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:

environment:
  - VAR_NAME=value
  - 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.

Practice

1. What is the main purpose of using environment in a Docker Compose file?

easy

A. To set environment variables inside containers

B. To define network settings for containers

C. To specify the container image version

D. To mount volumes from the host

Solution

1. Step 1: Understand the role of environment variables

   Environment variables allow customization of container behavior without changing the container image or code.

2. Step 2: Identify what environment does in Compose

   The environment key in Docker Compose sets these variables inside the container at runtime.

3. Final Answer:

   To set environment variables inside containers -> Option A

4. Quick Check:

   environment sets variables = C [OK]

Hint: Remember: environment = variables inside container [OK]

Common Mistakes:

• Confusing environment with network or volume settings
• Thinking environment sets image versions
• Mixing environment with Dockerfile commands

2. Which of the following is the correct syntax to pass an environment variable named API_KEY with value 12345 in a Docker Compose service?

easy

A. set_env:
- API_KEY: 12345

B. environment:
- API_KEY=12345

C. variables:
API_KEY=12345

D. env:
API_KEY: 12345

Solution

1. Step 1: Recall Docker Compose environment syntax

   Docker Compose uses the environment key with a list or dictionary to set variables.

2. Step 2: Match correct syntax for list format

   The correct list format is environment:\n - API_KEY=12345. Other keys like env, variables, or set_env are invalid.

3. Final Answer:

   environment: - API_KEY=12345 -> Option B

4. Quick Check:

   Correct key and list syntax = D [OK]

Hint: Use 'environment:' with dash and equals for variables [OK]

Common Mistakes:

• Using wrong keys like env or variables
• Mixing colon and equals signs incorrectly
• Indentation errors in YAML

3. Given this Compose snippet:

version: '3'
services:
  app:
    image: myapp
    environment:
      - MODE=${APP_MODE}

If the host environment variable APP_MODE is set to production, what will be the value of MODE inside the container?

medium

A. MODE

B. ${APP_MODE}

C. production

D. undefined

Solution

1. Step 1: Understand variable substitution in Compose

   Compose replaces ${APP_MODE} with the value of the host environment variable APP_MODE when starting containers.

2. Step 2: Apply substitution with given host variable

   Since APP_MODE is set to production on the host, MODE inside the container will be production.

3. Final Answer:

   production -> Option C

4. Quick Check:

   Host var substitution = production [OK]

Hint: Host vars replace ${VAR} in Compose environment [OK]

Common Mistakes:

• Thinking ${APP_MODE} stays as literal string
• Assuming variable is undefined inside container
• Confusing container env with host env

4. You wrote this Compose snippet:

services:
  web:
    image: nginx
    environment:
      - PORT: 80

But the container does not get the PORT variable set. What is the error?

medium

A. Missing quotes around 80

B. environment key should be under volumes

C. Image name is incorrect

D. Using colon instead of equals sign in environment list

Solution

1. Step 1: Check environment variable syntax in list form

   When using a list under environment, variables must be in KEY=VALUE format, not KEY: VALUE.

2. Step 2: Identify the error in the snippet

   The snippet uses - PORT: 80 which is invalid YAML for a list item; it should be - PORT=80.

3. Final Answer:

   Using colon instead of equals sign in environment list -> Option D

4. Quick Check:

   List env vars use = not : [OK]

Hint: Use '=' for list env vars, ':' only in dict format [OK]

Common Mistakes:

• Mixing list and dictionary syntax for environment
• Thinking quotes are mandatory for numbers
• Placing environment under wrong keys

5. You want to pass a default value for an environment variable LOG_LEVEL in Compose if the host variable is unset or empty. Which syntax correctly sets LOG_LEVEL to info by default?

hard

A. environment:
- LOG_LEVEL=${LOG_LEVEL:-info}

B. environment:
LOG_LEVEL: info

C. environment:
- LOG_LEVEL=info

D. environment:
- LOG_LEVEL=${LOG_LEVEL-info}

Solution

1. Step 1: Understand Compose variable default syntax

   Compose supports shell-style default values using ${VAR:-default} to use default if VAR is unset or empty.

2. Step 2: Identify correct syntax for default

   ${LOG_LEVEL:-info} sets LOG_LEVEL to host value if set, else info. Other options either ignore host variable or use incorrect syntax.

3. Final Answer:

   environment: - LOG_LEVEL=${LOG_LEVEL:-info} -> Option A

4. Quick Check:

   Use ${VAR:-default} for defaults [OK]

Hint: Use ${VAR:-default} to set default env var in Compose [OK]

Common Mistakes:

• Using ${VAR-default} which does not handle empty values
• Setting static value ignoring host variable
• Wrong YAML indentation or syntax