一个演算日期的shell程序

#!/bin/bash
usage()
{
echo " useage:"
echo " date_offset.sh YYYYMMDD offset"
echo " such as:"
echo " date_offset.sh 20011001 -1 : 20010930"
echo " date_offset.sh 20011001 5  : 20011006"
exit 2
}

#get the days of some month
get_mon_days()
{
        Y=`expr substr $1 1 4` #get the year string
        M=`expr substr $1 5 2` #get the month string

        r1=`expr $Y % 4`
        r2=`expr $Y % 100`
        r3=`expr $Y % 400`

        case $M in
        01|03|05|07|08|10|12) days=31;;
        04|06|09|11) days=30;;
        esac

    #check if the leap year
        if [ $M -eq 02 ]
        then
                if [ r1 -eq 0 -a r2 -ne 0 -o r3 -eq 0 ]
                then
                        days=29
                else
                        days=28
                fi
        fi

        echo $days
}
#get the date of yesterday
yesterday()
{
 Y=`expr substr $1 1 4`
 M=`expr substr $1 5 2`
 D=`expr substr $1 7 2`

 YY=`expr $Y - 1`
 MM=`expr $M - 1`
 DD=`expr $D - 1`

 MM=`printf "%02d" $MM`
 DD=`printf "%02d" $DD`

 dd=$Y$MM
 dad=`get_mon_days $dd`

 be_date=$Y$M$DD

 if [ $D -eq 01 ]
 then
    if [ $M -ne 01 ]
    then
       be_date=$Y$MM$dad
    fi
   if [ $M -eq 01 ]
   then
       be_date=$YY"1231"
    fi
fi
        echo $be_date

}

#get the date of tomorrow
tomorrow()
{
 Y=`expr substr $1 1 4`
 M=`expr substr $1 5 2`
 D=`expr substr $1 7 2`

 YY=`expr $Y + 1`
 MM=`expr $M + 1`
 DD=`expr $D + 1`

 MM=`printf "%02d" $MM`
 DD=`printf "%02d" $DD`

 r1=`expr $Y \% 4`
 r2=`expr $Y \% 100`
 r3=`expr $Y \% 400`

 next_date=$Y$M$DD

        if [ $D -eq 30 ]
        then
                case $M in
                04|06|09|11) next_date=$Y$MM"01";;
                esac
        fi
        if [ $D -eq 31 ]
        then
                next_date=$Y$MM"01"
                case $M in
                12) next_date=$YY"0101";;
                esac
        fi
        if [ $M -eq 02 ]
        then
                if [ r1 -eq 0 -a r2 -ne 0 -o r3 -eq 0 ]
                then
                        if [ $D -eq 29 ]
                        then
                                next_date=$Y$MM"01"
                        fi
                else
                        if [ $D -eq 28 ]
                        then
                                next_date=$Y$MM"01"
                        fi
                fi
        fi
        echo $next_date
}

#main function

# check the num of args
if [ $# -ne 2 ]
then
echo "error num of args!"
usage
fi

# check the first arg
if [ `expr length $1` -ne 8 ]
then
echo "error the first arg!"
usage
fi
 
TMP_YEAR=`expr substr $1 1 4`
TMP_MONTH=`expr substr $1 5 2`
TMP_DAY=`expr substr $1 7 2`

#echo $TMP_YEAR
#echo $TMP_MONTH
#echo $TMP_DAY
 
if [ `expr length $TMP_YEAR` -ne 4 ]
then
echo "error date format!"

usage
fi

if [ $TMP_MONTH -lt 1 ] || [ $TMP_MONTH -gt 12 ]
then
echo "error date format!"
usage
fi

LAST_DAY=`echo \`cal $TMP_MONTH $TMP_YEAR\`|tail -n1|awk '{print $NF}'`
if [ $TMP_DAY -lt 1 ] || [ $TMP_DAY -gt $LAST_DAY ]
then
echo "error date format!"
usage
fi

#check the second arg
expr $2 + 0 >/dev/null 2>&1
if [ ! $? ]
then
echo "arg error!"
usage
fi

TMP_DATE=$1

if [ $2 -lt 0 ]
then
 INC=-1
 COUNT=$2
else
 INC=1
 COUNT=`expr 0 - $2`
fi

#compute the date of yesterday or tomorrow each time
while [ $COUNT -lt 0 ]
do
 if [ $INC -gt 0 ]
 then
  TMP_DATE=`tomorrow $TMP_DATE`
 else
  TMP_DATE=`yesterday $TMP_DATE`
 fi
 COUNT=`expr $COUNT + 1`
done

echo $TMP_DATE