编写一个简单的命令解释器-shell功能-A simple shell-shell function

简单的shell解释器的实现，实现了管道，重定向功能，可以执行少数的内部指令，可以后台运行，注释清晰。-Simple shell interpreter to achieve, to achieve the pipes, redirection, you can perform a small number of internal commands can run in the background, clear notes.