＃date '+%Y%m%d' | awk '{ print $1 }'
20061019
＃date '+%Y%m%d' | awk '{ print $1-1 }'
20061018

...........................................
bash-2.03# more main.sh
#!/bin/sh
dt=`date '+%Y%m%d' | awk '{ print $1-1 }'`
mv billing_detail.xls billing_detail_${dt}.xls