Linux Port Resolver

Quick fix port conflicts on Linux without guesswork.

Quick Commands

# Find what's on a port
ss -tlnp | grep :PORT
# Alternative if ss not available
netstat -tlnp 2>/dev/null | grep :PORT
# Also check IPv6
ss -tlnp | grep :PORT

Resolution Steps

1. Identify the process

   ss -tlnp | grep ":<PORT> "
   # Output: LISTEN 0 128 0.0.0.0:PORT 0.0.0.0:* users:(("process",PID,))
   

2. Verify the PID - don't kill system processes

   ps -p PID -o pid,ppid,cmd,user
   

3. Kill gracefully, escalate if needed

   kill PID                  # SIGTERM - graceful
   sleep 2 && ss -tlnp | grep ":<PORT> "  # verify
   kill -9 PID               # SIGKILL - force, last resort
   

4. Or reconfigure - change the port in the service config instead

   • systemd: edit /etc/systemd/system/SERVICE.service, change port, systemctl daemon-reload && systemctl restart SERVICE
   • Docker: change ports: mapping in docker-compose.yml
   • Custom app: check config file or env var for port setting

5. Prevent recurrence with ExecStartPre in systemd

   [Service]
   ExecStartPre=/bin/sh -c 'kill -9 $(ss -tlnp | grep ":PORT " | grep -oP "pid=\\K\\d+") 2>/dev/null; true'
   

Safety Checklist

• [ ] Confirmed the port is actually in use
• [ ] Identified the PID correctly
• [ ] PID is not PID 1 or a critical system daemon
• [ ] Verified there's no other instance still running
• [ ] Checked both IPv4 and IPv6 listeners

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