说明：  应用UNIX的fork()等系统调用，编写一个c程序具有以下功能： a) 实现Shell的基本功能，包括有：打印提示符；接受和分析命令行（滤去无效的空格、tab符号以及换行符等）；执行命令（要有出错处理；输入exit或者bye退出）；返回父进程； b) 处理后台程序（不需要wait) c) 处理多行命令（分析命令行中的‘ ’并处理之） d)应用 dup(), pipe()系统调用具有输入输出重定向以及管道功能；
(Application of UNIX s fork () system call, etc., to prepare a c program has the following features: a) the realization of the basic functions of Shell, including: print prompt accepted and analysis of the command line (least squares void spaces, tab symbols, as well as for Bank address, etc.) the implementation of the command (must be an error processing enter exit or quit bye) return to the parent process b) procedures for dealing with the background (no wait) c) to deal with multi-line command (command-line analysis of the and to deal with of) d) Application of dup (), pipe () system call with input and output redirection and pipes function )