Bash比较运算符始终为true [英] Bash comparison operator always true

问题描述

我正在尝试编写一个小脚本，以将我的外部IP(前三个字节)与以下内容进行比较:

I'm trying to write a small script to compare my external IP (first three bytes) with the one below:

#!/bin/bash
MYFILE=/home/me/.config/i3/pia
while true
do
    IP_EX=$(wget http://ipinfo.io/ip -qO - | cut -d"." -f1,2,3) 
    if [[ "$IP_EX"=="173.199.65" ]]
    then
        echo file created
        touch $MYFILE 
    else
        echo file deleted
        rm -f $MYFILE   
    fi
    echo sleeping
    sleep 4
done

这总是返回file created，并且else语句从不执行.如果我将$IP_EX替换为whatever，就是 even 的情况.为什么会这样?

This always returns file created, and the else statement is never executed. This is the case even if I replace the $IP_EX with whatever. Why is that?

推荐答案

Bash命令对空格敏感.您需要在==周围添加空格.

Bash commands are sensitive to spaces. You need to add spaces around ==.

观察到这给出​​了错误的答案:

Observe that this gives the wrong answer:

$ IP_EX=abc; [[ "$IP_EX"=="173.199.65" ]] && echo True
True

相反，此版本带有空格，可以正常工作:

By contrast, this version, with spaces, works correctly:

$ IP_EX=abc; [[ "$IP_EX" == "173.199.65" ]] && echo True
$ 

问题在于bash将"$IP_EX"=="173.199.65"视为单个字符串.当给定这样一个参数时，[[如果字符串不为空，则返回true；如果为空，则为false:

The problem is that bash sees "$IP_EX"=="173.199.65" as a single string. When given such a single argument, [[ returns true if the string is not empty and false if it is empty:

$ [[ "" ]] && echo True
$ [[ "1" ]] && echo True
True

添加空格后，bash将"$IP_EX" == "173.199.65"视为三个参数，中间参数为==.因此，它测试是否相等.这就是你想要的.

With the spaces added in, bash sees "$IP_EX" == "173.199.65" as three arguments with the middle argument being ==. It therefore tests for equality. This is what you want.

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