> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Environment Variables in Docker Recap
> This article explains how to manage environment variables in Docker for Flask applications to enhance configuration flexibility and security.
Modern application development favors separating configuration from code. By using environment variables—especially within Docker containers—you can adhere to [The Twelve-Factor App](https://12factor.net/) methodology and deploy more flexibly.
## Table of Contents
1. [Why Use Environment Variables?](#why-use-environment-variables)
2. [Step 1: Read Environment Variables Locally](#step-1-read-environment-variables-locally)
3. [Step 2: Pass Variables into Docker](#step-2-pass-variables-into-docker)
4. [Docker Commands Reference](#docker-commands-reference)
5. [Further Reading](#further-reading)
***
## Why Use Environment Variables?
Hardcoding configuration values (such as colors, database URLs, or API keys) leads to brittle deployments and requires code changes for every tweak. Environment variables allow you to:
* Decouple code from deployment details
* Support multiple environments (dev, staging, prod) without modifying source
* Secure sensitive data outside of version control
Using environment variables is a best practice for twelve-factor compliant apps.
***
## Step 1: Read Environment Variables Locally
Below is a simple Flask application (`app.py`) that reads `APP_COLOR` from the environment, defaulting to `"red"` if unset.
```python theme={null}
import os
from flask import Flask, render_template
app = Flask(__name__)
# Read background color from environment; default to "red"
color = os.environ.get("APP_COLOR", "red")
@app.route("/")
def main():
print(f"Current color: {color}")
return render_template("hello.html", color=color)
if __name__ == "__main__":
app.run(host="0.0.0.0", port=8080)
```
To test locally:
```bash theme={null}
export APP_COLOR=blue
python app.py
```
Visit `http://localhost:8080`—the page background will reflect your `APP_COLOR`. If you don’t set `APP_COLOR`, it gracefully falls back to **red**.
Never commit sensitive environment variables (like credentials) to your code repository. Consider using a secrets manager for production.
***
## Step 2: Pass Variables into Docker
Once your app is containerized, leverage Docker’s `-e` flag (or `--env-file`) to inject runtime configuration.
1. **Build the image**
```bash theme={null}
docker build -t simple-webapp-color .
```
2. **Run with a custom color**
```bash theme={null}
docker run -e APP_COLOR=blue simple-webapp-color
```
3. **Scale with different settings**
```bash theme={null}
docker run -e APP_COLOR=green simple-webapp-color
docker run -e APP_COLOR=yellow simple-webapp-color
```
By externalizing `APP_COLOR`, you keep one image for all environments—no code changes required.
***
## Docker Commands Reference
| Command | Purpose | Example |
| ---------------------------------- | --------------------------------------- | ------------------------------------------------------ |
| `docker build` | Build an image from a `Dockerfile` | `docker build -t simple-webapp-color .` |
| `docker run -e VAR=VALUE` | Run a container with an environment var | `docker run -e APP_COLOR=blue simple-webapp-color` |
| `docker run --env-file ./env.list` | Load multiple vars from a file | `docker run --env-file ./env.list simple-webapp-color` |
***
## Further Reading
* [Flask Official Documentation](https://flask.palletsprojects.com/)
* [Docker ENV and ARG](https://docs.docker.com/engine/reference/builder/#env)
* [The Twelve-Factor App: Config](https://12factor.net/config)