类似这样自动给文件加修改日期啥的是怎么实现的？

经常看见很多文件都是在文件前面加上很多说明性的文字，和文件修改时间。这个是如何做到的？

类似下面这个文件

其中前面2行
# global xinitrc file:
# $Id: Xinitrc.global 2240 2013-05-27 16:52:51Z johnb $
显示了文件的修改时间和修改的人

后面的是类似版权说明的

twu5@ug01:~|⇒ cat /usr/local/rcfiles/Xinitrc.global
#!/bin/bash

# global xinitrc file:
# $Id: Xinitrc.global 2240 2013-05-27 16:52:51Z johnb $

# Copyright (c) 2013 University of Alberta Department of Computing Science <@cs.ualberta.ca>
#
# Permission to use, copy, modify, and distribute this software for any
# purpose with or without fee is hereby granted, provided that the above
# copyright notice and this permission notice appear in all copies.
#
# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
# OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.