Environment files (.env) in Docker - Commands & Configuration

Introduction

Environment files store settings like passwords or ports outside your main code. This keeps your app flexible and safe by changing settings without editing code.

When you want to keep database passwords separate from your app code to avoid exposing them.

When you need to run the same app on different servers with different settings like ports or API keys.

When you want to share your app code but keep your personal or secret settings private.

When you want to quickly change environment variables without rebuilding your Docker image.

When you want to organize many environment variables in one simple file for easier management.

Config File - .env

.env

APP_NAME=my-app
APP_ENV=production
APP_PORT=8080
DB_HOST=database
DB_USER=admin
DB_PASS=secret123

This file sets environment variables for your app.

APP_NAME names your app.

APP_ENV sets the environment type like production.

APP_PORT defines which port the app listens on.

DB_HOST, DB_USER, and DB_PASS configure database connection details.

Commands

This command runs an nginx container using the environment variables from the .env file and maps port 8080 on your computer to port 8080 in the container.

Terminal

docker run --env-file .env -p 8080:8080 nginx

Expected Output

Unable to find image 'nginx:latest' locally

latest: Pulling from library/nginx

Digest: sha256:...

Status: Downloaded newer image for nginx:latest

f1a2b3c4d5e6 nginx

→

--env-file - Loads environment variables from the specified file

→

-p - Maps a port from the host to the container

This command checks inside the running container to see if the APP_NAME environment variable is set correctly.

Terminal

docker exec f1a2b3c4d5e6 printenv APP_NAME

Expected Output

my-app

Key Concept

If you remember nothing else from this pattern, remember: environment files let you safely and easily change app settings without touching your code.

Common Mistakes

Not using the --env-file flag and trying to pass environment variables directly without quotes or proper syntax.

Docker won't load the variables correctly, causing your app to miss important settings.

Use --env-file with a properly formatted .env file or pass variables with -e VAR=value syntax.

Including spaces around the equal sign in the .env file like 'APP_NAME = my-app'.

Docker treats spaces as part of the variable name or value, causing errors or unexpected behavior.

Write variables without spaces: 'APP_NAME=my-app'.

Committing .env files with secrets to public repositories.

This exposes sensitive information like passwords to anyone who can see the code.

Add .env to .gitignore and share secrets securely.

Summary

Create a .env file to store environment variables like app name and database credentials.

Use docker run with --env-file to load these variables into your container.

Verify variables inside the container with docker exec and printenv.

Practice

1. What is the main purpose of a .env file in Docker?

easy

A. To list all running Docker containers

B. To write Docker commands for building images

C. To store environment variables outside the Dockerfile for easy configuration

D. To define network settings for Docker

Solution

1. Step 1: Understand the role of .env files

   .env files hold environment variables separately from code or Dockerfiles, making it easy to change settings without editing code.

2. Step 2: Identify the correct purpose in Docker context

   Docker uses .env files to load environment variables into containers, not for commands, container lists, or network settings.

3. Final Answer:

   To store environment variables outside the Dockerfile for easy configuration -> Option C

4. Quick Check:

   .env files = environment variables storage [OK]

Hint: Think: .env means environment variables outside code [OK]

Common Mistakes:

• Confusing .env with Dockerfile commands
• Thinking .env lists containers or networks
• Assuming .env files run commands

2. Which of the following is the correct syntax for a line in a .env file?

easy

A. MY_VAR = value

B. MY_VAR: value

C. MY_VAR=>value

D. MY_VAR=value

Solution

1. Step 1: Recall .env file syntax rules

   Each line must be a key-value pair with no spaces around the equal sign, like KEY=value.

2. Step 2: Check each option

   MY_VAR=value uses no spaces and an equal sign, which is correct. The other options use spaces, colon, or arrow separators, which are incorrect.

3. Final Answer:

   MY_VAR=value -> Option D

4. Quick Check:

   KEY=value with no spaces [OK]

Hint: No spaces around = in .env lines [OK]

Common Mistakes:

• Adding spaces around equal sign
• Using colon or arrow instead of equal sign
• Using quotes unnecessarily

3. Given a .env file with content:

APP_NAME=MyApp
APP_PORT=8080

and a Docker Compose service using env_file: .env, what will be the value of APP_PORT inside the container?

medium

A. 8080

B. Not set

C. APP_PORT

D. MyApp

Solution

1. Step 1: Understand env_file usage in Docker Compose

   Docker Compose loads variables from the .env file and sets them as environment variables inside the container.

2. Step 2: Identify the value of APP_PORT

   The .env file sets APP_PORT=8080, so inside the container, APP_PORT will be "8080" as a string.

3. Final Answer:

   8080 -> Option A

4. Quick Check:

   APP_PORT = 8080 from .env [OK]

Hint: env_file loads variables exactly as key=value [OK]

Common Mistakes:

• Confusing variable names with values
• Assuming variables are not set inside container
• Mixing up APP_NAME and APP_PORT values

4. You have a .env file with the line DB_PASS=secret pass. When you run your Docker container, the environment variable DB_PASS is not set correctly. What is the likely problem?

medium

A. The space in the value causes parsing issues; values should not contain spaces without quotes

B. The key DB_PASS is invalid because it contains uppercase letters

C. The .env file must be named env.txt to work

D. Docker does not support environment variables from .env files

Solution

1. Step 1: Check .env file value format rules

   Values with spaces must be quoted or use no spaces; otherwise, parsing breaks.

2. Step 2: Identify the issue with the given line

   "secret pass" has a space but no quotes, so Docker reads only "secret" or fails to parse correctly.

3. Final Answer:

   The space in the value causes parsing issues; values should not contain spaces without quotes -> Option A

4. Quick Check:

   Spaces in values need quotes or no spaces [OK]

Hint: No spaces in values unless quoted in .env [OK]

Common Mistakes:

• Ignoring spaces in values
• Thinking uppercase keys are invalid
• Renaming .env file incorrectly

5. You want to use a .env file to set environment variables for multiple Docker Compose services, but one service needs a different value for API_URL. How can you handle this situation?

hard

A. Put all variables in one .env file and override the variable in the Dockerfile

B. Use multiple .env files and specify the correct one per service with env_file in each service definition

C. Use only one .env file and change the variable manually before running Docker Compose

D. Docker Compose does not support different env files per service

Solution

1. Step 1: Understand env_file usage per service

   Docker Compose allows specifying env_file separately for each service to load different environment variables.

2. Step 2: Apply multiple .env files for different services

   Create separate .env files with different API_URL values and assign them to each service using env_file.

3. Final Answer:

   Use multiple .env files and specify the correct one per service with env_file in each service definition -> Option B

4. Quick Check:

   Multiple env_files per service = different variables [OK]

Hint: Use separate env_file per service for different variables [OK]

Common Mistakes:

• Trying to override variables only in Dockerfile
• Editing one .env file manually each time
• Believing Docker Compose can't use multiple env files