1.语法

if..then..elif..fi的语法：

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if [ 条件语句 ]；then
    执行内容
elif [ 条件语句 ]；then
    执行内容
else
    执行内容
fi

# 或

if [ 条件语句 ]
then
    执行内容
elif [ 条件语句 ]
then
    执行内容
else
    执行内容
fi

2.案例

1.if..then..elif..fi的案例，比较两个值是否相等。

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#!/bin/bash
a=10
b=20
if [ $a -eq $b ]
then
    echo "a is equal to b"
elif [ $a -gt $b ]    # a 大于 b 见表1
then
   echo "a is greater than b"
elif [ $a -lt $b ]    # a 小于 b 见表1
then
   echo "a is less than b"
else
   echo "None of the condition met"
fi

文件比较符，表1。

[ ]括号 （（））扩容 含义
-eq == 等于
-ne != 不等于
-gt > 大于
-ge >= 大于等于
-lt < 小于
-le <= 小于等于

2.通过条件语句实现的计算器。

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#!/bin/bash
# author:djangwoang
# filename:jisuanqi.sh

echo "please input your first number"
read a
echo "please input + - * /"
read b
echo "please input your second number"
read c

if [ "x$b" == "x" ];then
    echo "please input + - * /"
elif [ "$b" == "+" ];then
    tmp=$((a+c))
elif [ "$b" == "-" ];then
    tmp=$((a-c))
elif [ "$b" == "*" ];then
    tmp=$((a*c))
elif [ "$b" == "/" ];then
    tmp=$((a/c))
else
    echo "please input + - * /"
fi

echo "result is:${tmp}"