python 2.5.4，系统式centos 5.1_x8664
1、运行一个python:/usr/local/python/bin/python /jykj/transport/transport.py
     执行系统命令,结果
       [root@localhost ~]# ps -ef|grep python|grep transport.py
           root     15200 15124  2 11:32 pts/1    00:00:00 /usr/local/python/bin/python /jykj/transport/transport.py
           [root@localhost ~]#
     运行python使用popen结果
   >>> import os
    >>> aaa = "ps -ef|grep python|grep transport.py"
    >>> bbb = os.popen(aaa).read()
    >>> print bbb

    >>> bbb
    ''
    >>>
为什么bbb是空啊？