Signals Handling Inside Docker Container

Reading Time: 2 minutes

Linux supports both POSIX reliable signals and POSIX real-time signals.

Signal Dispositions: Each signal has a current disposition, which determines how the process behaves when it is delivered the signal.

We have different types of signals in Linux

The specified default disposition of few signals are below:

Term : default action is to terminate the process.
Ign : default action is to ignore the signal.
Stop : default action is to stop the process.
Cont : default action is to continue the process if it is currently stopped.

List of few standard signals :

Name          Default Action      Description
SIGHUP         Terminate Process   Terminal line hangup
SIGINT        Terminate Process       Interrupt program
SIGQUIT      Create Core Image       Quit Program
SIGABRT     Create Core Image        Abort Program
SIGKILL        Terminate Process        Kill Program
SIGTERM      Terminate Process Software Termination Signal
SIGUSR1     Terminate Process         User Defined Signal 1
SIGUSR2     Terminate Process         User Defined Signal 2

Note: The signal SIGKILL and SIGSTOP cannot be caught, blocked or ignored.

Case Study: Handle Sigterm Signal in docker container and terminate the process gracefully.

Let suppose we need to run scala application inside our container and based on the signal received we need to handle it and terminate the application gracefully.

Suppose our Dockerfile has an entrypoint as:
ENTRYPOINT [“/bin/bash”, “blog.sh”]

inside our blog.sh script we need to trap the Signal and handle it based on the use case of the case study.

	#!/usr/bin/env bash
	set -x

	pid=0

	sigterm_handler() {
	echo "sigterm handler called…"
	if [ $pid -ne 0 ]; then
	kill -TERM "$pid"
	echo "scala application terminated…"
	wait "$pid"
	fi
	exit 143;
	}

	trap 'kill ${!}; sigterm_handler' TERM

	#running scala application
	sbt run &
	pid="$!"
	echo "PID=$pid"

	# wait forever
	while true
	do
	tail -f /dev/null & wait ${!}
	done

	set +x