Shell中可以使用"[ ]"進行資料判斷，“[ ]”的用法跟test基本一致。

使用“[ ]”時要注意三點（來自鳥哥）：

1、在中括號"[]"內的每個元件都需要有空格鍵來分割。

2、在中括號內的變數，最好都以雙引號括起來。

3、在中括號內的常量，最好都以單引號或雙引號括起來

if else語句的結構為：

if [ condition ]; then

        #some statements

elif [ condition ]; then

        #some statements

else

        #some statements

fi

下面是一個小例子，其中[ "$yn" == "Y" ] || [ "$yn" == "y" ] 等價於[ "$yn" == "Y" -o "$yn" == "y" ]，“-o”表示“或”，連線兩個判斷。

#!/bin/bash

#read user's input and do something
read -p "please input (Y/N):" yn

if [ "$yn" == "Y" ] || [ "$yn" == "y" ]; then
        echo "OK, you say yes."
elif [ "$yn" == "N" ] || [ "$yn" == "n" ]; then
        echo "NO? goodbye."
else
        echo "I don't know what you say"
fi

#test whether a directory exist,if not,create it
if [ ! -d "keystoneclient" ]; then
        mkdir keystoneclient
fi
 

while [ condtion ]

do

        #some statements

done

下面是一個小例子：

#!/bin/bash

while [ "$yn" != "yes" -a "$yn" != "YES" ]
do
        read -p "please input YSE or yes to stop this loop." yn
done

echo "OK, you input the correct answer."

#!/bin/bash

while [ 1 ]
do
        date
        sleep 1
done