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Work with your first container

If you read the output from our hello world, they even recommend what to try next.

docker container run -it ubuntu bash
Let's see what happens:

Unable to find image 'ubuntu:latest' locally
latest: Pulling from library/ubuntu
31e907dcc94a: Pull complete
Digest: sha256:8a37d68f4f73ebf3d4efafbcf66379bf3728902a8038616808f04e34a9ab63ee
Status: Downloaded newer image for ubuntu:latest
root@0e539c9ccee4:/#

We are inside the docker container!

Tip

You need to use the -it option whenever you want to run a container in interactive mode. - The -i or --interactive option connects you to the input stream of the container, so that you can send inputs to bash; - The -t or --tty option makes sure that you get some good formatting and a native terminal-like experience by allocating a pseudo-tty.

Playing with a running container

This is a fully fledged Ubuntu host, and we can do anything we like in it. Let's explore it a bit, starting with asking for its hostname:

root@0e539c9ccee4:/# hostname
0e539c9ccee4

Tip

A container's hostname defaults to be the container's ID in Docker. You can override the hostname using --hostname.

Let's have a look at the /etc/hosts file too. 

root@0e539c9ccee4:/# cat /etc/hosts
127.0.0.1   localhost
::1 localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
172.17.0.2  0e539c9ccee4
Docker has also added a host entry for our container with its IP address. Let's also check out its networking configuration.

root@0e539c9ccee4:/# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
6: eth0@if7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default
    link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0
       valid_lft forever preferred_lft forever

ip: command not found

Install the package iproute2 that provides a collection of utilities for networking and traffic control. 

   apt update && apt install -y iproute2

As we can see, we have the lo loopback interface and the eth0@if7 network interface with an IP address of 172.17.0.2, just like any other host.

We can also check its running processes:

root@0e539c9ccee4:/# ps aux
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.0  0.0   4588  3968 pts/0    Ss   11:04   0:00 bash
root         334  0.0  0.1   7888  4224 pts/0    R+   11:08   0:00 ps aux

Note that the process bash has PID 1.

Now type exit or the CTRL-d key sequence...you'll return to the command prompt of your Ubuntu host. So what's happened to our container? The container only runs for as long as the command we specified, /bin/bash, is running. Once we exited the container, that command ended, and the container was stopped.

So the container still exists but it's stopped: 

docker container ps -a
CONTAINER ID   IMAGE         COMMAND    CREATED          STATUS                      PORTS     NAMES
0e539c9ccee4   ubuntu        "bash"     5 minutes ago    Exited (0) 15 seconds ago             frosty_satoshi
a621c49ed3d6   hello-world   "/hello"   15 minutes ago   Exited (0) 15 minutes ago             bold_galileo

Starting a stopped container

We can start again our stopped container with docker container start <container-id or container-name>:

docker container start 0e539c9ccee4
0e539c9ccee4
Our container will restart with the same options we had specified when we launched it with the docker run command.

Attaching to a container

The docker container attach command allows you to attach your terminal to the running container.

Tip

The command that is executed when starting a container is specified using the ENTRYPOINT and/or CMD instruction in the Dockerfile. The attach command allows you to connect and interact with the container’s main process which has PID 1. Remember that if you kill the main process the container will terminate.

This is useful when you want to see what is written in the standard output in real-time, or to control the process interactively.

So running the attach command on our Ubuntu container will bring us back to our bash prompt: 

docker attach 0e539c9ccee4
root@0e539c9ccee4:/#

You can detach from a container and leave it running using the CTRL-p CTRL-q key sequence.

What happens if you type exit?

Getting a shell to a container

The docker exec command allows you to run commands inside a running container. The command can be run in background using the option -d or interactively using the option -i.

Try the following command on your Ubuntu container:

docker exec -it 0e539c9ccee4 bash
root@0e539c9ccee4:/#
Let's look at the processes inside the container: 
root@0e539c9ccee4:/# ps aux
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.0  0.0   4588  3712 pts/0    Ss+  13:56   0:00 bash
root           9  0.1  0.0   4588  3968 pts/1    Ss   13:57   0:00 bash
root          17  0.0  0.1   7888  4096 pts/1    R+   13:57   0:00 ps aux
We can see that the exec command started a new shell session.

Tip

Usually the exec command is used to launch bash within the container and work with that. The attach command primarily is used if you quickly want to see the output of the main process (PID 1) directly and/or want to kill it.