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 15
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

  begin
    # Get original file permissions
    old_stat = stat(file_name)
  rescue Errno::ENOENT
    # No old permissions, write a temp file to determine the defaults
    check_name = join(dirname(file_name), ".permissions_check.#{Thread.current.object_id}.#{Process.pid}.#{rand(1000000)}")
    open(check_name, "w") { }
    old_stat = stat(check_name)
    unlink(check_name)
  end

  # Overwrite original file with temp file
  FileUtils.mv(temp_file.path, file_name)

  # Set correct permissions on new file
  chown(old_stat.uid, old_stat.gid, file_name)
  chmod(old_stat.mode, file_name)
end