Set Environment Variables in Dockerfile
To set environment variables in a Dockerfile, use the ENV directive. This allows developers to define key-value pairs accessible across containers spawned from the image.
Environment variables persist for each subsequent instruction during the build. This is useful for projects that need certain configurations in the image, yet might need them visible later for debugging or scaling.
Here's how to set environment variables in a Dockerfile:
FROM ubuntu:20.04
ENV APP_MODE=production
ENV DB_USER=root
DB_PASS=example
Build your docker image:
docker build -t my_app_image .
To verify the environment variables:
docker run --rm my_app_image env
For variable values that may change across environments, update during runtime:
docker run --rm --env APP_MODE=debug my_app_image env
This approach provides control and flexibility, allowing standardization while adapting to change when necessary.
Utilizing CLI for Environment Variables
The command line interface (CLI) offers flexibility for setting environment variables on the go. Use the --env or -e flag to pass variables when initiating a container:
docker run --rm --env APP_MODE=debug my_app_image
For multiple variables:
docker run --rm -e APP_MODE=staging -e DB_USER=admin -e DB_PASS=secure123 my_app_image
To use existing system environment variables:
export CONFIG_TYPE=test
docker run --rm -e CONFIG_TYPE my_app_image
This method provides agility for adjusting container configurations without altering the Dockerfile.
.env Files for Environment Variables
.env files offer an organized approach to managing environment variables. Create a .env file with your variables:
APP_MODE=production
DB_USER=root
DB_PASS=example
Use the --env-file flag to pass these variables to your container:
docker run --rm --env-file ./.env my_app_image
This method keeps sensitive information separate from your code and allows for easy configuration changes across different environments.
Remember to secure your .env file with appropriate file permissions.
Docker Compose for Variable Management
Docker Compose excels at managing environment variables across multiple containers. Define variables globally or per service in your docker-compose.yml file:
version: '3.8'
services:
web:
image: my_app_image
environment:
- APP_MODE=production
- DB_USER=${DB_USER}
db:
image: postgres
environment:
- POSTGRES_USER=${DB_USER}
- POSTGRES_PASSWORD=${DB_PASS}
Docker Compose reads from your .env file or shell environment, syncing settings to each service. You can set default values using ${VARIABLE:-default_value}.
This approach centralizes configuration management, streamlining deployment and enhancing maintainability.
Mastering environment variables in Docker can improve your development process, offering both flexibility and control. By understanding how to manage these settings effectively, you create a seamless workflow that adapts easily to different scenarios.
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- Docker Inc. Docker Compose environment variables. Docker Documentation. 2023.
- Kubernetes. Environment variables. Kubernetes Documentation. 2023.
- Mouat N. Using Docker: Developing and Deploying Software with Containers. O'Reilly Media; 2015.
