TIL: pkill to stop a process by name

Sometimes when I learn about a tool I think “wow, that would have been useful [when I had some problem in the past]”. That happened with pkill, which is a command you can use to stop a process accurately.

Many years ago I was slogging through debugging a Selenium test failure in a Rails application with my coworker, Ron. Every time we made a change we had to stop the server, restart it, and re-run the test suite. This had a bunch of steps so naturally we wrote a script to automate them. At the end of the script, we killed the Rails server by looking for the process using port 3000 with kill -9 $(lsof -t -i:3000).

  • lsof lists open files. In unix/linux, everything is a file including things like network connections (via sockets).
  • -t terse output–just the pid
  • -i:3000 process using port 3000

It worked! Until one run when, all of a sudden, my entire browser disappeared. It was unexpected, abrupt, and rude. We were using that browser! Ron and I were really baffled … until we realized that there were two processes using the port: the server and the browser (maybe with a keep alive connection?).

This is what the lsof output looks like when you look for processes using a port:

$ lsof -i:3000 -P

COMMAND  PID    TYPE  NODE  NAME
ruby3.2  42727  IPv6  TCP   *:3000                           (LISTEN)
ruby3.2  42727  IPv6  TCP   localhost:3000->localhost:57537  (ESTABLISHED)
Google   6439   IPv6  TCP   localhost:57537->localhost:3000  (ESTABLISHED)

Notice that process ID 42727 is using :3000 in the LISTEN state, while that same process and Chrome are both using it for a connection in the ESTABLISHED state. So we got unlucky on one run, grabbed the browser’s process ID, and unceremonously killed it. At the time, we worked around this by adding another flag, -sTCP:LISTEN. This filters to TCP sockets in the LISTEN state, so we’d always get the server process.

pkill would have been perfect for this! It stops processes by matching the process name. For example, pkill ruby would stop all running ruby processes. That’s a little aggressive if you only want to stop one process. With pkill -f you match the program arguments as well, like ruby bin/rails so pkill -9 -f rails would do it. (You could also argue that I should have the server process itself write its pid to a file to later stop it. You’re probably right!)

One thing I love is programs that have two modes to either (1) take action or (2) preview that action. Many systems expose this as a “dry run”. In the case of pkill, the dry-run equivalent is pgrep, which works the same way, but just outputs the pid instead of sending a kill signal.

pkill has a ton of other options so be sure to look at the tldr or man page.

Shell