[clug] how can I know umount command do success!

[=?gb2312?B?wunRqfLU?=]

hi,
  now my linux have mount two file system: the 'df' prints following information:
 
  Filesystem           1k-blocks      Used Available Use% Mounted on
  /dev/hda1              5044156    188084   4599840   4% /volume1
  /dev/hdb1              5044156    188084   4599840   4% /volume2

  I write a shell as followed:
  
  #/bin/sh
  umount /volume1
  rm -dfr /volume1
  umount /volume2
  rm -dfr /volume2

  but when the shell excutes , I often causes error! I think it may be the umount command hasn't
  finished, then the rm has worked!