将变量与字符串 bash 进行比较

Question

上下文

对于我们的很多CentOS服务器我想安装一些监控软件，该软件是基于CentOS版本的，我想检查发布版本并根据它安装软件。

问题

似乎 if 语句成功运行而没有错误，而结果对于两个或所有三个 if 语句都不应该为真。我查看了 if 命令和测试，看来我应该在 bash 中使用双括号和单 = 符号。我相信我在做一些非常简单的错误，但我就是找不到。

代码

#!/bin/bash
versionknown=false
version=$(</etc/centos-release)
known_version1="CentOS release 6.9 (Final)"
known_version2="CentOS Linux release 7.3.1611 (Core)"

if [[ "$version"="CentOS release 6.9 (Final)" ]] ; then
    echo "Same versions as expected"
    versionknown=true
    #run commands for this specific version
fi

if [[ "$version"="CentOS Linux release 7.3.1611 (Core)" ]] ; then
    echo "Same versions as expected v2"
    versionknown=true
    #run commands for this specific version
fi


if [[ "$versionknown"=false ]] ; then
    echo "Different version detected than known:"
    echo $version
    echo "Aborted"
fi

echo $versionknown

结果

Same versions as expected
Same versions as expected v2
Different version detected than known:
CentOS Linux release 7.3.1611 (Core)
Aborted
true

更新

在收到一些回复后，我更改了代码，在等号 (=) 周围添加了空格。仍然不能按预期工作，因为比较应该在第二个 if 语句中返回 true，而它没有。

代码 #2

#!/bin/bash
versionknown=false
version=$(</etc/centos-release)
known_version1="CentOS release 6.9 (Final)"
known_version2="CentOS Linux release 7.3.1611 (Core)"

if [[ "$version" = "CentOS release 6.9 (Final)" ]] ; then
        echo "Same versions as expected"
        versionknown=true
        #run script for this specific version
fi

if [[ "$version" = "CentOS Linux release 7.3.1611 (Core)" ]] ; then
        echo "Same versions as expected v2"
        versionknown=true
        #run script for this specific version
fi


if [[ "$versionknown" = false ]] ; then
        echo "Different version detected than known:"
        echo $version
        echo "Aborted"
fi

echo $versionknown

结果 #2

Different version detected than known:
CentOS Linux release 7.3.1611 (Core)
Aborted
false

更新

declare -p version 了解到/etc/centos-release 在脚本文件的末尾添加了一个空格，我相信在 CentOS 版本 6.9 (Final) 上并非如此。在字符串中添加空格，或者一起使用我的 known_version 变量并添加空格可以解决问题，脚本现在可以按预期工作。

Answer 1

= 符号周围需要空格；没有它，shell 会将“表达式”视为一个单词，而不是比较两个单词是否相等。

[[ "$version" = "CentOS release 6.9 (Final)" ]]

[[ a=b ]] 等价于[[ -n a=b ]]，也就是说，您只是在测试单个单词是否为非空字符串。

在[[ ... ]] 中使用= 还是== 是风格问题； bash 两者都支持。

我会完全避免使用versionknown 变量，并编写如下代码：

version=$(</etc/centos-release)
known_version1="CentOS release 6.9 (Final)"
known_version2="CentOS Linux release 7.3.1611 (Core)"

if [[ "$version" = "$known_version1" ]] ; then
    echo "Same versions as expected"
    #run commands for this specific version
elif [[ "$version" = "$known_version2" ]] ; then
    echo "Same versions as expected v2"
    #run commands for this specific version
else
    echo "Different version detected than known:"
    echo "$version"
    echo "Aborted"
fi

或case 声明

case $version in
   "$known_version1")
       echo "Same versions as expected"
       # ...
       ;;
   "$known_version2")
       echo "Same versions as expected v2"
       # ...
       ;;
   *)
      echo "Different version detected than known:"
      echo "$version"
      echo "Aborted"
esac

Answer 2

作为一般经验法则，可能在所有编程语言中，一个等号 (=) 用于赋值，而不是用于相等检查，在这种情况下是双等号 (=)。

您输入所有if 语句的原因是您的赋值表达式的结果是true，因此if 语句被评估为true 并被执行。

更新

感谢@chepner 的评论，当与单方括号[ ] 或双方括号[[ ]] 关联时，使用单个等号(=) 进行相等检查。 bash 还支持在 [ ] 或 [[ ]] 两种情况下使用双等号 (=) 作为相等检查。

你想要的大概是这样的：

#!/bin/bash
versionknown=false
version=$(</etc/centos-release)
known_version1="CentOS release 6.9 (Final)"
known_version2="CentOS Linux release 7.3.1611 (Core)"

if [[ "$version" == "CentOS release 6.9 (Final)" ]] ; then
    echo "Same versions as expected"
    versionknown=true
    #run commands for this specific version
fi

if [[ "$version" == "CentOS Linux release 7.3.1611 (Core)" ]] ; then
    echo "Same versions as expected v2"
    versionknown=true
    #run commands for this specific version
fi


if [[ "$versionknown" == "false" ]] ; then
    echo "Different version detected than known:"
    echo $version
    echo "Aborted"
fi

echo $versionknown

Answer 3

您在 if 语句中做错了什么。

简单回答：

1. 如果要比较字符串，请确保使用==。
2. 请使用if [ "$version" == "xyz" ] 而不是if [[ "$version" = "xyz" ]]

总结：if [ "$version"=="CentOS release 6.9 (Final)" ]; then 并且成功了！

问候，弗兰克坦克