【Shell】脚本日期问题

脚本完成输入一个日期，输出日期前一天和后一天日期

# get the date of next day#!/bin/bashfunction nextday{    read -p "input a correct day（YYYY mm dd）: " year month day        today=$year$month$day        if [ ${#year} -eq 0 -a ${#month} -eq 0 -a ${#day} -eq 0 ];then                echo "the next day is : " `date -d "+1day" +"%Y %m %d"`                exit        elif [ ${#year} -ne 4 -o ${#month} -ne 2 -o ${#day} -ne 2 ];then                echo "Error: date format error"                exit        elif [ "$today" != "$(date -d $today +%Y%m%d 2>/dev/null)" ];then                echo "Error: no such date"                exit        fi    month=`expr $month + 0`    next_year=`expr $year + 0`    next_month=`expr $month + 0`    next_day=`expr $day + 1`        case $month in            1|3|5|7|8|10|12) max_day=31;;            4|6|9|11)        max_day=30;;            2)               max_day=28               [ `expr $year % 4`    -eq 0 ] && max_day=29               [ `expr $year % 100`  -eq 0 ] && max_day=28               [ `expr $year % 400` -eq 0 ] && max_day=29            ;;        esac    if [ $day -eq $max_day ]    then        next_month=`expr $month + 1`                next_day=1        [ $next_month -eq 13 ] &&  next_year=`expr $year + 1` && next_month=1 && next_day=1    fi    [ $next_month -lt 10 ] && next_month="0$next_month"    [ $next_day   -lt 10 ] && next_day="0$next_day"        tomorrow="$next_year $next_month $next_day"        echo "the next day is :"$tomorrow     last_year=`expr $year + 0`    last_month=`expr $month + 0`    last_day=`expr $day - 1`    if [ $day -eq 1 ]    then       last_month=`expr $month - 1`                    last_day=$max_day       [ $last_month -eq 0 ] && last_year=`expr $year - 1` && last_month=12 && last_day=$max_day    fi    [ $last_month -lt 10 ] && last_month="0$last_month"    [ $last_day   -lt 10 ] && last_day="0$last_day"        yesterday="$last_year $last_month $last_day"        echo "the last day is :"$yesterday }nextday