#Go

March 2, 2019

A no-nonsense guide to environment variables in Go

Environment variables are the best way to set configuration values for your software application as they can be defined at system-level, independently of the software. This is one of the principles of the Twelve-Factor App methodology and enables applications to be built with portability.

Using environment variables

All you need to interact with environment variables is the standard os package. Here is an example of how you can access the system PATH environment variable.

{% gist e79ed163b202ed12f664ceafd93484e4 %}

It’s equally easy to set environment variables:

{% gist 025290c6e61a5c58d12608d7c55a6d4b %}

Loading environment variables from a .env file

It is not always practical to set environment variables on development machines where multiple projects are running.

godotenv is a Go port of the Ruby dotenv library. This allows you to define your application’s environment variables in a .env file.

To install the package run:

$ go get github.com/joho/godotenv

Add your configuration values to a .env file at the root of your project:

GITHUB_USERNAME=craicoverflow
GITHUB_API_KEY=TCtQrZizM1xeo1v92lsVfLOHDsF7TfT5lMvwSno

Then you can use these values in your application:

{% gist ea6d06a2d819c706029e888f1a6a7d28 %}

It’s important to note that if an environment variable is already defined in the system, then Go will prefer use that instead of the value in .env.


Wrapping environment variables in a configuration package

It’s all well and good accessing environment variables directly like this, but having to maintain that doesn’t seem fun, does it? Every value is a string - and imagine having to update every reference when an environment key is modified!

To deal with this, let’s create a configuration package to access environment variables in a much more centralized and maintainable way.

Here is a simple config package which will return configuration values in a Config struct. We have the option to define a default value, so when an environment variable does not exist this will be used instead.

{% gist 88a3866f6c06d34d154c963404cdc044 %}

Next, we should add different types to the Config struct. The current implementation can only handle string types, which isn’t very practical for larger applications.

Let’s add functions to handle bool, slice and integer types.

{% gist f487b83a5bae318a7a94ba6d326219c1 %}

Update your .env file with these environment variables.

GITHUB_USERNAME=craicoverflow
GITHUB_API_KEY=TCtQrZizM1xeo1v92lsVfLOHDsF7TfT5lMvwSno
MAX_USERS=10
USER_ROLES=admin,super_admin,guest
DEBUG_MODE=false

And now you can access these values from the rest of your application:

{% gist 602027502a75e06c531c921fe79daf83 %}

And there you have it

There are several libraries out there that claim to offer a configuration “solution” for your Go application. But is it really a solution when it’s just as easy to make one yourself?

How do you manage configuration in your Go applications?

© Enda Phelan 2020

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