How to send signal to program run in a docker container?

• docker kill used to send signal to main container process i.e process with PID 1.
• Any application with PID 1 can handle signals directly. Below command kill the main docker process: $ docker kill --signal="SIGTERM" container-id/name
• But application which does not have PID 1 i.e application is background process:
  • We can not send single directly to any background process running within docker container.
  • In this case we need to trap and handle the user defined signal in the shell script running as entry point.
• Let's we have the following Dockerfile. (Update it as per the application)

---

FROM centos:6.7
# Install/Deploye the service below.

# Copy the shell script.
COPY entrypoint.sh /home
EXPOSE 8080
ENTRYPOINT ["/home/entrypoint.sh"]

---

• Below is the entrypoint.sh. (Update it it as per the application). Suppose we wants to restart a init.d service.

  #start the service
  /etc/init.d/<servicename> start
  pid="$!"
  
  # SIGUSR1- Single handler
  my_handler() {
      /etc/init.d/<servicename> restart
  }
  
  # Trap and handle the user defind singnal.
  trap 'my_handler' SIGUSR1
  
  # wait forever(Alive container.)
  while true
  do
      tail -f /dev/null & wait ${!}
  done
  

• Build the docker image and run the container.
• Now you can restart the service from Host machine: $docker kill --signal="SIGUSR1" container-id/name

You can use docker kill --signal="<signal>" <container name or id> to send any signal to the root process of a given container.

See https://docs.docker.com/engine/reference/commandline/kill/#send-a-custom-signal--to-a-container

You can use nsenter to get into your container space and send your signal.

PID=$(docker inspect --format {{.State.Pid}} <container_name_or_ID>)
nsenter --target $PID --mount --uts --ipc --net --pid kill -SIGINT <PID of your program inside your container>

More info : http://jpetazzo.github.io/2014/06/23/docker-ssh-considered-evil/