Docker is one of the container technologies

Containers vs Images

This is a very simplified explanation, which hopefully clears up for you. As with all simplifications, they aren’t entirely correct.

Containers put processes, files, and networking into a space where they are secluded from the rest. You main OS is called the host and the container is called the guest. You can selectively share resources with the guest. To use an analogy, if you house were the computer with linux, if you took a room, put tools and resources for those tools into it, put workers into it, got them to start working and locked the door, they’d be contained in the room, unable to break out. If you want to give workers access to resources, you either a window, a corridor, or even a door depending on much access you want to give them.

Containers are created from an image. Think of it as the tools, resources, and configuration required every time you create a room in your house for workers to do a job. The woodworkers will need different tools and resources than say metalworkers.

Most images are stored on DockerHub. So when you do docker pull linuxserver/sonarr you download the image. When you do docker run linuxserver/sonarr you create a container from an image.

Installation

You’re on Cinnamon Mint which is linux distribution derived from another linux distribution called debian. You have to follow the installation instructions. Everything is there. If something doesn’t work, it’s most likely because you skipped a step. The most important ones are the post-installation steps:

• Adding your user to the docker group
• Logging out and back in (or simply restarting)

Those are the most commonly missed steps. I’ve fallen for this trap too!

Local help

To use linux, you need to learn about ways to help yourself or find help. On linux, most well-written programs print a help. Simply running the command without any arguments most often output a help text --> running docker does so. If they don’t, then the --help flag often does --> docker --help. The shorthand is -h --> docker -h.

Some commands have sub commands e.g docker run, docker image, docker ps, … . Those subcommands also take flags of which -h and --help are available.

The help output is often not extensive and programs often have a manual. To access it the command is man --> man find will output the manual for the find command. Docker doesn’t have a local manual but an online one.

For clarification when running a command there are different ways to interpret the text after the command:

Flags/Options

These are named parameters to the command. Some do not take input like -h and --help which are called flags. Some do like --file /etc/passwd and are often called options.

Arguments

These are unnamed parameters and each command interprets them differently. echo "hello world" --> echo is the command and "hello world" is the argument. Some commands can take multiple arguments

Running containers

Imperatively

As described above docker run linuxserver/sonarr runs an image in a container. However, it runs in the foreground (as opposed to the background in what is most often called a “daemon”). Starting in the foreground is most likely not how you want to run things as that means if you close your terminal, you end the process too. To run something in the background, you use docker run --detatch linuxserver/sonarr.

You can pass options like -v or --volume to make a file or folder from your host system available in the guest e.g -v /path/on/host:/tmp/path/in/guest. Or -p / --port to forward a host port to a guest port e.g -p 8080:80. That means if you access port 8080 on your host, the traffic will be forwarded to port 80 in the guest.

These are imperatives as in you command the computer to do a specific action. Run that docker image, stop that docker container, restart these containers, start a container with this port forward and that volume with this user …

Declaratively

If you don’t want to keep typing the same commands, you can declare everything about your containers up front. Their volumes, ports, environment variables, which image is used, which network card/interface they have access to, which other network they share with other containers, and so on.

This is done with docker-compose or docker compose for newer docker versions (not all operating systems have the new docker version).

This already a long text, so if you want to know more, the best resource is the docker compose manual and the compose file reference.

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Hopefully this helped with the basics and understanding what you’re doing. There are probably great video resources out there that explain it more didactically than I do with steps you can follow along.

Good luck!

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