Try this script

[[ -z $1 ]]&& echo "Argument needed "&& exit
[[ ! -s $1 ]]&& echo "File not found "&& exit
rm -f $1New
var=`wc -l <$1`
echo "Enter line no :\c"
read lineno
echo "Enter Text:\c"
read text
head -$lineno $1 >$1New
echo "$text" >>$1New
var1=$((var-lineno))
tail -$var1 $1 >>$1New