说明：  一个C++编写的LINUX SHELL,支持的命令有cat cd rm mkdir ls pwd pid mv cp,"exit"命令用于退出
( The program is to execute some common commands of Linux,available commands are: cat cd rm mkdir ls pwd pid mv cp,same function as the standard linux shell command.There’s also a command “exit” to let the user exit the program.The programe also shows how long did the last executed command take(in second).)