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Working inside a container

Container images provide you with minimal stuff, but you can always install new tools to work with.

Let's see if we can spin up a container and curl https://aliyousuf.com.

Getting inside a container

$ docker run --interactive --tty ubuntu:latest

Here's some explanation:

  • --interactive (or -i): attaches container's stdin, stdout
  • --tty (or -t): attaches a psuedo-terminal with the current terminal

In short, it allows you interact with the container easily.

You might see something like this:

root@<container id>:/#

This represents that you're inside a container now.

Let's do something

root@<container id>:/# ls

No problem, right? Let's curl now, it shouldn't be a problem.

root@<container id>:/# curl https://aliyousuf.com
bash: curl: command not found
root@<container id>:/#

What? But why did it fail?

Remember, I told you that Docker Images provides you with minimal tools. But you can install curl using apt-get.

root@<container id>:/# apt-get update && apt-get install curl -y
root@<container id>:/# curl https://aliyousuf.com

OK, since its working now, let's curl and save the data in a file and exit from the container because it's almost time to go home.

root@<container id>:/# mkdir mydata && cd mydata
root@<container id>:/mydata# curl https://aliyousuf.com > index.html
root@<container id>:/mydata# cat index.html
root@<container id>:/mydata# exit

Great, we downloaded a file, made sure it's there and exited from the container.

Let's make sure if the container is still there.

$ docker ps -a

Yup, its there but EXITED.

Also note the COMMAND column in docker ps -a output, "/bin/bash". More about it later.

Reuse EXITED container

If you run docker run -it ubuntu:latest, it will spin up another container with fresh data. This container will not have /mydata/index.html in it.

Kick start the older container so that you get your data back.

$ docker start <container id>

OK, but it didn't show anything? Because now it's running in the background, doing nothing. Let's get inside the container again by running /bin/bash.

$ docker exec -it <container id> /bin/bash

You have executed a command manually inside a container and gain the access back. Make sure if your file is still there.

root@<container id>:/# cat mydata/index.html
root@<container id>:/# exit

The file is there. Good.

Note, if you do docker ps this time, it will show the container as RUNNING. That's something new.

This is because the command that you executed by docker exec is not the primary command that was used to run the container.

You might think that /bin/bash is exactly the same as primary command, but it's not the first one.

Copy data from host to a container

Oh did you say you already have something on your host machine and you want it inside the container?

$ docker cp <src path> <container id>:<dst path>

Let's create a file and copy it to the container and see if it's there.

$ echo "hello world" > myfile.txt
$ docker cp myfile.txt <container id>:/mydata/hello-world.txt
$ docker exec -it <container id> cat /mydata/hello-world.txt

Stop the container

You can stop the container now.

$ docker stop <container id>

It's back to EXITED state. Check it using docker ps -a.

Save the changes

Use docker commit.

What? I have heard about git commit, what on Earth is docker commit?

Every command that you executed on a running container to install new tools or copied files to it, wouldn't it be nice to save them as a new image? Why not?

$ docker commit <container id> my-first-image:latest

Yes, it's that simple. Now you can run container from this image again and again.

$ docker run -it my-first-image:latest

Do you know what's simpler? Dockerfiles.