echo `date '+%y%m%d'` -> 060728
echo `date '+%Y%m%d'` -> 20060728
echo `date '+%Y%m%d_%H%M%S'` -> 20060728_170000
echo `date +"%Y%m%d%H%M%S"`
echo `date +%Y%m%d --date '1 day ago'`