目标：熟悉掌握工作原理的壳牌命令解释器…

代码说明：

目的：熟悉shell命令解释程序的工作原理，掌握使用fork和exec等系统调用设计多进程应用程序的基本技术。-Objective: shell command interpreter familiar with the working principle, to master the use of fork and exec system calls, such as the design of multi-process applications, the basic technology.