Environment variables in Compose in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of environment variable substitution in Compose

Given the following docker-compose.yml snippet, what will be the value of the environment variable APP_MODE inside the container if the host environment variable APP_MODE is not set?

services:
  app:
    image: myapp
    environment:
      - APP_MODE=${APP_MODE:-production}

A APP_MODE will be empty inside the container

B APP_MODE will cause an error and container won't start

C APP_MODE will be set to 'production' inside the container

D APP_MODE will be set to the literal string '${APP_MODE:-production}'

💡 Hint

Look at the syntax ${VAR:-default} for environment variables in Compose.

The syntax ${APP_MODE:-production} means use the value of APP_MODE from the host environment if set; otherwise, use 'production'. Since APP_MODE is not set on the host, 'production' is used.

❓ Configuration

intermediate

Correct syntax for passing environment variables from a file

Which option correctly loads environment variables from a file named .env into a docker-compose service?

A

services:
  web:
    image: nginx
    environment:
      - .env

B

services:
  web:
    image: nginx
    env_file:
      - .env

C

services:
  web:
    image: nginx
    environment_file: .env

D

services:
  web:
    image: nginx
    envfiles:
      - .env

💡 Hint

Check the exact key name for loading env files in Compose.

The correct key is 'env_file' (singular, underscore), which accepts a list of files. Other options are invalid keys or syntax.

❓ Troubleshoot

advanced

Why does the container not see updated environment variables?

You updated the host environment variable API_KEY and restarted your container using docker-compose up -d. However, inside the container, API_KEY still has the old value. What is the most likely reason?

A Docker Compose caches environment variables at startup; you must recreate the container with <code>docker-compose up -d --force-recreate</code>

B Environment variables cannot be updated once the container is created; you must rebuild the image

C The <code>API_KEY</code> variable must be set inside the Dockerfile, not in Compose

D You need to restart the Docker daemon to apply environment variable changes

💡 Hint

Think about how Compose handles container recreation and environment variables.

Docker Compose reads environment variables when creating containers. Restarting without recreating keeps old containers with old variables. Using --force-recreate ensures new containers with updated variables.

🔀 Workflow

advanced

Order of precedence for environment variables in Compose

Given these sources of environment variables:
1. Variables defined in the environment section of docker-compose.yml
2. Variables loaded from an env_file
3. Variables from the host shell environment

Which order correctly describes their precedence inside the container (highest to lowest)?

A 2, 3, 1

B 3, 2, 1

C 2, 1, 3

D 1, 2, 3

💡 Hint

Think about which source overrides the others in Compose.

Variables defined directly in the environment section override those from env_file, which override host environment variables.

🧠 Conceptual

expert

Effect of variable expansion in Compose with missing variables

Consider this docker-compose.yml snippet:

services:
  db:
    image: postgres
    environment:
      POSTGRES_PASSWORD: "${DB_PASS}"

If DB_PASS is not set in the host environment, what will happen when you run docker-compose up?

A Compose will fail with an error about missing variable DB_PASS

B POSTGRES_PASSWORD will be set to an empty string inside the container

C POSTGRES_PASSWORD will be set to the literal string '${DB_PASS}'

D Compose will prompt you to enter a value for DB_PASS

💡 Hint

Check Compose behavior when a variable is referenced but not defined.

By default, Compose requires variables used in ${} to be set; otherwise, it errors out. To provide defaults, you must use the syntax ${VAR:-default}.