership failed, moving on.
  end
end

def self.probe_stat_in(dir) #:nodoc:

:nodoc:
Private utility method. 
def self.probe_stat_in(dir) #:nodoc:
  basename = [
    '.permissions_check',
    Thread.current.object_id,
    Process.pid,
    rand(1000000)
  ].join('.')
  file_name = join(dir, basename)
  FileUtils.touch(file_name)
  stat(file_name)
ensure
  FileUtils.rm_f(file_name) if file_name
end