Running Applications

Table of Contents

1. Create a small project
2. Install dependencies
3. Write the server
4. Run it, bound to all interfaces
5. Publish the port
6. Access it from your host
7. Stop and restart
8. Beginner Pitfalls

1. Create a small project

Inside the sandbox (sbx run --name sbx-crash-course if you're not already attached), create a project in your workspace so the files are visible from your host too:

mkdir -p ~/sbx-crash-course/app && cd ~/sbx-crash-course/app
npm init -y

2. Install dependencies

npm install express

This downloads and installs into node_modules inside the sandbox, exactly like it would on your host — except it's isolated here, and gone the moment you remove the sandbox.

3. Write the server

cat > index.js << 'EOF'
const express = require("express");
const app = express();
const port = 3000;

app.get("/", (req, res) => {
  res.json({ message: "Hello from inside the sandbox!", pid: process.pid });
});

app.listen(port, "0.0.0.0", () => {
  console.log(`Listening on http://0.0.0.0:${port}`);
});
EOF

4. Run it, bound to all interfaces

node index.js

Note: the "0.0.0.0" in app.listen matters. A published port only reaches a service that's listening on all interfaces inside the sandbox — a server bound to 127.0.0.1 (the sandbox's own loopback) is unreachable from the outside no matter what you publish.

Leave this running and open a second terminal on your host for the next steps.

5. Publish the port

From your host, publish sandbox port 3000 to a host port:

sbx ports sbx-crash-course --publish 3000:3000

Check it took effect:

sbx ports sbx-crash-course

SANDBOX PORT   HOST ADDRESS
3000           127.0.0.1:3000

sbx ls shows the same mapping alongside the sandbox's status.

6. Access it from your host

curl http://localhost:3000

{"message":"Hello from inside the sandbox!","pid":1234}

Or just open http://localhost:3000 in your browser. The request left your host, crossed into the sandbox through the published port, and hit the Express server running there — the same round trip you'd get from a container, just with a whole isolated Linux VM behind it instead.

7. Stop and restart

Back in the sandbox's terminal, Ctrl-C stops the server. Restart it the same way:

node index.js

To run it in the background instead of holding a terminal open, use sbx exec from your host with -d (detached):

sbx exec -d sbx-crash-course bash -c "cd ~/sbx-crash-course/app && node index.js"

Leave it running in the background — the mini project in Part 5 builds alongside it, and Part 6 replaces it with a fuller backend.

8. Beginner Pitfalls

• Forgetting "0.0.0.0" and leaving the default (which for most frameworks is 127.0.0.1 or localhost) is the single most common reason "it works inside the sandbox but I can't reach it from my browser."
• sbx run only applies --publish when it creates a sandbox. Since sbx-crash-course already existed, we used sbx ports to publish afterward — remember that distinction.
• An explicit host port (like 3000:3000 above) has to be free on your host. If it's taken, publish without a host port (--publish 3000) and let the OS assign one, then check it with sbx ports.