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The -e flag

Learn to pass environment variables with -e.

The -e flag

Environment variables are the way to run the same image with different settings in different environments (development, test, production). -e NAME=value gives the container a variable; several -e flags can be written:

docker run --rm -e AD=Ayşe -e DIL=tr alpine sh -c 'echo "$AD $DIL"'
# Ayşe tr

If no value is written (-e TOKEN), the variable is taken from the host variable of the same name.

Seeing the variables

All environment variables inside a container are seen with env. docker inspect shows them too; that is why passwords given with -e can be read by anyone with access to the container:

$ docker exec db env | grep POSTGRES
POSTGRES_PASSWORD=gizli
POSTGRES_DB=app
$ docker inspect -f '{{.Config.Env}}' db

In production, Docker secrets or Kubernetes Secrets are preferred for secret values.

Practice A configured database

Start a container named db from postgres:16 in the background with three environment variables: POSTGRES_USER=app, POSTGRES_PASSWORD=gizli, POSTGRES_DB=magaza_db.

Starter code

terminal
# buraya yaz

Expected output

92a3b4c5d6e7

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 In docker run -e TOKEN img, where does TOKEN's value come from?

  • From the TOKEN environment variable on the host Correct answer
  • It is always empty
  • From the image
  • It errors

Explanation: This is one way to keep a password out of the command history.

2 Give the container an environment variable.

Question 
docker run --rm ____ DIL=tr alpine env
Answer 
docker run --rm -e DIL=tr alpine env

Explanation: -e (env) takes NAME=value.

3 Match.

Term Meaning
-e NAME=value Assign a variable
-e AD Take from the host
env List the variables
docker inspect Shows the settings

All lessons in Beginner

1 What is Docker?

  1. What is Docker?
  2. Containers and virtual machines
  3. Images and containers
  4. Docker architecture
  5. Use cases
  6. Summary - What is Docker

2 Installation and first commands

  1. Installing Docker
  2. docker version
  3. docker info
  4. The hello-world container
  5. Docker Desktop
  6. Summary - Installation

3 First container

  1. docker run basics
  2. Running in the background
  3. docker ps
  4. docker stop and rm
  5. docker logs
  6. Summary - First container

4 Dockerfile basics

  1. What is a Dockerfile?
  2. The FROM instruction
  3. The RUN instruction
  4. COPY and ADD
  5. CMD and ENTRYPOINT
  6. Summary - Dockerfile

5 Build and images

  1. docker build
  2. Tagging images
  3. Build context
  4. Layers and cache
  5. .dockerignore
  6. Summary - Build

6 docker run options

  1. Interactive mode
  2. Naming containers
  3. Restart policy
  4. Working directory
  5. Switching users
  6. Summary - docker run

7 Image management

  1. docker images
  2. docker rmi
  3. docker pull
  4. docker push
  5. docker tag
  6. Summary - Image management

8 Port mapping

  1. The port concept
  2. The -p flag
  3. Multiple ports
  4. Random ports
  5. IP binding
  6. Summary - Port mapping

9 Volumes and data

  1. Data persistence
  2. Named volumes
  3. The -v flag
  4. Bind mounts
  5. Volume management
  6. Summary - Volumes

10 Environment variables

  1. The -e flag
  2. The ENV instruction
  3. The ARG instruction
  4. env-file
  5. The WORKDIR instruction
  6. Summary - Environment

11 Project: docker-compose

  1. Compose design
  2. Service definition
  3. Database service
  4. Volume and network
  5. Putting it together - compose

12 Docker Hub and registry

  1. What is Docker Hub?
  2. docker login
  3. Searching images
  4. Official images
  5. Private registry
  6. Summary - Registry

13 Network basics

  1. Network drivers
  2. docker network ls
  3. Bridge network
  4. Container connectivity
  5. Publishing ports
  6. Summary - Network

14 Beginner final

  1. Challenge - Dockerfile
  2. Challenge - Build and run
  3. Challenge - Volume
  4. Challenge - Compose
  5. Challenge - Beginner project