Environment variables in Compose in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of environment variables in Docker Compose?

Environment variables in Docker Compose allow you to set dynamic values for configuration, such as ports, passwords, or service names, making your setup flexible and reusable.

beginner

How do you define environment variables directly inside a docker-compose.yml file?

You use the environment key under a service, listing variables as key-value pairs, for example:<br>

environment:<br>  - VAR_NAME=value<br>  - ANOTHER_VAR=another_value

intermediate

What is the role of an .env file in Docker Compose?

An .env file stores environment variables outside the Compose file. Docker Compose automatically loads it to substitute variables in the docker-compose.yml file, helping keep secrets or settings separate.

intermediate

How can you reference an environment variable inside the docker-compose.yml file?

You use the syntax ${VARIABLE_NAME} to insert the value of an environment variable defined in the shell or .env file into the Compose file.

intermediate

What happens if an environment variable used in docker-compose.yml is not set?

If a variable is not set and no default is provided, Docker Compose will replace it with an empty string, which may cause errors or unexpected behavior in your services.

Which key in docker-compose.yml is used to set environment variables for a service?

A variables

B environment

C env_vars

D settings

The correct key is environment to define environment variables inside a service.

Where does Docker Compose look for environment variables to substitute in the Compose file by default?

A In the .env file in the same directory

B In the Dockerfile

C In the system PATH variable

D In the docker-compose.override.yml file

Docker Compose automatically loads environment variables from a file named .env in the same directory as the Compose file.

How do you reference an environment variable named PORT in docker-compose.yml?

A ${PORT}

B $PORT

C %PORT%

D #PORT#

The correct syntax for variable substitution in docker-compose.yml is ${PORT}.

What is the effect of setting environment variables inside the environment section of a service?

A They update the docker-compose.yml file automatically

B They change the host machine's environment

C They become available inside the running container

D They modify the Docker daemon settings

Environment variables set under environment are passed into the container and available to processes inside it.

If an environment variable is missing and no default is set, what does Docker Compose do?

A Uses a random value

B Throws an error and stops

C Prompts the user to enter a value

D Replaces it with an empty string

Docker Compose replaces missing variables with empty strings, which can cause issues if the variable is required.

Explain how environment variables are used in Docker Compose and why they are helpful.

Think about how you can change settings without editing the Compose file directly.

Describe the difference between setting environment variables in the environment section and using an .env file in Docker Compose.

One affects the container runtime, the other affects Compose file processing.