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
345e3491a907: Pull complete
57671312ef6f: Pull complete
5e9250ddb7d0: Pull complete
Digest: sha256:adf73ca014822ad8237623d388cedf4d5346aa72c270c5acc01431cc93e18e2d
Status: Downloaded newer image for ubuntu:latest

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@a5a2c01df566:/# hostname
a5a2c01df566

We can see that our container's hostname is the container ID. Let's have a look at the /etc/hosts file too.

root@a5a2c01df566:/# 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  a5a2c01df566

Docker has also added a host entry for our container with its IP address. Let's also check out its networking configuration.

root@a5a2c01df566:/# 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
85: eth0@if86: <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@if86 network interface with an IP address of 172.17.0.2, just like any other host.

We can also check its running processes:

root@a5a2c01df566:/# ps aux
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.0  0.0   4216  3588 pts/0    Ss   08:46   0:00 bash
root         601  0.0  0.0   5896  2852 pts/0    R+   10:17   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
a5a2c01df566   ubuntu    "bash"    2 hours ago   Exited (0) 6 seconds ago             festive_cerf

Starting a stopped container¶

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

docker container start a5a2c01df566
a5a2c01df566

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 container attach a5a2c01df566
root@a5a2c01df566:/#

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 a5a2c01df566 bash
root@a5a2c01df566:/#

Let's look at the processes inside the container:

root@a5a2c01df566:/# ps aux
USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root           1  0.0  0.0   4108  3420 pts/0    Ss+  14:05   0:00 bash
root           9  0.6  0.0   4108  3468 pts/1    Ss   14:06   0:00 bash
root          17  0.0  0.0   5896  2848 pts/1    R+   14:06   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.