Docker Compose env variable defined but empty inside container

Set DATABASE_URL in my shell, ran docker compose up, but inside the container:

bash

echo $DATABASE_URL
# (empty)

The variable was definitely set in my terminal:

bash

echo $DATABASE_URL
# postgresql://user:pass@localhost:5432/mydb

Root cause:

My docker-compose.yml had:

yaml

services:
  api:
    environment:
      - DATABASE_URL

This syntax tells Docker Compose to pass the DATABASE_URL from the host shell. But I was running Compose with sudo in one terminal and had set the variable in a different user's session.

Actually — the real issue:

I was running with sudo docker compose up. When you sudo, the environment variables of the regular user don't carry over.

Fix 1: Use .env file (recommended)

bash

# .env file (in same directory as docker-compose.yml)
DATABASE_URL=postgresql://user:pass@localhost:5432/mydb

Docker Compose automatically loads .env from the project directory.

Fix 2: Inline the value in docker-compose.yml

yaml

services:
  api:
    environment:
      - DATABASE_URL=postgresql://user:pass@localhost:5432/mydb

Not ideal for secrets but works for debugging.

Fix 3: Pass sudo with env

bash

sudo -E docker compose up  # -E preserves environment variables

Lesson: Docker Compose's env_file and .env handling is more reliable than relying on shell environment, especially when sudo is involved. Always use a .env file for local development.