#!/bin/bash
###################################
# Script Name : finday.sh                                               
# Author : Kamaraj Subramanian                                 
# Website : www.unix.com                              
# Bugs : this thread   
###################################
USAGE()
{
    echo "----------------------------------------------------------------"
    echo "USAGE :: findday YEAR MONTH DAY WEEK"
    echo "Example :"
    echo "findday 2012 10 SU 3"
    echo "The above will tell the date of 3rd sunday in October 2012"
    echo "Valid values for YEAR ( 1 to 9999 )"
    echo "Valid values for MONTH ( 1 to 12 )"
    echo "Valid values for DAY ( SU, MO, TU, WE, TH, FR, SA )"
    echo "Valid values for WEEK ( 1 to 5 )"
    echo "----------------------------------------------------------------"
    exit 1
}

YearCheck()
{
    echo "$1" | grep -v "^[0-9]*$" >/dev/null 2>&1 && echo "Please enter the number 1 to 9999" && USAGE
    if [[ ! "${1}" -le "10000" || "${1}" -eq "0" ]]
    then
        echo "Enter the correct Year [1-9999]"
        USAGE
    fi
    
}

MonthCheck()
{
    echo "$1" | grep -v "^[0-9]*$" >/dev/null 2>&1 && echo "Please enter the number 1 to 12" && USAGE
    if [[ ! "${1}" -le "12" || "${1}" -eq "0" ]]
    then
        echo "Enter the correct Month [1-12]"
        USAGE
    fi
}
DayCheck()
{
    echo "$1" | egrep -v "^SU$|^MO$|^TU$|^WE$|^TH$|^FR$|^SA$" >/dev/null 2>&1 && echo "Valid values for DAY ( SU, MO, TU, WE, TH, FR, SA )" && USAGE
    [ "$1" == "SU" ] && WNO=7 && PNO=1
    [ "$1" == "MO" ] && WNO=6 && PNO=2
    [ "$1" == "TU" ] && WNO=5 && PNO=3
    [ "$1" == "WE" ] && WNO=4 && PNO=4
    [ "$1" == "TH" ] && WNO=3 && PNO=5
    [ "$1" == "FR" ] && WNO=2 && PNO=6
    [ "$1" == "SA" ] && WNO=1 && PNO=7
    
}
WeekCheck()
{
    if [[ ! "${1}" -le "5" || "${1}" -eq "0" ]]
    then
        echo "Enter the correct WEEK [1-5]"
        USAGE
    fi
}

if [ "$#" -ne "4" ]
then
    USAGE
else
    YearCheck $1
    MonthCheck $2
    DayCheck $3
    WeekCheck $4
    echo "---------------"
    echo "Given Inputs"
    echo "---------------"
    echo "YEAR  :: $1"
    echo "MONTH :: $2"
    echo "DAY   :: $3"
    echo "WEEK  :: $4" 
    echo "---------------"
    cal $2 $1 | awk -v n="$4" -v WNO="$WNO" -v PNO="$PNO" '
    {
    if(NR==3)
    {
        if(NF==WNO)
        {
            a=2;
        }
        else
        {
            a=3;
        }    
    }
    if(a && NR==(a+n))
    {
        if(length($PNO)>0)
        {
            printf("Output Date : %s\n",$PNO);
        }
        else
        {
            printf("Requested week date is not available for the given month year combination\n");
        }
        exit;
    }
    }'
fi

$ ./findday.sh 2012 12 SA 5
---------------
Given Inputs
---------------
YEAR  :: 2012
MONTH :: 12
DAY   :: SA
WEEK  :: 5
---------------
Output Date : 29
 
$ cal 12 2012
   December 2012      
Su Mo Tu We Th Fr Sa  
                   1  
 2  3  4  5  6  7  8  
 9 10 11 12 13 14 15  
16 17 18 19 20 21 22  
23 24 25 26 27 28 29  
30 31                 
 
$ ./findday.sh 2012 12 FR 5
---------------
Given Inputs
---------------
YEAR  :: 2012
MONTH :: 12
DAY   :: FR
WEEK  :: 5
---------------
Requested week date is not available for the given month year combination