Docker Tutorial · Docker Compose · Lesson 21 of 26

Environment Files, Profiles and Overrides

Configure Compose per environment with .env files, variable interpolation, profiles and override files.

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Variables

Compose reads a .env file next to compose.yaml and substitutes ${VAR} and ${VAR:-default} in the file. env_file: passes a file's variables into a container.

Profiles

Services with profiles: [debug] start only when that profile is enabled (docker compose --profile debug up), handy for admin tools you don't always want running.

Overrides

compose.override.yaml is merged automatically, so development settings (bind mounts, debug ports) can live there while compose.yaml stays production-like. docker compose config shows the final merged result.

Interpolation, defaults and profiles

cat > compose.yaml <<'EOF'
services:
  api:
    image: alpine:3.24
    environment:
      LOG_LEVEL: ${LOG_LEVEL:-info}
      DB_HOST: ${DB_HOST}
  adminer:
    image: alpine:3.24
    profiles: [debug]
EOF
echo "DB_HOST=db.internal" > .env
echo "--- services by default: $(docker compose config --services | tr '\n' ' ')"
echo "--- with --profile debug: $(docker compose --profile debug config --services | sort | tr '\n' ' ')"
echo "--- resolved environment:"
docker compose config --format json | python3 -c 'import json,sys; e=json.load(sys.stdin)["services"]["api"]["environment"]; [print(f"  {k}={v}") for k,v in sorted(e.items())]'
LOG_LEVEL=debug docker compose config --format json | python3 -c 'import json,sys; print("  with LOG_LEVEL=debug in the shell:", json.load(sys.stdin)["services"]["api"]["environment"]["LOG_LEVEL"])'

Output

--- services by default: api 
--- with --profile debug: adminer api 
--- resolved environment:
  DB_HOST=db.internal
  LOG_LEVEL=info
  with LOG_LEVEL=debug in the shell: debug

Run this on your computer — see “Get Started” for setup.

See Environment variables in Compose and Profiles.

Test yourself

How do you see the final configuration after .env interpolation and override files?

docker compose ps
docker compose config
cat compose.yaml
docker inspect

Show answer

Answer: docker compose config

config prints the fully resolved, merged model.

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