Beginner

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: ...

Stop scrolling through endless output. Master docker compose logs options to find issues fast and monitor services effectively. Basic commands # All service logs docker compose logs # Single service docker compose logs web # Multiple services docker compose logs web worker Follow logs in real-time Watch logs as they happen: # Follow all services docker compose logs -f # Follow specific service docker compose logs -f api # Start fresh and follow docker compose logs -f --since 1m Tail recent logs Get last N lines: ...

Keep production and development configs separate. Docker Compose automatically merges compose.override.yml for local development tweaks. The magic Compose automatically loads two files: compose.yml (base configuration) compose.override.yml (local overrides) # These are equivalent: docker compose up docker compose -f compose.yml -f compose.override.yml up Basic setup compose.yml (production-ready): services: web: image: myapp:latest ports: - "80:80" environment: NODE_ENV: production LOG_LEVEL: warn compose.override.yml (developer-friendly): services: web: build: . # Build locally instead of using image ports: - "3000:80" # Different port for development volumes: - .:/app # Mount source code environment: NODE_ENV: development LOG_LEVEL: debug DEBUG: "true" Real development example compose.yml: ...