用python cmd模块实现一个unix管道？

from cmd import Cmd

class PipelineExample(Cmd):

    def do_greet(self,person):
        if person:
            greeting = "hello," + person
        else:
            greeting = 'hello'
        self.output = greeting

    def do_echo(self,text):
        self.output = text

    def do_pipe(self,args):
        buffer = None
        for arg in args:
            s = arg
            if buffer:
                # This command just adds the output of a previous command as the last argument
                s += ' ' + buffer
            self.onecmd(s)
            buffer = self.output

    def postcmd(self,stop,line):
        if hasattr(self,'output') and self.output:
            print self.output
            self.output = None
        return stop

    def parseline(self,line):
        if '|' in line:
            return 'pipe',line.split('|'),line
        return Cmd.parseline(self,line)

    def do_EOF(self,line):
        return True

if __name__ == '__main__':
    PipelineExample().cmdloop()

这是一个示例会话：

(Cmd) greet wong
hello,wong
(Cmd) echo wong | greet
hello,wong
(Cmd) echo wong | greet | greet
hello,hello,wong