模仿linux中shell的功能，简单的ls，dir等命令，也包括双管道，输入输出重定向，同时支持shell的定义变量以及变量赋值。同时做了简单的错误输入提示。-Linux in the shell to imitate the function, a simple ls, dir, etc. commands, but also dual-channel input and output redirection, while supporting the definition of shell variables and variable assignments. At the same time made a simple error input prompts.