88 lines
3.3 KiB
Markdown
88 lines
3.3 KiB
Markdown
|
# Lesson 02: Hello World
|
||
|
|
|
||
|
|
Yes — Docker has a `hello-world` image, just like programming languages have a "hello world" program. Running it is the canonical first step.
|
||
|
|
|
||
|
|
Open a terminal and run:
|
||
|
|
|
||
|
|
```bash
|
||
|
|
docker run hello-world
|
||
|
|
```
|
||
|
|
|
||
|
|
You should see something like:
|
||
|
|
|
||
|
|
```
|
||
|
|
Unable to find image 'hello-world:latest' locally
|
||
|
|
latest: Pulling from library/hello-world
|
||
|
|
...
|
||
|
|
Status: Downloaded newer image for hello-world:latest
|
||
|
|
|
||
|
|
Hello from Docker!
|
||
|
|
This message shows that your installation appears to be working correctly.
|
||
|
|
|
||
|
|
To generate this message, Docker took the following steps:
|
||
|
|
1. The Docker client contacted the Docker daemon.
|
||
|
|
2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
|
||
|
|
3. The Docker daemon ran a new container from that image, which runs the
|
||
|
|
executable that produces the output you are currently reading.
|
||
|
|
4. The Docker daemon streamed that output to the Docker client, which
|
||
|
|
sent it to your terminal.
|
||
|
|
...
|
||
|
|
```
|
||
|
|
|
||
|
|
## What just happened
|
||
|
|
|
||
|
|
`docker run hello-world` is a small command doing a surprising amount of work:
|
||
|
|
|
||
|
|
1. You asked Docker for an image named `hello-world`.
|
||
|
|
2. Docker checked your local image cache. Not there.
|
||
|
|
3. Docker reached out to **Docker Hub** — a public registry of images — and downloaded it.
|
||
|
|
4. Docker created a new container from that image and started the program inside.
|
||
|
|
5. The program printed its message and exited.
|
||
|
|
6. The container stopped (but still exists; we'll deal with that in lesson 04).
|
||
|
|
|
||
|
|
The whole thing took a few seconds and didn't install anything onto your system the way a normal program would. The `hello-world` "program" lives entirely inside the image's filesystem.
|
||
|
|
|
||
|
|
## The shape of every `docker run` command
|
||
|
|
|
||
|
|
```bash
|
||
|
|
docker run [options] IMAGE [command and arguments]
|
||
|
|
```
|
||
|
|
|
||
|
|
- `IMAGE` is the name of the image to run (`hello-world`, `ubuntu`, `python:3.12`, etc.).
|
||
|
|
- `[options]` go *before* the image name. (Order matters; this catches everyone at first.)
|
||
|
|
- `[command and arguments]` go *after* the image, and override the image's default command.
|
||
|
|
|
||
|
|
For `hello-world` the image's default command is "print the welcome message and exit", and we didn't override it.
|
||
|
|
|
||
|
|
## Run it again
|
||
|
|
|
||
|
|
```bash
|
||
|
|
docker run hello-world
|
||
|
|
```
|
||
|
|
|
||
|
|
This time it skips the download — the image is already in your local cache — and just runs the container. Almost instant.
|
||
|
|
|
||
|
|
## A first peek at what's there
|
||
|
|
|
||
|
|
Docker stores every image it has downloaded. List them:
|
||
|
|
|
||
|
|
```bash
|
||
|
|
docker images
|
||
|
|
```
|
||
|
|
|
||
|
|
You should see `hello-world` with a size of around 14 kilobytes. That's the entire program plus its filesystem. (For comparison, an installed copy of, say, Python 3 takes hundreds of megabytes.)
|
||
|
|
|
||
|
|
You can also list every container that's ever existed, even stopped ones:
|
||
|
|
|
||
|
|
```bash
|
||
|
|
docker ps -a
|
||
|
|
```
|
||
|
|
|
||
|
|
You'll see two entries — one for each time you ran `hello-world`. They both ran for a fraction of a second and then exited. Don't worry about them; lesson 04 covers cleaning up.
|
||
|
|
|
||
|
|
## Try it yourself
|
||
|
|
|
||
|
|
1. Run `docker run hello-world` a third time. Notice how fast it is now that the image is cached.
|
||
|
|
2. Run `docker images` and find the row for `hello-world`. The `IMAGE ID` column shows its content hash — that's how Docker uniquely identifies images under the hood.
|
||
|
|
3. Move on to [`03_running_containers.md`](03_running_containers.md), where we'll run something more interesting than a "hello".
|