Running and experimenting with Node.js apps, I often play around and forgot I started the app already or it’s already running on the port I wanted to start it up on now.
1. What’s the PID so I can kill it 🥊?
lsof is a powerful command utility for this, it stands for “list open files”. Despite the name, it can list much more than just files - it shows all open file descriptors, including network connections, which is exactly what we need for port troubleshooting.
Basic usage:
lsof -i :portUsing lsof with -i to list network connections and :port to look at a specific port (like 5000, 8000, 3000, etc.), I can find the PID and use kill -9 [pid] to force terminate the application I no longer want running on that port.
Example workflow:
# Check what's running on port 3000
lsof -i :3000
# Output might look like:
# COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
# node 1234 user 20u IPv4 0x123 0t0 TCP *:3000 (LISTEN)
# Kill the process
kill -9 1234Understanding the kill command:
kill -9sends a SIGKILL signal to the process- Unlike
kill(SIGTERM), SIGKILL cannot be ignored by the process - With
kill -9you’re not asking the application to terminate gracefully - you’re telling the OS to immediately stop running the program regardless of what the application is doing - Use with caution - the process won’t have a chance to clean up resources, close files, or save data
Better approaches:
# Try graceful termination first (SIGTERM)
kill [pid]
# If that doesn't work after a few seconds, then force kill
kill -9 [pid]
# Alternative: Find and kill by process name
pkill -f "node.*3000" # Kill node processes with 3000 in command
# On some systems, you might prefer:
killall node # Kills all node processes (be careful!)Other useful lsof examples:
# List all network connections
lsof -i
# List connections on specific interface
lsof -i tcp:80
# Show what files a specific process has open
lsof -p [pid]
# Show what processes are using a specific file
lsof /path/to/file