Avoid fragmenting your configuration

Nowadays configuration often is done using environment (aka ENV) variables. They work great using docker/containers, in development and production, on all platforms and using all languages. In short I think environment variables are great for configuration of many aspects of an application.

However, I encountered a pattern in several different applications that I really dislike: Several, fragmented ENV variables for one configurable aspect of the application.

Let us have a look at two examples to see what I mean, then I will try to explain where it could come from and why I think it is bad practice. Finally I will show a better alternative – at least in my opinion.

First real world example

In one javascript app a websocket url was made configurable using 4 (!) ENV variables like this:

WS_PREFIX || "wss://";
WS_HOST || "hostname";
WS_PORT || "";
WS_PATH || "/ws";

function ConnectionString(prefix, host, port, path) {
  return {
    attrib: {
    string: prefix + host + port + path,

We immediately see, that the author wrote a function to deal with the complex configuration in the rest of the application. Not only the devops team or administrators need to supply many ENV variables but they have to supply them in a peculiar way:

The port needs to be specified as :8888, using a leading colon (or the host needs a trailing colon…) which is more than unexpected. The alternative would be a better and more sophisticated implementation of ConnectionString…

Another real example

In the following example the code there are again three ENV variables dealing with hosts, urls and websockets. This examples feels quite convoluted, is hard to understand and definitely needs a refactoring.


const defaultHost = window.TANGOGQL_HOST ?? "localhost:5004";
const defaultSocketUrl = window.TANGOGQL_SOCKET ?? ws://${defaultHost}/socket;

// dealing with config peculiarities somewhere else
const socketUrl = React.useMemo(() =>
        config.host.replace(/.*:\/\//, "ws://") + "/socket"
    , [config.host]);


The examples show clearly that something simple like a configuration for an URL can lead to complicated and hard to use solutions. Most likely the authors tried to not repeat themselves and factored the URLs into the smallest sensible components. While this may sound like a good idea it puts burden on both the developers and the devops team configuring the application.

In my opinion it would be much simpler and more usable for both parties to have complete URLs for the different use cases. Of course this could mean repeating protocols, hostnames and ports if they are the same in the different situations. But just having one or two ENV variables like


would be straightforward to use in code and to be configured in the runtime environment. At the same time the chance for errors and the complexity in the configuration is reduced.

Even though certain parts of the URLs are duplicated in the configuration I highly prefer this approach over the presented real world solutions.

Fun with docker container environment variables

Docker (as one specific container technology product) is a basic ingredient of our development infrastructure that steadily gained ground from the production servers over the build servers on our development machines. And while it is not simple when used for operations, the complexity increases a lot when used for development purposes.

One way to express complexity is by making the moving parts configurable and using different configurations. A common way to make things configurable with containers are environment variables. Running a container might look like a endurance typing contest if used extensibly:

docker run --rm \
-e POSTGRES_USER=myuser \
-e POSTGRES_PASSWORD=mysecretpassword \
-e POSTGRES_DB=mydatabase \
-e PGDATA=/var/lib/postgresql/data/pgdata \
ubuntu:22.04 env

This is where our fun begins.

Using an env-file for extensive configurations

The parameter –env-file reads environment variables from a local text file with a simple key=value format:

docker run --rm --env-file my-vars.env ubuntu:22.04 env

The file my-vars.env contains all the variables line by line:


If we run the command above in a directory containing the file, we get the following output:


The HOSTNAME might vary, but the FIRST and SECOND environment variables are straight from our file.

The only caveat is that the env-file really has to exist, or we get an error:

docker: open my-vars2.env: Das System kann die angegebene Datei nicht finden.

My beloved shell

The env-file can be empty, contain only comments (use # to begin them) or whitspace, but it has to be present.

Please be aware that the env-files are different from the .env-file(s) in docker-compose. A lot of fun is lost by this simple statement, like variable expansion. As far as I’m aware, there is no .env-file mechanism in docker itself.

But we can have some kind of variable substitution, too:

Using multiple env-files for layered configurations

If you don’t want to change all your configuration entries all the time, you can layer them. One layer for the “constants”, one layer for global presets and one layer for local overrides. You can achieve this with multiple –env-files parameters, they are evaluated in your specified order:

docker run -it --rm --env-file first.env --env-file second.env ubuntu:22.04 env

Let’s assume that the content of first.env is:


And the content of second.env is:


The results of our container call are (abbreviated):


You can see that the second TEST assignment wins. If you switch the order of your parameters, you would read TEST=1.

Now imagine that first.env is named global.env and second.env is named local.env (or default.env and development.env) and you can see how this helps you with modular configurations. If only the files need not to exist all the time, it would even fit well with git and .gitignore.

The best thing about this feature? You can have as many –env-file parameters as you like (or your operating system allows).

Mixing local and configured environment variables

We don’t have explicit variable expansion (like TEST=${FIRST} or something) with –env-files, but we have a funny poor man’s version of it. Assume that the second.env from the example above contains the following entries:


You’ve seen that right: The first entry has no value (and no equal sign)! This is when the value is substituted from your operating system:


By just declaring, but not assigning an environment variable it is taken from your own environment. This even works if the variable was already assigned in previous –env-files.

If you don’t believe me, this is a documented feature:

If the operator names an environment variable without specifying a value, then the current value of the named variable is propagated into the container’s environment


And even more specific:

When running the command, the Docker CLI client checks the value the variable has in your local environment and passes it to the container. If no = is provided and that variable is not exported in your local environment, the variable won’t be set in the container.


This is a cool feature, albeit a little bit creepy. Sadly, it doesn’t work in all tools that allow to run docker containers. Last time I checked, PyCharm omitted this feature (as one example).


I’ve presented you with three parts that can be used to manage different configurations for docker containers. There are some pain points (non-optional file existence, feature loss in tools, no direct variable expansion), but also a lot of fun.

Do you know additional tricks and features in regard to environment variables and docker? Comment below or link to your article.