shell判断数字

用shell来判断输入的是否是数字。
#cat isnum.sh

#!/bin/sh

file_size=128
echo "Please input a number for testing:"
read n
input_num="$(echo "$n" | sed -n "/^[0-9]\+$/p")"
if [ -n "$input_num" ];then
        echo "Input is a number!"
        file_size="$input_num"
        echo "The number is: $file_size."
else
        echo "Input isn't a number!"
        echo "Use defaule number $file_size."
fi