Fix "Address Already in Use" Error (All Languages)

The error by language

Every language throws this error differently, but the cause is always the same: another process is bound to the port.

Universal fix: portrm

Regardless of which language threw the error, the fix is the same:

$ ptrm fix 3000
  Port 3000 in use
  node (PID 48291) - Next.js dev server
  SAFE - development server
  Killed PID 48291. Port 3000 is free.

Fix by language

Node.js (EADDRINUSE)

// Handle gracefully
server.on('error', (e) => {
  if (e.code === 'EADDRINUSE') {
    console.error(`Port ${port} in use. Run: ptrm fix ${port}`);
    process.exit(1);
  }
});

Python (OSError)

# Enable SO_REUSEADDR
import socket
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sock.bind(('0.0.0.0', 8000))

# Flask: use a different port
app.run(port=8001)

# Django: specify port
$ python manage.py runserver 8001

Go

// Check error type
ln, err := net.Listen("tcp", ":8080")
if err != nil {
    if opErr, ok := err.(*net.OpError); ok {
        if sysErr, ok := opErr.Err.(*os.SyscallError); ok {
            if sysErr.Err == syscall.EADDRINUSE {
                log.Fatal("Port 8080 in use. Run: ptrm fix 8080")
            }
        }
    }
    log.Fatal(err)
}

Java (BindException)

// Spring Boot: change port
// application.properties
server.port=9090

// Or catch the exception
try {
    ServerSocket ss = new ServerSocket(8080);
} catch (BindException e) {
    System.err.println("Port 8080 in use. Run: ptrm fix 8080");
}

Ruby

# Rails: use a different port
$ rails server -p 3001

# Sinatra
set :port, 3001

Manual fix (all OS)

# macOS / Linux
$ lsof -i :3000
$ kill -9 <PID>

# Windows
> netstat -ano | findstr :3000
> taskkill /PID <PID> /F

Prevention

Use ptrm preflight in your start scripts to check port availability before launching:

# package.json
{
  "scripts": {
    "dev": "ptrm preflight 3000 && next dev",
    "start:api": "ptrm preflight 8080 && node server.js"
  }
}