How to fix ERROR: Missing required environment variable in Docker

# Run docker-compose to see the error
docker-compose config

# Example output:
# ERROR: Missing required environment variable "DATABASE_URL"
# Or: required variable DATABASE_URL is missing a value: Database connection required

# Find required variables in your compose file
grep -E '\$\{[^}]+\?' docker-compose.yml

# Export the variable
export DATABASE_URL="postgres://user:pass@localhost:5432/mydb"

# Then run docker-compose
docker-compose up

DATABASE_URL="postgres://user:pass@localhost:5432/mydb" docker-compose up

echo 'export DATABASE_URL="postgres://user:pass@localhost:5432/mydb"' >> ~/.bashrc
source ~/.bashrc

echo $DATABASE_URL

# Create .env file in your project directory
cat > .env << 'EOF'
# Required environment variables
DATABASE_URL=postgres://user:pass@localhost:5432/mydb
API_KEY=your-api-key-here
REDIS_URL=redis://localhost:6379
EOF

cp .env.example .env
# Then edit .env to fill in actual values

# This command shows the resolved configuration
docker-compose config

docker-compose --env-file ./config/.env up

services:
  app:
    environment:
      - DATABASE_URL=${DATABASE_URL?Database URL is required}

services:
  app:
    environment:
      # Use :- for default when unset or empty
      - DATABASE_URL=${DATABASE_URL:-postgres://localhost:5432/default}
      # Use - for default only when unset
      - LOG_LEVEL=${LOG_LEVEL-info}

# .github/workflows/deploy.yml
jobs:
  deploy:
    runs-on: ubuntu-latest
    env:
      DATABASE_URL: ${{ secrets.DATABASE_URL }}
      API_KEY: ${{ secrets.API_KEY }}
    steps:
      - uses: actions/checkout@v4
      - name: Deploy with Docker Compose
        run: docker-compose up -d

# .gitlab-ci.yml
deploy:
  variables:
    DATABASE_URL: $DATABASE_URL  # From CI/CD settings
  script:
    - docker-compose up -d

# Create .env file from environment
cat > .env << EOF
DATABASE_URL=${DATABASE_URL}
API_KEY=${API_KEY}
EOF
docker-compose up -d

# Check if variable is set and show its value
echo "DATABASE_URL='$DATABASE_URL'"

# In .env file, ensure no spaces around =
# Correct:
DATABASE_URL=postgres://localhost/db

# Wrong (empty value):
DATABASE_URL=
DATABASE_URL=
DATABASE_URL=""

# Fails if empty or unset
- DEBUG_MODE=${DEBUG_MODE:?Debug mode required}

# Only fails if unset (allows empty)
- DEBUG_MODE=${DEBUG_MODE?Debug mode required}

# Check which variables are needed in each file
grep -E '\$\{[^}]+\?' docker-compose.yml docker-compose.override.yml docker-compose.prod.yml

# Ensure all required variables are set for the files you're using
docker-compose -f docker-compose.yml -f docker-compose.prod.yml config

# Development
docker-compose --env-file .env.dev up

# Production
docker-compose --env-file .env.prod -f docker-compose.yml -f docker-compose.prod.yml up

services:
  app:
    # Always runs
    environment:
      - DATABASE_URL=${DATABASE_URL:?Required}

  debug-tools:
    profiles: ["debug"]
    # Only validates when profile is active
    environment:
      - DEBUG_TOKEN=${DEBUG_TOKEN:?Required for debug}