docker run vs create+start: why are created containers different?

Question

Related to

docker container started in Detached mode stopped after process execution

https://serverfault.com/questions/661909/the-right-way-to-keep-docker-container-started-when-it-used-for-periodic-tasks

I do understand the difference between docker run and create + start, but don't understand how the actual containers created in these two ways differ.

Say I create and run a container with

docker run -dit debian:testing-slim

and then stop it. The created container can later be started with

docker start silly_docker_name

and it'll run in the background, because the entry command for the image is bash.

But when a container is first created

docker create --name silly_name debian:testing-slim

and then started with

docker start silly_name

then it'll exit immediately. Why isn't bash started, or how come it exits in this case?

Answer 1

The difference for a container process that is a shell (like bash in your debian example) is that a shell started without a terminal+interactive "mode" exits without doing anything.

You can test this by changing the command of a create'd container to something that doesn't require a terminal:

$ docker create --name thedate debian date

Now if I run thedate container, each time I run it it outputs the date (in the logs) and exits. docker logs thedate will show this; one entry for each run.

To be explicit, your docker run command has flags -dit: detached, interactive (connect STDIN), and tty are all enabled.

If you want a similar approach with create & start, then you need to allocate a tty for the created container:

$ docker create -it --name ashell debian

Now if I start it, I ask to attach/interactively to it and I get the same behavior as run:

$ docker start -ai ashell root@6e44e2ae8817:/#

NOTE: [25 Jan 2018] Edited to add the -i flag on create as a commenter noted that as originally written this did not work, as the container metadata did not have stdin connected at the create stage