File
Methods
A
Class Public methods
atomic_write(file_name, temp_dir = dirname(file_name))

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

This method needs to create a temporary file. By default it will create it in the same directory as the destination file. If you don't like this behavior you can provide a different directory but it must be on the same physical filesystem as the file you're trying to write.

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 19
def self.atomic_write(file_name, temp_dir = dirname(file_name))
  require "tempfile" unless defined?(Tempfile)

  Tempfile.open(".#{basename(file_name)}", temp_dir) do |temp_file|
    temp_file.binmode
    return_val = yield temp_file
    temp_file.close

    old_stat = if exist?(file_name)
      # Get original file permissions
      stat(file_name)
    elsif temp_dir != dirname(file_name)
      # If not possible, probe which are the default permissions in the
      # destination directory.
      probe_stat_in(dirname(file_name))
    end

    if old_stat
      # Set correct permissions on new file
      begin
        chown(old_stat.uid, old_stat.gid, temp_file.path)
        # This operation will affect filesystem ACL's
        chmod(old_stat.mode, temp_file.path)
      rescue Errno::EPERM, Errno::EACCES
        # Changing file ownership failed, moving on.
      end
    end

    # Overwrite original file with temp file
    rename(temp_file.path, file_name)
    return_val
  end
end