Basic Docker Syntax

Docker can seem overwhelming at first. However, the commands are pretty intuitive, and with a bit of practice, you’ll be a Docker wizard in no time.

The syntax for Docker can be categorised into four main groups:

• Running a container

• Managing & Inspecting containers

• Managing Docker images

• Docker daemon stats and information

We will break down each of these categories in this task.

Managing Docker Images

Docker Pull

Before we can run a Docker container, we will first need an image. Recall from the “Intro to Containerisation” room that images are instructions for what a container should execute. There’s no use running a container that does nothing!

In this room, we will use the Nginx image to run a web server within a container. Before downloading the image, let’s break down the commands and syntax required to download an image. Images can be downloaded using the docker pull command and providing the name of the image.

For example, docker pull nginx. Docker must know where to get this image (such as from a repository, which we’ll come onto in a later task).

Continuing with our example above, let’s download this Nginx image!

cmnatic@thm:~$ docker pull nginx
Using default tag: latest
latest: Pulling from library/nginx
-- omitted for brevity --
Status: Downloaded newer image for nginx:latest
cmnatic@thm:~$

By running this command, we are downloading the latest version of the image titled “nginx”. Images have these labels called tags. These tags are used to refer to variations of an image. For example, an image can have the same name but different tags to indicate a different version. I’ve provided an example of how tags are used within the table below:

Docker Image	Tag	Command Example	Explanation
ubuntu	latest	docker pull ubuntu (same as docker pull ubuntu:latest)	This command will pull the latest version of the "ubuntu" image. If no tag is specified, Docker will assume you want the "latest" version if no tag is specified. It is worth remembering that you do not always want the "latest". This image is quite literally the "latest" in the sense it will have the most recent changes. This could either fix or break your container.
ubuntu	22.04	docker pull ubuntu:22.04	This command will pull version "22.04 (Jammy)" of the "ubuntu" image.
ubuntu	20.04	docker pull ubuntu:20.04	This command will pull version "20.04 (Focal)" of the "ubuntu" image.
ubuntu	18.04	docker pull ubuntu:18.04	This command will pull version "18.04 (Bionic)" of the "ubuntu" image.

When specifying a tag, you must include a colon : between the image name and tag, for example, ubuntu:22.04 (image:tag). Don’t forget about tags - we will return to these in a future task!

Docker Image x/y/z

The docker image command, with the appropriate option, allows us to manage the images on our local system. To list the available options, we can simply do docker image to see what we can do. I’ve done this for you in the terminal below:

cmnatic@thm:~$ docker image

Usage:  docker image COMMAND

Manage images

Commands:
  build       Build an image from a Dockerfile
  history     Show the history of an image
  import      Import the contents from a tarball to create a filesystem image
  inspect     Display detailed information on one or more images
  load        Load an image from a tar archive or STDIN
  ls          List images
  prune       Remove unused images
  pull        Pull an image or a repository from a registry
  push        Push an image or a repository to a registry
  rm          Remove one or more images
  save        Save one or more images to a tar archive (streamed to STDOUT by default)
  tag         Create a tag TARGET_IMAGE that refers to SOURCE_IMAGE

Run 'docker image COMMAND --help' for more information on a command.
cmnatic@thm:~$

• In this room, we are only going to cover the following options for docker images:

  • pull (we have done this above!)

  • ls (list images)

  • rm (remove an image)

  • build (we will come onto this in the “Building your First Container” task)

Docker Image ls

This command allows us to list all images stored on the local system. We can use this command to verify if an image has been downloaded correctly and to view a little bit more information about it (such as the tag, when the image was created and the size of the image).

cmnatic@thm:~$ docker image ls
REPOSITORY   TAG       IMAGE ID       CREATED       SIZE
ubuntu       22.04     2dc39ba059dc   10 days ago   77.8MB
nginx        latest    2b7d6430f78d   2 weeks ago   142MB
cmnatic@thm:~$

For example, in the terminal above, we can see some information for two images on the system:

Repository	Tag	Image ID	Created	Size
ubuntu	22.04	2dc39ba059dc	10 days ago	77.8MB
nginx	latest	2b7d6430f78d	2 weeks ago	142MB

Docker Image rm

If we want to remove an image from the system, we can use docker image rm along with the name (or Image ID). In the following example, I will remove the "ubuntu" image with the tag "22.04". My command will be docker image rm ubuntu:22.04:

It is important to remember to include the tag with the image name.

cmnatic@thm:~$ docker image rm ubuntu:22.04
Untagged: ubuntu:22.04
Untagged: ubuntu@sha256:20fa2d7bb4de7723f542be5923b06c4d704370f0390e4ae9e1c833c8785644c1
Deleted: sha256:2dc39ba059dcd42ade30aae30147b5692777ba9ff0779a62ad93a74de02e3e1f
Deleted: sha256:7f5cbd8cc787c8d628630756bcc7240e6c96b876c2882e6fc980a8b60cdfa274
cmnatic@thm:~$

If we were to run a docker image ls, we would see that the image is no longer listed:

cmnatic@thm:~$ docker image ls
REPOSITORY   TAG       IMAGE ID       CREATED       SIZE
nginx        latest    2b7d6430f78d   2 weeks ago   142MB
cmnatic@thm:~$