Upstart job starts the script twice

Question

I am using Ubuntu 14.04.4 LTS (GNU/Linux 3.13.0-85-generic x86_64) where I created a simple Upstart job:

# content of /etc/init/listener.conf

start on runlevel [2345]
stop on runlevel [!2345]
respawn

exec /home/mk/log_listener

The script itself looks like this:

#!/bin/sh

tail -F /var/log/vsftpd.log | while read line; do
    if echo "$line" | grep -q 'OK UPLOAD:'; then
         curl http://some.url/
    fi
done

But for some reason the job starts two processes:

$ ps jax | grep lis
   1  1234  1234  1234 ?           -1 Ss       0   0:00 /bin/sh /home/mk/log_listener
1234  1236  1234  1234 ?           -1 S        0   0:00 /bin/sh /home/mk/log_listener

I suppose this is not a "normal" behavior? How can I find out who and why is starting the second process and how to properly call the script in my Upstart job?

add a 'j' flag to ps (ps jax | grep listener) to confirm or deny the idea that the processes are related (parent/child). — Jeff Schaller, Apr 14 '16 at 16:24

score 1 · Accepted Answer · edited Apr 13 '17 at 12:36

1

We see that those two processes are related (the parent of PID 1236 is 1234) and that PID 1234 is the session leader (lower-case 's' in the STAT field).

See https://unix.stackexchange.com/a/18178/117549 where we see that pipelines in shell scripts create a new process group.

Thus, I believe that Upstart is executing your script correctly, and you are seeing two processes because of the tail ... | while read pipeline.

If I can suggest a small improvement to your script, you don't need to echo every line against grep inside your loop; instead:

tail -F /var/log/vsftpd.log | grep --line-buffered 'OK UPLOAD:' | while read unused
do
  curl http://some.url/
done

edited Apr 13 '17 at 12:36

Community

1

answered Apr 14 '16 at 16:58

Jeff Schaller

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Thanks for your answer! I didn't quite understand the conclusion, so, this IS normal behavior (because both processes shown are part of the same PID)? Thanks for the script suggestion, I might update it also :) – errata Apr 15 '16 at 09:47
Normal behavior; you can see it happen separately from upstart as well. – Jeff Schaller Apr 15 '16 at 09:53

Upstart job starts the script twice

1 Answers1