mount概念

0,  // cd to top dir

    $ mkdir -v {content, old, new0, new1}

    $ cd content

    $ touch  {content0, content1, content2}

    $ cd old

    $ touch {old0, old1, old2}

    $ mkdir -v old-child

    $ cd old-child

    $ touch {old-child0, old-child1, old-child2}

1, // cd to top dir

    $ mount --bind content old/old-child

    $ mount --bind old new

    $ ll old/old-child

       content0

       content1

       content2

    $ ll new/old-child

       old-child0

       old-child1

       old-child2

   结论：old挂载到new上，但从new挂载点看不到原来挂载在old目录中old-child挂载点下的文件系统

2， //cd to top dir

     $ mount --bind old old

     $ mount --make-shared old     //此句必须在下一句之前

     $ mount --bind old new0

     $ mount --bind content old/old-child

     $ ll new0/old-child

        content0

        content1

        content2