控制流 if 和 then 介绍

beyes

示例代码：

#!/bin/bash
#fitest

if [ "10" -lt "12" ]
then
        echo "yes,10 is less than 12"
fi

注意：if 和 [ 间要有空格，"10" 和　[　间要有空格，"12"　和　]　间要有空格　-lt 和　"10" 和　"12"　间可以没有空格。

beyes

#!/bin/bash
#ifcp

echo -n "Enter your name:"
read NAM
#if [ -z $NAME ] ||                         //　-z the length of STRING is zero(为空）
if  [ "$NAME" = "" ];then                   // = 或者 ==　都可以
        echo "You did not enter a name"
elif [ $NAME = "root" ]; then
        echo "Hello root"
elif [ $NAME = "beyes" ]; then
        echo "Hello beyes"
else
        echo "you are not root or beyes,but hi, $NAME"
fi

beyes

代码：

#!/usr/bash

echo -n "Please enter your name:"     // -n 不换行

read NAME                          // 读输入名称到变量

if [ "$NAME" == "" ];then
        echo "You did not enter your name"      //输入为空提示
else
        echo "Your name is ${NAME}"

fi

beyes

测试代码：

#!/bin/bash

echo "Is it morning? Please answer yes or no"
read timeofday

if [ $timeofday = "yes" ]
then
echo "Good moring"

elif [ $timeofday = "no" ]; then
echo "Good afternoon"
else
echo "Sorry, $timeofday not recognized. Enter yes or no"
        exit 1
fi

exit 0

在上面的代码中，如果不是输入字符串，而是直接回车，那么就会出现以下的错误：

[beyes@localhost shell]$ ./check_elif.sh
Is it morning? Please answer yes or no

./check_elif.sh: line 6: [: =: unary operator expected
./check_elif.sh: line 10: [: =: unary operator expected

这是因为，当直接回车时，if 测试式就会变成：

if [ = "yes" ]

这不是正确的表达式，所以原句需要改成：

if [ "$timeofday" = "yes" ]        // $timeofday 加了双引号，其意是把回车也当作字符串来处理