Beginner

docker compose up -d starts services in the background, but it returns immediately — before services are actually ready. --wait solves this by blocking until all services are healthy. The problem Without --wait, you often end up with fragile sleep-based scripts: # Fragile: how long is enough? docker compose up -d sleep 10 npm test The solution docker compose up --wait npm test --wait starts services in detached mode and blocks until every service with a healthcheck reports healthy. If a service fails to become healthy, the command exits with a non-zero status. ...

When using dynamic port mapping or multiple services, it’s not always obvious which host port maps to which container port. docker compose port tells you exactly. Basic usage # Which host port maps to container port 80 on the web service? docker compose port web 80 # Output: 0.0.0.0:8080 Why dynamic ports matter When you let Docker assign ports automatically, the host port changes on every docker compose up: services: web: image: nginx ports: - "80" # Dynamic host port, container port 80 docker compose port web 80 # Output: 0.0.0.0:55432 (assigned dynamically) This is common in CI or when running multiple instances of the same project. ...

Docker Compose supports shell-style variable substitution in your Compose files. Combined with defaults and error messages, it makes your configurations flexible and safe. Basic substitution Reference environment variables or .env file values: services: app: image: myapp:${TAG} environment: DATABASE_URL: postgres://${DB_USER}:${DB_PASS}@db/${DB_NAME} # .env TAG=2.1.0 DB_USER=admin DB_PASS=secret DB_NAME=myapp Default values Provide fallback values when a variable is unset or empty: services: app: image: myapp:${TAG:-latest} environment: LOG_LEVEL: ${LOG_LEVEL:-info} PORT: ${PORT:-3000} deploy: replicas: ${REPLICAS:-1} Two syntaxes with a subtle difference: ...

Know the difference between exec and run! Each has its place in your debugging toolkit. The key difference exec: Runs commands in an existing container run: Creates a new container # Exec: enters running container docker compose exec web bash # Run: starts new container docker compose run web bash When to use exec Use exec for debugging running services: # Debug a running web server docker compose exec web bash # Check logs inside container docker compose exec web tail -f /var/log/app.log # Run database queries docker compose exec db psql -U postgres # Check process list docker compose exec web ps aux # Test connectivity from inside docker compose exec web curl http://api:3000/health Important: Container must be running! ...

Stop managing long docker run commands! Convert them to maintainable Compose files. Basic conversions Common flag mappings: # Docker run command docker run -d \ --name myapp \ -p 3000:3000 \ -e NODE_ENV=production \ -e API_KEY=secret123 \ -v $(pwd)/data:/app/data \ -v /var/run/docker.sock:/var/run/docker.sock \ --restart unless-stopped \ myapp:latest Becomes: services: myapp: image: myapp:latest container_name: myapp ports: - "3000:3000" environment: NODE_ENV: production API_KEY: secret123 volumes: - ./data:/app/data - /var/run/docker.sock:/var/run/docker.sock restart: unless-stopped Network configurations # Host network docker run --network host nginx # Custom network docker run --network mynet --ip 172.20.0.5 app # Network alias docker run --network mynet --network-alias db postgres Compose equivalent: ...