实现Shell，能处理命令

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（getpid()系统调用）

logout ：退出shell

ls [-l]

rm [-r]

mkdir, mv, cp-achieve Shell, can handle orders cd : the process of change current path pwd : View current path process pid : View the current process (getpid () system call) logout : withdraw from the shell ls [-l] rm [-r] mkdir, mv, cp