无法在我的Node.js应用中读取环境变量

我在Ubuntu 12.04上，我只是在学习环境变量.我试图从我的应用程序中读取一个自定义变量，但它始终显示为undefined.这是我的测试应用程序的代码:

I am on Ubuntu 12.04 and I'm just learning about environment variables. I am trying to read a custom variable from within my application but it always shows up as undefined. Here is the code of my test app:

// app.js

console.log('Value: ' + process.env.NODE_ENV);

如果我运行以下命令，您将看到该变量具有一个值:

If I run the following commands you will see that the variable has a value:

$ NODE_ENV=production
$ echo $NODE_ENV
production

我可以整天echo $NODE_ENV，它将继续显示生产"，但是当我在Node应用程序中执行process.env.NODE_ENV时，它始终显示未定义".

I can echo $NODE_ENV all day and it will continue to show me "production", but when I do process.env.NODE_ENV in my Node application it always displays "undefined".

$ node app.js
Value: undefined

但是，这是一个奇怪的部分，如果我显示另一个我知道已经存在的环境变量，例如process.env.PATH，那么它将起作用.

Here is the odd part though, if I display another environment variable that I know already exists, say process.env.PATH, then it works.

$ node app.js
Value: /usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games

另一个怪癖是，尽管echo $NODE_ENV向我显示了正确的值，但命令printenv list似乎没有包含我的自定义变量NODE_ENV. printenv NODE_ENV也没有显示任何内容，但是printenv PATH显示了正确的值，就像我在节点应用程序中访问PATH时一样.

Another quirk is that the command printenv list doesn't appear to contain my custom variable NODE_ENV despite the fact that echo $NODE_ENV shows me the correct value. printenv NODE_ENV shows nothing as well, but printenv PATH shows the proper value just as it did when I accessed PATH in my node application.