Dynamic App Link Printing for Dockerised Environments

Docker Career Node.js

Improve developer experience by printing the correct local app URL dynamically at startup.

🔗 Purpose

In containerised development environments, it's easy to lose track of what port an app is running on — especially when host-to-container port mappings differ.

To solve this, we introduce a simple pattern that dynamically prints the correct app link (e.g. http://localhost:4001) when the server starts, based on the current environment variable (e.g. HOST_PORT).

This reduces guesswork and ensures developers always know exactly where to access the application — whether it's running locally or inside Docker.

⚙️ High-Level Approach

  1. Keep the app startup script clean and reusable

    Define your actual server start command in a dedicated script (e.g. start-server.sh):

    next start -p 3001

    This script contains no logging or environment logic and can be reused in CI, production, or other automation.

  2. Add a dynamic wrapper

    Create a wrapper script (e.g. start.ts or start.js) that:

    • Spawns the actual start script
    • Waits for the server "ready" signal
    • Detects the port from the environment (e.g. HOST_PORT)
    • Prints the resolved URL like:
    App ready — visit: http://localhost:4001

✅ Why This Matters

  • 🧭 Instant feedback: Clear startup logs help devs find the running app without searching Docker configs or port mappings.
  • 💻 Portable setup: Works the same in Docker, local dev, or cloud shell environments.
  • 🔄 DRY principle: Keeps logic clean and reusable across environments.
  • 🧪 CI/CD friendly: Start logic remains testable and scriptable.

🧪 Example Output

In Docker:

> App starting...
> App ready — visit: http://localhost:4001

Locally:

> App ready — visit: http://localhost:3001

🗂️ Suggested File Structure

scripts/
├── start-server.sh      # Minimal entry point (no logs, no logic)
└── start.js or start.ts # Dynamic wrapper (prints resolved URL)
.env                     # Defines HOST_PORT or fallback
package.json

✨ Tip

Inject HOST_PORT into your environment via Docker or shell:

HOST_PORT=4001 node scripts/start.js

This makes it easy to reuse across local and containerised setups.

🧾 Summary

This simple pattern helps teams:

  • Remove confusion around Docker port mapping
  • Improve developer onboarding and DX
  • Maintain clean separation of concerns between start logic and messaging

Feel free to adapt it to your own stack — whether you're using Node.js, Python, or any web server.