Shell Programming and Scripting

#!/usr/bin/sh

check_done() {
        if [ $month -eq 1 -o $month -eq 3 -o $month -eq 5 -o $month -eq 7 -o $month -eq 8 -o $month -eq 10 -o $month -eq 12 ]
        then
                daysofmth=31
        elif [ $month -eq 2 ]
        then
                if [ `expr $year % 100` -eq 0 -a `expr $year % 400` -eq 0 ]
                then
                        daysofmth=29
                elif [ `expr $year % 100` -ne 0 -a `expr $year % 4` -eq 0 ]
                then
                        daysofmth=29
                else
                        daysofmth=28
                fi
        elif [ $month -eq 4 -o $month -eq 6 -o $month -eq 9 -o $month -eq 11 ]
        then
                daysofmth=30
        fi
        julday=`expr $julday - $daysofmth`
        if [ $julday -lt 0 ]
        then
                done=1
                julday=`expr $daysofmth + $julday`
        elif [ $julday -eq 0 ]
        then
                done=1
                julday=$daysofmth
        fi
}
#########  main script starts here
if [ $# -ne 1 ]
then
        echo "Usage: tonfro <delta>"
        exit 1
fi
julday=`expr \`date +%j\` + $1`
year=`date +%Y`
month=0
while [ $julday -le 0 ]
do
        year=`expr $year - 1`
        if [ `expr $year % 100` -eq 0 -a `expr $year % 400` -eq 0 ]
        then
                julday=`expr $julday + 366`
        elif [ `expr $year % 100` -ne 0 -a `expr $year % 4` -eq 0 ]
        then
                julday=`expr $julday + 366`
        else
                julday=`expr $julday + 365`
        fi
done

done=0

while [ $done -ne 1 ]
do
        month=`expr $month + 1`
        if [ $month -eq 13 ]
        then
                year=`expr $year + 1`
                month=1
        fi
        check_done
done

printf "%d-%.2d-%.2d\n" $year $month $julday

  Arithmetic Expansion
       Arithmetic expansion allows the evaluation of an arithmetic  expression
       and  the  substitution of the result.  The format for arithmetic expan-
       sion is:

              $((expression))

       The expression is treated as if it were within  double  quotes,  but  a
       double  quote  inside  the  parentheses  is not treated specially.  All
       tokens in the expression undergo parameter expansion, string expansion,
       command  substitution, and quote removal.  Arithmetic substitutions may
       be nested.

       The evaluation is performed according to the rules listed  below  under
       ARITHMETIC EVALUATION.  If expression is invalid, bash prints a message
       indicating failure and no substitution occurs.

                if [ `expr $year % 100` -eq 0 -a `expr $year % 400` -eq 0 ]
                then
                        daysofmth=29
                elif [ `expr $year % 100` -ne 0 -a `expr $year % 4` -eq 0 ]
                then
                        daysofmth=29
                else
                        daysofmth=28
                fi

                if [ (($(($year % 100)) -eq 0 -a $(($year % 400)) -eq 0)) -o \
                (($(($year % 100)) -ne 0 -a $(($year % 4)) -eq 0)) ]
                then
                        daysofmth=29
                else
                        daysofmth=28
                fi

       ((expression))
              The expression is evaluated according  to  the  rules  described
              below  under ARITHMETIC EVALUATION.  If the value of the expres-
              sion is non-zero, the return status is 0; otherwise  the  return
              status is 1.  This is exactly equivalent to let "expression".