Environment files (.env) in Docker - Step-by-Step Execution

Process Flow - Environment files (.env)

Create .env file

↓

Add KEY=VALUE pairs

↓

Reference .env in Docker Compose or Dockerfile

↓

Docker reads .env file

↓

Inject environment variables into containers

↓

Containers use variables at runtime

This flow shows how a .env file is created, filled with variables, referenced by Docker, and used inside containers.

Execution Sample

Docker

MY_VAR=hello
VERSION=1.0

# docker-compose.yml snippet
services:
  app:
    environment:
      - MY_VAR
      - VERSION

Defines environment variables in a .env file and uses them in a Docker Compose service.

Process Table

Step	Action	File/Command	Result/Output
1	Create .env file	.env	File created with MY_VAR=hello and VERSION=1.0
2	Write variables	.env	MY_VAR and VERSION set in file
3	Reference .env in docker-compose.yml	docker-compose.yml	Environment keys MY_VAR and VERSION listed
4	Run docker-compose up	docker-compose up	Docker reads .env and injects variables
5	Container starts	docker container	Container environment has MY_VAR=hello, VERSION=1.0
6	Use variables inside container	docker exec app env	Output shows MY_VAR=hello and VERSION=1.0
7	Exit	-	Process ends after container runs with env variables

💡 Container started with environment variables injected from .env file

Status Tracker

Variable	Start	After Step 2	After Step 5	Final
MY_VAR	undefined	hello	hello	hello
VERSION	undefined	1.0	1.0	1.0

Key Moments - 2 Insights

Why doesn't Docker automatically know about variables in the .env file without referencing them?

Docker only injects variables listed in the docker-compose.yml environment section or Dockerfile. The .env file defines variables but they must be explicitly referenced to be used, as shown in step 3 and 4.

What happens if a variable is in the .env file but not listed in docker-compose.yml?

That variable is ignored and not passed to the container environment. Only variables listed under environment in docker-compose.yml are injected, as seen in step 4 and 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of MY_VAR inside the container at step 6?

A undefined

B hello

C VERSION

D empty string

💡 Hint

Check the 'Use variables inside container' row in execution_table showing MY_VAR=hello

At which step does Docker read the .env file and inject variables into the container?

A Step 4

B Step 2

C Step 1

D Step 6

💡 Hint

Look at the 'Run docker-compose up' step where Docker reads .env and injects variables

If you remove VERSION from the environment list in docker-compose.yml, what happens to VERSION inside the container?

A It remains 1.0

B It becomes empty string

C It becomes undefined

D Container fails to start

💡 Hint

Refer to key_moments explaining variables not listed in docker-compose.yml are ignored

Concept Snapshot

Environment files (.env) store key=value pairs.
Docker reads .env when running docker-compose or Dockerfile.
Variables must be referenced in docker-compose.yml or Dockerfile to be used.
Containers get these variables injected at runtime.
Useful for separating config from code.

Full Transcript

This visual execution shows how environment files (.env) work with Docker. First, you create a .env file and add variables like MY_VAR=hello and VERSION=1.0. Then, you reference these variables in your docker-compose.yml under the environment section. When you run docker-compose up, Docker reads the .env file and injects the variables into the container environment. Inside the container, you can see these variables with commands like 'env'. Variables not listed in docker-compose.yml are ignored. This method helps keep configuration separate and easy to change without modifying code.