Unable to trap TERM signal from 'docker stop'

Question

This is my bash script used in CMD

#!/bin/bash
set -eo pipefail

echo "Setting trap"
echo $$
echo $BASHPID
trap 'cleanup' TERM
trap 'cleanup' KILL

cleanup() {
    echo "Cleaning up..."
    kill -TERM `jobs -p`
}


# To start the essential services
service ntp start
service awslogs start

cd /app
python -m job_manager  &

wait

The Docker file is not very interesting

FROM ubuntu:16.04

RUN apt-get update --fix-missing && apt-get install -y \
  git \
  python \
  python-pip \
  ntp \
  curl

ENV APP_HOME /app
RUN mkdir -p ${APP_HOME}
COPY src/ ${APP_HOME}/

# job-cmd.sh is kept here    
COPY docker/helper-files/* /   

CMD /job-cmd.sh

The idea is trap the TERM signal inside job-cmd.sh and then pass on to the python task.

I have tried a number of time and it did not work. After I add these call

echo $$
echo $BASHPID

I realised the pid of the CMD process is actually 7 instead of 1 as I would expect.

My questions:

1) Why the bash process is assigned PID 7?

2) How can I fix the my job script/dockerfile?

Answer 1

I think this is happening because you are using the shell form of the CMD instruction. From https://docs.docker.com/engine/reference/builder/#cmd:

If you want to run your command without a shell then you must express the command as a JSON array and give the full path to the executable. This array form is the preferred format of CMD.

So, replace your CMD instruction in Dockerfile with:

CMD ["/job-cmd.sh"]

Then your Bash process will be assigned PID 1. Your TERM handler will work, but you can't trap the KILL signal. From man trap:

Trapping SIGKILL or SIGSTOP is syntactically accepted by some historical implementations, but it has no effect. Portable POSIX applications cannot attempt to trap these signals.

FYI, I explained more about the PID 1 problem here: https://serverfault.com/questions/869543/bash-script-entrypoint-pid-1-kills-tail-sub-process-only-if-a-fake-trap-whi/870872#870872