VB 调用 Shell 执行 CMD 命令的问题

Public Const aa As String = "c:\text.txt"
'
Sub a()
    Dim bb As String
    bb = MyPat & "\123.txt"
 '   MsgBox bb ' 这里，bb变量能得到实际的值
    Shell ("cmd /c ping www.baidu.com >>" & bb), vbHide ' 执行不成功,,不知道如何修改才可以？
    Shell ("cmd /c ping www.baidu.com >>" & aa), vbHide ' 执行成功
End Sub

Public Function MyPat() As String ' 获取我的文档目录
    Dim WSHShell As Object
    Set WSHShell = CreateObject("Wscript.Shell")
    MyPat = WSHShell.SpecialFolders("MyDocuments")
    Set WSHShell = Nothing
End Function