Scenario:

You have a container image myapp:grace built from /home/interview/Dockerfile that runs an application requiring cleanup on shutdown. When you run docker stop, the container is killed after the 10-second timeout instead of shutting down gracefully because the application doesn't properly handle the SIGTERM signal.

Task:

Fix the application script and Dockerfile to properly handle SIGTERM signals, implement cleanup logic, and ensure the container exits gracefully within 20 seconds when docker stop is executed.

Step 1: Test current container shutdown behavior

docker run -d --name test_grace myapp:grace
time docker stop test_grace

Should take ~10 seconds (default timeout) before force kill. Docker sends SIGTERM, waits, then sends SIGKILL.

Step 2: Check exit code and logs

docker inspect test_grace --format '{{.State.ExitCode}}'
docker logs test_grace
docker rm test_grace

Exit code 137 indicates SIGKILL (128 + 9). Logs should show no cleanup messages.

Step 3: Examine current configuration

cat /home/interview/Dockerfile
cat /home/interview/server.sh

Check if Dockerfile uses shell form vs exec form CMD, and if the script has signal handling (trap).

Step 4: Add signal handling to the application script

nano /home/interview/server.sh

Add trap to catch SIGTERM and perform cleanup:

#!/bin/sh

cleanup() {
    echo "Received SIGTERM, cleaning up..."
    sleep 2  # Simulate cleanup work
    echo "Cleanup complete, exiting gracefully"
    exit 0
}

trap cleanup TERM

echo "Application started"

while true; do
    sleep 1
done

Step 5: Fix Dockerfile to use exec form

nano /home/interview/Dockerfile

Change from shell form to exec form. Shell form runs as /bin/sh -c "command" which doesn't forward signals. Exec form runs the command directly as PID 1:

# Before (shell form):
# CMD /app/server.sh

# After (exec form):
CMD ["/app/server.sh"]

Step 6: Rebuild the image

docker build -t myapp:grace .

Step 7: Test the fixed container

docker run -d --name test_fixed myapp:grace
sleep 3
time docker stop --timeout 20 test_fixed

Should exit quickly (within a few seconds) instead of waiting for timeout.