One-line invocation to kill certain running processes in bash

Context

I was making some live changes in a review environment and to test I needed to restart the web server. Since the web server was running multiple processes in parallel, killing each one manually was too cumbersome.

Implementation

Let’s use Passenger web server as an example. Assume that you have the following Passenger processes running on the server:

ps aux | grep Passenger
root        17  0.0  0.0 303940 14660 ?        Ssl  18:24   0:00 Passenger watchdog
root        20  2.4  0.1 1693060 21144 ?       Sl   18:24   1:45 Passenger core
root       168  0.4  4.9 1517332 800028 ?      Sl   18:25   0:18 Passenger RubyApp: /hello (review)
root       190  0.0  3.5 1308056 569668 ?      Sl   18:25   0:00 Passenger RubyApp: /hello (review)
root       211  0.0  3.4 1308156 556684 ?      Sl   18:25   0:00 Passenger RubyApp: /hello (review)
root       234  0.0  3.4 1308256 556692 ?      Sl   18:25   0:00 Passenger RubyApp: /hello (review)
root       255  0.0  3.4 1308356 556700 ?      Sl   18:25   0:00 Passenger RubyApp: /hello (review)
root       276  0.0  3.4 1308456 556708 ?      Sl   18:25   0:00 Passenger RubyApp: /hello (review)

Then, a potential one-liner to kill all the above processes is:

ps aux | grep -i "Passenger RubyApp" | grep -v grep | awk '{ print $2 }' | xargs kill -9

Here’s what the above commands do:

ps aux lists all current processes.
grep -i "Passenger RubyApp" filters this list to include only lines that contain “Passenger RubyApp” (the -i flag makes the search case-insensitive).
grep -v grep is used to exclude the grep process itself from the list.
awk '{ print $2 }' prints only the second field (by default, fields in awk are separated by whitespace), which for the ps aux command is the PID (Process ID).
xargs kill -9 takes the PIDs piped from the previous command and uses kill -9 to terminate these processes.

Notes

Using kill -9 immediately kills the process and may lead to data loss (if the process was in the middle of I/O operations). It’s generally better to use kill -15 (or kill without any signal which defaults to -15) first, which sends the TERM (terminate) signal allowing the process to finish its current task and clean up before exiting