mount bind

       The bind mounts.
              Since Linux 2.4.0 it is possible to remount part of the  file  hierarchy  somewhere
              else. The call is
                     mount --bind olddir newdir
              or shortoption
                     mount -B olddir newdir
              or fstab entry is:
                     /olddir /newdir none bind

              After  this  call  the  same  contents  is  accessible in two places.  One can also
              remount a single file (on a single file).

              This call attaches only (part of) a single filesystem, not possible submounts.  The
              entire file hierarchy including submounts is attached a second place using
                     mount --rbind olddir newdir
              or shortoption
                     mount -R olddir newdir

              Note  that the filesystem mount options will remain the same as those on the origi-
              nal mount point, and cannot  be  changed  by  passing  the  -o  option  along  with
              --bind/--rbind. The mount options can be changed by a separate remount command, for
              example:

                     mount --bind olddir newdir
                     mount -o remount,ro newdir