All-Things-Docker-and-Kubernetes

Lab 011: Shell Environment Variables

Pre-requisites

Introduction

In this lab, we’ll understand how to use shell environment variables. There are other ways to define variables in Docker such as defining through an environment file, a dockerfile, or a docker-compose file.

We’ll containerize an application which will display a simple website that will change colors based on the color we passed on the terminal.

Create the files

Create the templates directory and the simple hello.html inside it.

$ mkdir templates
$ vim templates/hello.html
hello.html ```html <!doctype html> Hello from Flask
```

Here’s the app.py written in Python.

app.py ```python from flask import Flask from flask import render_template import socket import random import os import argparse app = Flask(__name__) color_codes = { "red": "#e74c3c", "orange": "#ffa500", "yellow": "#ffff4d", "green": "#00e600", "blue": "#55a6d9", "violet": "#ee82ee", "indigo": "#4b0082", "pink": "#ffc0cb", "white": "#ffffff", "black": "#000000", "gray": "#cccccc" } SUPPORTED_COLORS = ",".join(color_codes.keys()) # Get color from Environment variable COLOR_FROM_ENV = os.environ.get('APP_COLOR') # Generate a random color COLOR = random.choice(["red", "green", "blue", "blue2", "darkblue", "pink"]) @app.route("/") def main(): # return 'Hello' return render_template('hello.html', name=socket.gethostname(), color=color_codes[COLOR]) if __name__ == "__main__": print(" This is a sample web application that displays a colored background. \n" " A color can be specified in two ways. \n" "\n" " 1. As a command line argument with --color as the argument. Accepts one of " + SUPPORTED_COLORS + " \n" " 2. As an Environment variable APP_COLOR. Accepts one of " + SUPPORTED_COLORS + " \n" " 3. If none of the above then a random color is picked from the above list. \n" " Note: Command line argument precedes over environment variable.\n" "\n" "") # Check for Command Line Parameters for color parser = argparse.ArgumentParser() parser.add_argument('--color', required=False) args = parser.parse_args() if args.color: print("Color from command line argument =" + args.color) COLOR = args.color if COLOR_FROM_ENV: print("A color was set through environment variable -" + COLOR_FROM_ENV + ". However, color from command line argument takes precendence.") elif COLOR_FROM_ENV: print("No Command line argument. Color from environment variable =" + COLOR_FROM_ENV) COLOR = COLOR_FROM_ENV else: print("No command line argument or environment variable. Picking a Random Color =" + COLOR) # Check if input color is a supported one if COLOR not in color_codes: print("Color not supported. Received '" + COLOR + "' expected one of " + SUPPORTED_COLORS) exit(1) # Run Flask Application app.run(host="0.0.0.0", port=8080) ```

Next, create the dockerfile.

FROM python:3.6

RUN pip install flask

WORKDIR /opt

COPY ./ /opt

EXPOSE 8080

ENTRYPOINT ["python", "app.py"]

Our project directory should now look like this:

$ tree

├── app.py
├── dockerfile
└── templates
    └── hello.html

1 directory, 3 files

Build the Image

Build the image from the dockerfile and give it the name “my-flask-app”. It will take a few minutes to build the image since it will pull down the Python container image from Dockerhub. It will then use the Python image as the base image. You should see the “Successful built” returned once it’s done.

$ docker build . -t my-flask-app 

Successfully built d40b9ef9ada7
Successfully tagged my-flask-app:latest

Verify that the image is created. As mentioned, the Python image is pulled down to our local machine.

$ docker images
REPOSITORY     TAG       IMAGE ID       CREATED              SIZE
my-flask-app   latest    d40b9ef9ada7   About a minute ago   913MB
python         3.6       54260638d07c   6 months ago         902MB

Run the Container

Before we run the container, get the IP of your machine.

$ curl ipecho.net/plain; echo

We’ll use the command below to run the container. Use the “-e” flag to define the APP_COLOR variable and specify a color. Make sure to append the image name/repository at the end.

$ docker run -p 82:8080 -e APP_COLOR=blue my-flask-app

It should return the following output:

docker run - output ```bash This is a sample web application that displays a colored background. A color can be specified in two ways. 1. As a command line argument with --color as the argument. Accepts one of red,orange,yellow,green,blue,violet,indigo,pink,white,black,gray 2. As an Environment variable APP_COLOR. Accepts one of red,orange,yellow,green,blue,violet,indigo,pink,white,black,gray 3. If none of the above then a random color is picked from the above list. Note: Command line argument precedes over environment variable. No Command line argument. Color from environment variable =blue * Serving Flask app 'app' (lazy loading) * Environment: production WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead. * Debug mode: off * Running on all addresses. WARNING: This is a development server. Do not use it in a production deployment. * Running on http://172.17.0.2:8080/ (Press CTRL+C to quit) ```

</br>

Open a web browser and navigate to the IP address followed by the port number, like this:

54.151.154.238:82/

You should see the website displayed.

Back in your terminal, hit Ctrl-C to quit and run the container again. Specify “red” this time.

$ docker run -p 82:8080 -e APP_COLOR=red my-flask-app

The website color should now change to red.

Try running containers and use different colors. Note that the code only accepts eleven colors. If you specified a color that is not declared in the code, you’ll get the error message below:

$ docker run -p 82:8080 -e APP_COLOR=darkblue my-flask-app 

No Command line argument. Color from environment variable =darkblue
Color not supported. Received 'darkblue' expected one of red,orange,yellow,green,blue,violet,indigo,pink,white,black,gray

What if we didn’t specify the environment variable?

$ docker run -p 82:8080 my-flask-app

As seen, the website will randomly choose a color from declared list of colors. Note that this catch-all is defined in the application code so that it won’t return an error if it doesn’t find any “color”.

In general, Docker will complain and return an error message if the environment variable doesn’t have a value assigned to it.

Cleanup

When you’re done with the lab, you can stop all running containers by running the command below.

$ docker stop $(docker ps)

Once all containers have “Exited” status, remove them.

$ docker ps  -a 

$ docker container prune -f

Finally, remove all images.

$ docker image prune -af
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