Program in UNIX to print prime number series.

Write a shell script to print prime number series.
echo –n Enter a num
read n

num=2
while [ $num -le $n ]
do
i=2
flag=0
while [ $i -le `expr $num / 2` ]
do
if [ `expr $num % $i` -eq 0 ]
then
flag=1
break
else
i=`expr $i + 1`
fi
done
if [ $flag -eq 0 ]
then
echo $num
fi

num=`expr $num + 1`
done