shell关于日期的加减

$cat sh.sh 
#!/bin/bash

if [ $# == 0 ] ; then
    ydate=`date -d -1days "+%Y%m%d"`
    ydateother=`date -d -1days "+%Y-%m-%d"`
    tdate=`date -d -0days "+%Y%m%d"`
    tdateother=`date -d -0days "+%Y-%m-%d"`
elif [ $# == 1 ] ; then 
    ydate=$1
    ydateother=`date -d ${ydate} +"%Y-%m-%d"`
    tdate=`date -d "${ydate} 1days" +"%Y%m%d"`
    tdateother=`date -d ${tdate} +"%Y-%m-%d"`
elif [ $# == 2 ] ; then 
    ydate=$1
    ydateother=`date -d ${ydate} +"%Y-%m-%d"`
    tdate=$2
    tdateother=`date -d ${tdate} +"%Y-%m-%d"`
else
    echo "please do not input if calcute yedterday."
    echo "please input like if calcute 20170510 one day: 20170510"
    echo "please input like if calcute 20170510 and 20170512 two days : 20170510 20170511"
    exit -1
fi 
echo ${ydate}'--'${ydateother}'--'${tdate}'---'${tdateother}

传入字符串，修改为日期；

根据传入的字符串识别的日期，进行加减