Methods
A
Class Public methods
atomic_write(file_name, temp_dir = Dir.tmpdir)

Write to a file atomically. Useful for situations where you don't want other processes or threads to see half-written files.

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

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

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

Source: show | on GitHub 

# File activesupport/lib/active_support/core_ext/file/atomic.rb, line 17
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
    # Changing file ownership failed, moving on.
  end
end