IF语句

选择执行：

注意：if语句可嵌套

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1.单分支

if 判断条件;then

	条件为真的分支代码

fi

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2.双分支

if 判断条件; then

	条件为真的分支代码

else

	条件为假的分支代码

fi

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3.多分支

if 判断条件1; then

	条件1为真的分支代码

elif 判断条件2; then

	条件2为真的分支代码

elif 判断条件3; then

	条件3为真的分支代码

else

	以上条件都为假的分支代码

fi

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逐条进行判断，第一次遇为“真”条件时，执行其分支，而后结束整个if语句

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例子1：年龄判别

需求：

第1段：0到18（18岁以下，不包含18岁）好好学习，天天向上

第2段：18到60（18岁到60岁，包含60岁）努力工作

第3段：60-150（60岁到150岁，包含150岁）享受生活

其他情况：你不是来自地球

代码如下：

#!/bin/bash

read -p "Please input your age: " AGE

[[ "$AGE" =~ ^[1]+$ ]] || { echo -e "33[1;31m ${AGE} is not legal digit33[0m"; exit; }

if [ "$AGE" -lt 18 ];then

	echo "good good study;day day up"

elif [ "$AGE" -le 60 ];then

	echo "work hard"

elif [ "$AGE" -le 150 ];then

	echo "enjoy your life"

else

	echo "you do not come from the earth!"

fi

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例子2：回答yes or no

代码如下：

read -p "Please input your answer: " ANSWER

if [[ $ANSWER =~ [Y|y]([E|e][S|s])? ]];then

    echo "your answer is yes"

elif [[ $ANSWER =~ [N|n][O|o]? ]];then

    echo "your answer is no"

else

    echo "your answer is wrong"

fi

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例子3：嵌套

效果图，判断是否富有与帅

代码如下：

read -p "Are you rich: " ANSWER
if [[ "$ANSWER" =~ ^[Y|y]([E|e][S|s])?$ ]];then
        echo "OK!"
elif [[ "$ANSWER" =~ ^[N|n][O|o]?$ ]];then
        read -p "Are you handsome: " ANSWER
        if [[ "$ANSWER" =~ ^[Y|y]([E|e][S|s])?$ ]];then
                        echo "OK!"
        elif [[ "$ANSWER" =~ ^[N|n][O|o]?$ ]];then                                                            
                echo "work hard!"
        else 
                echo "Your answer is wrong!"
        fi
else
        echo "Your answer is wrong!"    
fii 

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1. [:digit:] ↩︎