TIL - lsof & kill to Clear a Port

I’ve probably looked this up and done it a million times over the years, but it never somehow stuck in my head. But a good way to find what is running on a port (say, port 3000) is lsof -i :3000 and that will give you the pid or process number running on that port. Then to stop that running process, you can do kill -9 <pid> where you replace <pid> with the actual process number.

Example:

❯ lsof -i :3000
COMMAND   PID       USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
ruby    10192 mmcdermott   12u  IPv4 0x17ec2e84526c5d0b      0t0  TCP localhost:hbci (LISTEN)
ruby    10192 mmcdermott   13u  IPv6 0x17ec2e7f89b9dfbb      0t0  TCP localhost:hbci (LISTEN)
❯ kill -9 10192

A nice one liner for this is:

kill -9 $(lsof -t -i:3000)

Some background:

• lsof is “list of open files”.

  • The -i is for “internet files or sockets”
  • if you do -i :<port> it gives you “sockets bound to a specific port”.

• kill is a command to manually terminate a process.

  • It sends a “signal” specified by what follows the dash.
  • It terminates the pid (process id) specified after the signal.
  • -9 is SIGKILL, which kills the process immediately and all child processes.
  • You can also use -15, which is SIGTERM, which attempts to kill the process more gracefully and does not kill child processes.

• $() runs what ever is inside in a subshell and returns the value.

Sources (accessed 9/20/24)

• Stack Exchange: “Finding the PID of the process using a specific port?”
• Stack Overflow: “How to kill a process running on particular port in Linux?”
• Stack Exchange: “What does the “i” flag mean in lsof?”
• Stack Overflow: “What does it mean in shell when we put a command inside dollar sign and parentheses: $(command)”
• Geeks For Geeks “How to Kill a Process in Linux”
• Linux Handbook: “SIGTERM vs SIGKILL: What’s the Difference?”