linux环境下的shell模拟程序，程序起名为ysh其特征为：能执行外部程序命令，命令可以带参数。能执行fg，bg，cd，history，exit等内部命令，使用管道和输入输出重定向-shell under linux environment simulation program, the program named ysh characterized by: external procedures to carry out orders, the order parameter can take. To the implementation of fg, bg, cd, history, exit the internal order, use the pipeline and to redirect input and output. . . . . . . .