I deleted some very important files by using rm command.
I refer the tutoral http://abloz.com/2013/09/12/linux-rm-rf-file-recovery-record.html
But at the last step, the command should be –recovery-directory
But it will NOT work if we do not unmount the disk. So finally, it did not work.
PS, the command to check the disk of a directory is
dh -Th directory_path