学习如何编写一个UNIX的外壳——Shell，即命令解释程序。学习怎样接受命令，解释命令、执行命令，特别是创建子进程的方式来执行一个程序，以及父进程如何继续子进程的工作。-Learn how to write a UNIX shell- Shell, the command interpreter. Learning how to receive an order to explain the command, the command, in particular, the way is to create a child process to execute a program, and the parent how to continue the work of the child process.