Nachos上开发的一个初级shell。Shell支持cd, dir, clr, environ, echo, help, pause and quit命令；上述命令之外的其他命令通过系统调用fork和exec创建子进程实现；

-Nachos on the development of a primary shell. Shell support cd, dir, clr, environ, echo, help, pause and quit order　the order other than an order through the system call fork and exec to create the realization of sub-process