#! /bin/sh
###########################################################################################
## This scripts runs in 2 ways								  #
## First way: without command line arguments. It calculates yesterday's date		  #
## Second way: 3 numeric argumnets(dd mm yyyy). Calculates date prior to the input date
###########################################################################################

case $# in
3) 
   [[ `echo $1 | grep [[:alpha:]]` || `echo $1 | grep [[:punct:]]` ]] && echo -e "$1 is not a number.\nQuitting" && exit 1
   [[ `echo $2 | grep [[:alpha:]]` || `echo $2 | grep [[:punct:]]` ]] && echo -e "$2 is not a number.\nQuitting" && exit 1
   [[ `echo $3 | grep [[:alpha:]]` || `echo $3 | grep [[:punct:]]` ]] && echo -e "$3 is not a number.\nQuitting" && exit 1

   YEAR=$3
   NMONTH=$2
   DATE=$1 
   # Add a 0 in front if month or date is single digit; i.e. < 10
   [[ ${#NMONTH} -eq 1 ]] && NMONTH="0$NMONTH"
   [[ ${#DATE} -eq 1 ]] && DATE="0$DATE"
   
   [[ $DATE > 31 || $DATE < 01 ]] && echo -e "Invalid date.\n Quitting" && exit 1
   [[ $NMONTH > 12 || $NMONTH < 01 ]] && echo -e "Invalid month.\n Quitting" && exit 1
   [[ ${#YEAR} -ne 4 ]] && echo -e "Invalid Year.\n Quitting" && exit 1
   ;;
*)
YEAR=`date +"%Y"`
NMONTH=`date +"%m"`
DATE=`date +"%d"`;;
esac
if [[ $DATE == 01 || $DATE == 1 ]]; then # If its the first day of the month, set yesterday's date accordingly.

case $NMONTH in
01|02|04|06|08|09|11)         
	DATE=31
        if [[ $NMONTH == "01" || $NMONTH == "1" ]]; then
                YEAR=`expr $YEAR - 1` 
                NMONTH=12;LMONTH="December"
        else
	 NMONTH=`expr $NMONTH - 1`
	fi;;


03)     if [[ `expr $YEAR % 4` -eq  0 ]]; then
                DATE=29
        else
                DATE=28
        fi  
	NMONTH=`expr $NMONTH - 1`;;
*)
  	DATE=30
	NMONTH=`expr $NMONTH - 1` ;;
esac

else
       DATE=`expr $DATE - 1` 
fi

# If numeric month or date is <10, add a 0 in front
if [ ${#NMONTH} -eq 1 ]; then
  NMONTH="0$NMONTH"
fi
if [ ${#DATE} -eq 1 ]; then 
 DATE="0$DATE"
fi

case $NMONTH in # set long month name
01)     LMONTH="January";;
02)     LMONTH="February";;
03)     LMONTH="March";;
04)     LMONTH="April";;
05)     LMONTH="May";;
06)     LMONTH="June";;
07)     LMONTH="July";;
08)     LMONTH="August";;
09)     LMONTH="September";;
10)     LMONTH="October";;
11)     LMONTH="November";;
12)     LMONTH="December";;
esac

echo $DATE-$NMONTH-$YEAR #Output date-month-year

[/tmp]$ ./try.sh 31 04 2006
30-04-2006
[/tmp]$ ./try.sh 31 02 2006
30-02-2006
[/tmp]$