将变量与字符串进行比较 bash

Question

上下文

我们很多CentOS服务器我想安装一些监控软件，该软件是基于CentOS版本的，我想查看发布版本并以此为基础安装软件。

问题

似乎 if 语句运行成功且没有错误，而结果永远不会对两个或所有三个 if 语句都为真。我查看了 if 命令和测试，似乎我应该在 bash 中使用双括号和单个 = 符号。我相信我正在做一些非常简单的错误，但我就是找不到它。

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

收到一些回复后，我更改了我的代码，在等号 (=) 周围添加了 space。仍然没有按预期工作，因为比较应该 return 在第二个 if 语句上为真，但它没有。

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 告诉我 /etc/centos-release 有一个 space 添加到脚本文件的末尾，我相信在 CentOS 6.9 版（最终版）上情况并非如此.在字符串中添加 space，或者一起使用我的 known_version 变量并添加 space 解决了问题，脚本现在按预期工作。

Answer 1

作为一般经验法则，在可能所有的编程语言中，单个等号 (=) 用于赋值而不用于相等检查，在这种情况下是双等号 (=) 符号。

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 (=) 进行相等性检查。在 [ ] 或 [[ ]].

两种情况下，bash 还支持双等号 (=) 作为相等性检查

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 2

你的 if 语句有问题。

简单回答：

如果要比较字符串，请确保使用 ==。
请使用if [ "$version" == "xyz" ]代替if [[ "$version" = "xyz" ]]

总结：if [ "$version"=="CentOS release 6.9 (Final)" ]; then 它应该有效！

此致，FrankTheTank

Answer 3

= 符号周围需要空格；没有它，shell 将 "expression" 视为单个词，而不是两个词的相等性比较。

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

[[ a=b ]]等同于[[ -n a=b ]]，也就是说，你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 语句

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

将变量与字符串进行比较 bash

Comparing variables with strings bash

linux

bash

if-statement

string-comparison

上下文

问题

代码

结果

更新

代码#2

结果 #2

更新