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[#]: subject: (Set and use environment variables in FreeDOS)
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[#]: via: (https://opensource.com/article/21/6/freedos-environment-variables)
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[#]: author: (Jim Hall https://opensource.com/users/jim-hall)
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[#]: collector: (lujun9972)
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[#]: translator: (robsean)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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Set and use environment variables in FreeDOS
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======
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Environment variables are helpful in almost every command-line
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environment, including FreeDOS.
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![Looking at a map for career journey][1]
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A useful feature in almost every command-line environment is the _environment variable_. Some of these variables allow you to control the behavior or features of the command line, and other variables simply allow you to store data that you might need to reference later. Environment variables are also used in FreeDOS.
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### Variables on Linux
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On Linux, you may already be familiar with several of these important environment variables. In the [Bash][2] shell on Linux, the `PATH` variable identifies where the shell can find programs and commands. For example, on my Linux system, I have this `PATH` value:
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```
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bash$ echo $PATH
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/home/jhall/bin:/usr/lib64/ccache:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin
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```
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That means when I type a command name like `cat`, Bash will check each of the directories listed in my `PATH` variable, in order:
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1. `/home/jhall/bin`
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2. `/usr/lib64/ccache`
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3. `/usr/local/bin`
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4. `/usr/local/sbin`
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5. `/usr/bin`
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6. `/usr/sbin`
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And in my case, the `cat` command is located in the `/usr/bin` directory, so the full path to that command is `/usr/bin/cat`.
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To set an environment variable on Linux, you type the name of the variable, then an equals sign (`=`) and then the value to store in the variable. To reference that value later using Bash, you type a dollar sign (`$`) in front of the variable name.
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```
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bash$ var=Hello
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bash$ echo $var
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Hello
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```
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### Variables on FreeDOS
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On FreeDOS, environment variables serve a similar function. Some variables control the behavior of the DOS system, and others are useful to store some temporary value.
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To set an environment variable on FreeDOS, you need to use the `SET` keyword. FreeDOS is _case insensitive_, so you can type that using either uppercase or lowercase letters. Then set the variable as you might on Linux, using the variable name, an equals sign (`=`), and the value you want to store.
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However, referencing or _expanding_ an environment variable's value in FreeDOS is quite different from how you do it on Linux. You can't use the dollar sign (`$`) to reference a variable in FreeDOS. Instead, you need to surround the variable's name with percent signs (`%`).
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![Use % \(not $\) to reference a variable's value][3]
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(Jim Hall, [CC-BY SA 4.0][4])
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It's important to use the percent signs both before and after the name because that's how FreeDOS knows where the variable name begins and ends. This is very useful, as it allows you to reference a variable's value while immediately appending (or prepending) other text to the value. Let me demonstrate this by setting a new variable called `reply` with the value `yes`, then referencing that value with the text "11" before and "22" after it:
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![Set and reference an environment variable][5]
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(Jim Hall, [CC-BY SA 4.0][4])
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Because FreeDOS is case insensitive you can also use uppercase or lowercase letters for the variable name, as well as the `SET` keyword. However, the variable's value will use the letter case as you typed it on the command line.
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Finally, you can see a list of all the environment variables currently defined in FreeDOS. Without any arguments, the `SET` keyword will display all variables, so you can see everything at a glance:
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![Show all variables at once with SET][6]
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(Jim Hall, [CC-BY SA 4.0][4])
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Environment variables are a useful staple in command-line environments, and the same applies to FreeDOS. You can set your own variables to serve your own needs, but be careful about changing some of the variables that FreeDOS uses. These can change the behavior of your running FreeDOS system:
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* **DOSDIR**: The location of the FreeDOS installation directory, usually `C:\FDOS`
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* **COMSPEC**: The current instance of the FreeDOS shell, usually `C:\COMMAND.COM` or `%DOSDIR%\BIN\COMMAND.COM`
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* **LANG**: The user's preferred language
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* **NLSPATH**: The location of the system's language files, usually `%DOSDIR%\NLS`
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* **TZ**: The system's time zone
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* **PATH**: A list of directories where FreeDOS can find programs to run, such as `%DOSDIR%\BIN`
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* **HELPPATH**: The location of the system's documentation files, usually `%DOSDIR%\HELP`
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* **TEMP**: A temporary directory where FreeDOS stores output from each command as it "pipes" data between programs on the command line
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* **DIRCMD**: A variable that controls how the `DIR` command displays files and directories, typically set to `/OGNE` to order (O) the contents by grouping (G) directories first, then sorting entries by name (N) then extension (E)
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If you accidentally change any of the FreeDOS "internal" variables, you could prevent some parts of FreeDOS from working properly. In that case, simply reboot your computer, and FreeDOS will reset the variables from the system defaults.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/6/freedos-environment-variables
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作者：[Jim Hall][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/jim-hall
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/career_journey_road_gps_path_map_520.png?itok=PpL6jJgY (Looking at a map for career journey)
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[2]: https://opensource.com/article/19/8/using-variables-bash
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[3]: https://opensource.com/sites/default/files/uploads/env-path.png (Use % (not $) to reference a variable's value)
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[4]: https://creativecommons.org/licenses/by-sa/4.0/
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[5]: https://opensource.com/sites/default/files/uploads/env-vars.png (Set and reference an environment variable)
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[6]: https://opensource.com/sites/default/files/uploads/env-set.png (Show all variables at once with SET)
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@ -0,0 +1,109 @@
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[#]: subject: (Set and use environment variables in FreeDOS)
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[#]: via: (https://opensource.com/article/21/6/freedos-environment-variables)
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[#]: author: (Jim Hall https://opensource.com/users/jim-hall)
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[#]: collector: (lujun9972)
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[#]: translator: (robsean)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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在 FreeDOS 中设置和使用环境变量
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======
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环境变量几乎在每种命令行环境中都是很有帮助的，自然包括 FreeDOS 。
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![Looking at a map for career journey][1]
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几乎在每个命令行环境中的一个有用的功能是 _环境变量_。其中的一些变量允许你控制命令行的行为或功能，其它的变量仅允许你存储可能稍后需要的数据，在 FreeDOS 中也使用了环境变量。
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### 在 Linux 上的变量
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在 Linux 上，你可能已经熟悉其中的一些重要的环境变量。在 Linux 上的 [Bash][2] shell 中，`PATH` 变量标示着 shell 可以在哪里找到程序和命令。例如，在我的 Linux 系统上，我有这个 `PATH` 值：
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```
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bash$ echo $PATH
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/home/jhall/bin:/usr/lib64/ccache:/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin
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```
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这意味着，当我输入一个像 `cat` 这样的命令的名称时，Bash 将会按顺序检查我在 `PATH` 变量中所列出的每个目录：
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1. `/home/jhall/bin`
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2. `/usr/lib64/ccache`
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3. `/usr/local/bin`
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4. `/usr/local/sbin`
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5. `/usr/bin`
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6. `/usr/sbin`
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在我的实例中，`cat` 命令位于 `/usr/bin` 目录，因此，完整的路径是 `/usr/bin/cat` 。
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为在 Linux 上设置一个环境变量，你可以输入一个变量的名称，接着输入一个等于符号 (`=`) ，接着输入一个要存储在变量中的值。为了随后使用 Bash 引用这个值，你需要在变量的名称前输入一个美元符号 (`$`) 。
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```
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bash$ var=Hello
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bash$ echo $var
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Hello
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```
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### 在 FreeDOS 上的变量
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在 FreeDOS 上，环境变量提供一种类似的功能。一些变量控制 DOS 系统的行为，另一些变量用于存储一些临时值。
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为在 FreeDOS 上设置一个环境变量，你需要使用 `SET` 关键字。FreeDOS 是 _不区分大小写的_ ，因此你可以输入大写字母也可以使用小写字母。接下来，像你在 Linux 上一样设置变量，使用变量名称，一个等于符号 (`=`) ，你想要存储的值。
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不过，在 FreeDOS 中引用或 _扩展_ 一个环境变量的值的方法，与你在 Linux 上所使用的方法是完全不同的。在 FreeDOS 中，你不能使用美元符号 (`$`) 来引用一个变量。你反而需要使用百分符号 (`%`) 来包围变量的名称。
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![Use % \(not $\) to reference a variable's value][3]
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(Jim Hall, [CC-BY SA 4.0][4])
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在名称前后使用百分符号是非常重要，因为这就是 FreeDOS 知悉变量名称在哪里开始和结束的方式。这是非常有用的，因为它会允许你引用一个变量的值，与此同时，它会立即附加 (或预置) 其它的文本到值中。让我通过设置一个新的名称为 `reply` 的值为 `yes` 的变量，然后在 "11" 之前和"22" 之后引用这个值来演示这一点：
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![Set and reference an environment variable][5]
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(Jim Hall, [CC-BY SA 4.0][4])
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因为 FreeDOS 是不区分大小写的，所以你可以使用大写字母称或小写字母的变量名称以及 `SET` 关键字。不过，变量的值将使用你在命令行中所输入的字母。
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最后，你可以看到当前在 FreeDOS 中定义的所有的环境变量。不使用任何参数的 `SET` 关键字将显示所有的变量，因此你可以一目了然：
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![Show all variables at once with SET][6]
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(Jim Hall, [CC-BY SA 4.0][4])
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环境变量是一个有用的基本的命令行环境，同样适用于 FreeDOS 。你可以设置你自己的变量以满足你自己的需要，但是要仔细地更改 FreeDOS 使用的一些变量。这些变量会更改你正在运行的 FreeDOS 系统的行为：
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* **DOSDIR**：FreeDOS 安装目录的位置，通常是 `C:\FDOS`
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* **COMSPEC**: FreeDOS 的 shell 的当前实例，通常是 `C:\COMMAND.COM` 或 `%DOSDIR%\BIN\COMMAND.COM`
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* **LANG**: 用户的首选语言
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* **NLSPATH**: 系统语言文件的位置，通常是 `%DOSDIR%\NLS`
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* **TZ**: 系统的时区
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* **PATH**: 一个目录列表，FreeDOS 可以在其中找到要运行的程序，例如 `%DOSDIR%\BIN`
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* **HELPPATH**: 系统文档文件的位置，通常是 `%DOSDIR%\HELP`
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* **TEMP**: 一个临时目录，FreeDOS 在其中存储来自每个命令的输出，如同它在命令行上的程序之间的 "管道" 数据
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* **DIRCMD**: 一个控制 `DIR` 命令如何显示文件和目录的变量，通常设置 `/OGNE` 来排序 (O) 内容，先通过分组 (G) 目录，接下来按照名称 (N) 、扩展名 (E) 来排序条目
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如果你偶然间更改了任意的 FreeDOS 的 "内部" 变量，你可能会阻碍 FreeDOS 的一些部分的正常工作。在这种情况下，只需要简单地重新启动你的计算机，FreeDOS 将会按照系统默认值重新设置变量。
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/6/freedos-environment-variables
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作者：[Jim Hall][a]
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选题：[lujun9972][b]
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译者：[robsean](https://github.com/robsean)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/jim-hall
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/career_journey_road_gps_path_map_520.png?itok=PpL6jJgY (Looking at a map for career journey)
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[2]: https://opensource.com/article/19/8/using-variables-bash
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[3]: https://opensource.com/sites/default/files/uploads/env-path.png (Use % (not $) to reference a variable's value)
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[4]: https://creativecommons.org/licenses/by-sa/4.0/
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[5]: https://opensource.com/sites/default/files/uploads/env-vars.png (Set and reference an environment variable)
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[6]: https://opensource.com/sites/default/files/uploads/env-set.png (Show all variables at once with SET)
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