Linux中Shell的几条简单命令的实现,基本框架已经搭好了，如果需要可以在此基础上进行扩充（代码中增加了多条命令同时执行的功能)

试验内容:

实现简单的Shell，至少能处理四条命令

cd ：改变进程当前路径

pwd ：查看进程当前路径

pid：查看当前进程号

logout ：退出shell

-a few simple order to achieve the basic framework has been put up. If need be on the basis of this expansion (code added to the number of orders carried out simultaneously function) test : Shell simple realization, at least four orders can handle cd : the process of change the current path pwd : View the current path process pid : View the current process, logout : withdraw from the shell