shell 指定范围产生随机数

#/bin/bash
echo "---------------产生随机数---------------"
read -p "请输入起始数:" a
read -p "请输入终止数:" b
if [ $a -lt $b ]
then
c= expr $(date +%N) % $[ $b - $a +  ] +  $a
echo "$c"
else
echo "---------------请重新输入!---------------"
echo
echo
echo
sh .sh
fi