Program to calculate the fibonacci series using iterative loop in UNIX

Iterative method
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#                     Script Starts Here                            #
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if [ $# -eq 1 ]
then
    Num=$1
else
    echo -n "Enter a Number :"
    read Num
fi

f1=0
f2=1

echo "The Fibonacci sequence for the number $Num is : "

for (( i=0;i<=Num;i++ ))
do
     echo -n "$f1 "
     fn=$((f1+f2))
     f1=$f2
     f2=$fn
done

echo

Output:
Enter a Number :20
The Fibonacci sequence for the number 20 is :
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 6765