Main process (PID 1)

A container lives exactly as long as its main process, the one started by CMD or ENTRYPOINT. When that process exits, the container stops, whatever else runs inside.

How it works

The main process runs as PID 1 in the container. It receives the stop signal of docker stop and must handle it. A server has to stay in the foreground: a service that puts itself in the background returns at once, and the container exits.

What breaks

With the shell form CMD node server.js, Docker starts /bin/sh -c, and the shell does not pass SIGTERM on to node: docker stop waits 10 seconds then kills everything, cutting the requests in flight. The exec form CMD ["node", "server.js"] makes node itself PID 1.

In a terminal

docker run -d --name web nginx:1.27
docker top web
docker inspect -f '{{.Config.Cmd}}' web

See it break, then fix it

Write the Dockerfile or the compose file, type the docker commands, and watch a simulated server run them through reboots, upgrades and traffic. Free to start.

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