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So before starting the question here are my understanding for the docker.

  1. Docker has 3 components
  2. Images
  3. Containers
  4. Dockerfile

Now Images are the ones on which the Containers are created and Dockerfile is like a flow what to do. In simple words Images are Classes and Containers are Objects of Images.

Now I don't want to take the approach of the Dockerfile where you specify the steps to perform while creating the container.

I want to install some of the basic entities over Linux like MongoDb,Redis etc and run my server over them.

So I started like this:

  1. I downloaded the Ubuntu image from Docker Hub via docker pull ubuntu which returned me 18261df960118..7a16(big hex key)

  2. Now I have to create a container for this image, to achieve that I did:

    docker create -h abc.com --name abc.com 18261df960118..7a16

which returned me the id of the container.

  1. In order to go into the container I have to first start it and then get attached to it, so for that here are the commands docker start containerId followed by docker attach containerId.

But every time it is saying:

You cannot attach to a stopped container, start it first.

Nimantha
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Ankur Verma
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4 Answers4

9

Edit: In my original post I mention: "try to think like with VMs". I recently fell into this, which says not to do so:

Stop thinking about a container as a mini-VM and instead start thinking about it as just a process.

also, worth-reading article: Containers are not VMs

Original post:

The logic with Docker containers is that they are supposed to have a service up and running. If this service stops, they exit and go to "stopped" state. (As you learn more about Docker, you will understand how this works and you will be able to use ENTRYPOINT and CMD). But let's skip this for a while and try to think like with VMs, run a new container and get inside to type some commands...

this succeeds:

docker container create -it --name test ubuntu
445cad0a3afea97494635361316e5869ad3b9ededdd6db46d2c86b4c1461fb75
$ docker container start test
test
$ docker container exec -it test bash
root@445cad0a3afe:/# your are inside, you can type your commands here!

why yours failed...

when you created the container, you didn't use the -i flag which helps Keep STDIN open even if not attached. This practically means that when the container starts, it uses the CMD set in the official ubuntu Dockerfile, which is bash and then exits immediately.

docker attach VS docker exec --it bash

You can test this with an image like nginx. If you run a new nginx container and try to attach to it, you will see that logs from nginx are being printed out and you are not able to type any command in the shell. This happens because the CMD of the image is the following:

# Define default command.
CMD ["nginx"]

To be able to "attach" to a container like that but also be able to use the shell (some others may also mention this like doing something equivalent to ssh to the container), you will have to run:

docker container exec -it your_container_name bash

I suggest you also read:

tgogos
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  • Thanks! These are concepts that were also difficult for me to understand at the beginning... and still learning... Hope I made it a bit more clear to you. – tgogos Jul 20 '17 at 15:13
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    Yeah I started learning this ghost just now (I know I am late ;) ) so getting lost in its tentacles. Anyways thanks again. – Ankur Verma Jul 20 '17 at 15:16
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Check if your Docker container is running, executing below command:

docker ps

If yes, you can attach using container-id, as follows:

docker attach <docker-container-id>

Container not created?

docker run -d -it <docker-image-name>

In your case it is: docker run -d -it ubuntu:latest

Attach Docker Container

docker attach <docker-container-id>

Convert Docker Container to Docker Image

Get your docker container id, as:

docker ps -a

Commit it then:

docker commit <docker-container-id> abc-image

Keep it locally or push to remote using docker push

Arpit Aggarwal
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  • What if I want to do all my installations in my container and can attach to it and exit it when I want to. The command you gave creates new container every time. – Ankur Verma Jul 20 '17 at 14:05
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Dockefile builds the image, so the instructions in the Dockerfile only runs at build time. The only exception is CMD and/or ENTRYPOINT. Those instruct the container what command to run when you create a container.

Not using a Dockerfile to build your own image kind of defeats the purpose of using docker in the first place. You can of course treat containers as a VM and install things manually, but why not just use virtualbox for that?

I also would strongly recommend starting by using official images from docker hub. Modify them by extending those images. It's a lot of work to create great images.

Grimmy
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  • I have some commitments that needs to be solved so creating a container and installing all the stuff and do the work is now required but yeah coming to your point I will definitely do the stuff correct way once my basic work is done. – Ankur Verma Jul 20 '17 at 14:08
  • That can make sense, but i still think you will be done a lot quicker by just using the official mongodb and redis images. You can be up and running on a mater of minutes with basic setups. – Grimmy Jul 20 '17 at 14:13
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You're doing things the wrong way. To build your own image you need to use Dockerfile. Then you can build your image with a Dockerfile that look something like this:

FROM <your based image>
RUN <some commands to adapt the based image>
ENTRYPOINT (or CMD) <your command>

To understand difference between ENTRYPOINT and CMD directives go here

It's recommended to run only one process per container. If you need to run several process you should have a look to Docker compose and then make different containers working together.

D. Gonçalves
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