Shell中的if結合正則表達式使用

1、if 語句 判讀變量是否是數字

#!/bin/bash
read -p "plz input a letter:" letter
while :
do
if [[ $letter =~ ^[a-zA-Z]+$ ]];then
echo $letter && break
fi
read -p "not letter,plz input a letter agian:" letter
done

2、選出當前目錄下，正則判斷找出jar文件

#!/bin/bash
for filename in `ls`
do
file=$(echo "${filename}" | grep "jar$")
if [[ "${file}" =~ "jar" ]];then
echo ${file}
fi
done

注：只有bash才支持[[ ]]