Ubuntu Question posted in
The official Ubuntu documentation can be found here.

Ubuntu – Make Docker a proper UNIX citizen

I’m trying to write a wrapper over Docker to propagate signals and standard output in a sensible way. First, Docker doesn’t handle SIGPIPE nicely:

$ docker run --rm ubuntu yes | head
y
y
y
y
y
y
y
y
y
y
write /dev/stdout: broken pipe
$

The extra output "write /dev/stdout: broken pipe" would be fine even if a little annoying, but the Dockerized process isn’t stopped. I wrote a wrapper to mount a FIFO as a volume and redirect the command’s standard output:

fifo=`basename $0`.$$
mkfifo $fifo
cat $fifo &
docker run --rm -v$PWD/$fifo:$PWD/$fifo ubuntu sh -c "$* >$PWD/$fifo"
rm $fifo

This does what I’m hoping for standard output; however, now, ctrl-C won’t stop the container. Apparently the shell won’t send SIGINT to the foreground child. I expected the following to work around this latest issue:

trap 'kill $!' INT TERM
fifo=`basename $0`.$$
mkfifo $fifo
cat $fifo &
docker run --rm -v$PWD/$fifo:$PWD/$fifo ubuntu sh -c "$* >$PWD/$fifo" &
wait
rm $fifo

However, on ctrl-C, the script exits, with the Dockerized process (and cat) continuing to spray output on my terminal.

Is it clear what I’m trying to do? I’m a big fan of UNIX workflow, but a lot of modern projects are depending on Docker (to do what the OS should already be doing). More specifically, I’m trying to benchmark a few Docker applications that are using a shared resource. I’d like to terminate one instance once it has output a certain number of lines and terminate all the other instances by sending SIGTERM.

Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    One solution is to kill the cat background child rather than the Docker background child in the trap handler. Eventually, the Docker process gets a SIGPIPE, which works because of the FIFO volume.

    I'm guessing somehow the Docker daemon doesn't propagate SIGTERM to the underlying process in this script.

    There's gotta be something cleaner...


  2. 0

    Another solution is to use docker run‘s –detach option and then docker stop the container in the trap handler.

    fifo=`basename $0`.$$
trap 'rm -f $fifo && docker stop $docker' INT TERM
mkfifo $fifo
docker=`docker run --detach --rm -v$PWD/$fifo:$PWD/$fifo ubuntu sh -c "$* >$PWD/$fifo"`
cat $fifo
rm $fifo

    Could the FIFO volume be replaced with a docker kill --signal=PIPE solution?

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search