Environment files (.env) in Docker - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of Docker Compose with .env variable substitution

Given a .env file with APP_PORT=8080 and the following docker-compose.yml snippet, what will be the exposed port on the host after running docker-compose up?

Docker

version: '3.8'
services:
  web:
    image: nginx
    ports:
      - "${APP_PORT}:80"

A The container port 8080 is mapped to host port 80

B The container port 80 is mapped to host port 80

C Docker Compose will fail with an error about undefined variable

D The container port 80 is mapped to host port 8080

💡 Hint

Check how Docker Compose uses variables from the .env file for substitution.

Docker Compose automatically loads variables from the .env file in the same directory. The ${APP_PORT} in docker-compose.yml is replaced by 8080, so port 8080 on the host maps to port 80 in the container.

🧠 Conceptual

intermediate

Purpose of .env files in Docker projects

What is the main purpose of using a .env file in Docker and Docker Compose projects?

A To list all running Docker containers on the host

B To store environment variables that can be substituted in Docker Compose files and containers

C To define the Dockerfile instructions for building images

D To configure Docker daemon settings like storage driver

💡 Hint

Think about how you can avoid hardcoding values in your compose files.

The .env file holds environment variables that Docker Compose reads to substitute variables in the compose file or pass into containers. It helps keep configuration flexible and separate from code.

❓ Configuration

advanced

Correct syntax for multiline values in .env files

Which option correctly represents a multiline environment variable value in a .env file for Docker Compose?

A MULTILINE_VAR="line1\nline2\nline3"

B

MULTILINE_VAR=line1
line2
line3

C

MULTILINE_VAR='line1
line2
line3'

D MULTILINE_VAR=line1\nline2\nline3

💡 Hint

Remember that .env files treat each line as a separate variable unless escaped properly.

In .env files, to represent newlines inside a variable, you must use escaped newline characters \n inside quotes. Option A correctly uses double quotes and escaped newlines.

❓ Troubleshoot

advanced

Why is my .env variable not substituted in docker-compose.yml?

You have a .env file with DB_USER=admin but when running docker-compose up, the variable ${DB_USER} in docker-compose.yml is empty. What is the most likely cause?

A Docker Compose requires variables to be exported with 'export' keyword in .env

B The variable name in .env must be lowercase to match substitution

C The .env file is not in the same directory as the docker-compose.yml file

D The .env file must be named 'env' without the dot

💡 Hint

Check the location of your .env file relative to your compose file.

Docker Compose only automatically loads the .env file if it is in the same directory as the docker-compose.yml file. If it is elsewhere, variables won't be substituted.

✅ Best Practice

expert

Best practice for sensitive data in .env files

Which is the best practice for handling sensitive data like passwords in .env files used with Docker Compose?

A Add the .env file to .gitignore to avoid committing secrets to version control

B Commit the .env file to version control but encrypt it with a password

C Hardcode sensitive data directly in docker-compose.yml for clarity

D Use environment variables only on the host machine and never use .env files

💡 Hint

Think about how to keep secrets safe when sharing code.

It is best to keep sensitive data out of version control by adding .env files to .gitignore. This prevents accidental exposure of secrets. Other methods like encryption or host-only variables are more complex.