Docker
System information
docker help
docker info
docker version
docker version --format '{{json .}}' | jq # requires jq installed
docker version --format '{{.Client.Version}}'
docker version --format '{{.Server.Version}}'
docker --version
Run a stateless DEMO application
docker run --rm -p "8080:80" itsziget/phar-examples:1.0
# Press Ctrl-C to quit
Play with the “hello-world” container
Start “hello-world” container
docker run hello-world
# or
docker container run hello-world
Output:
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.
(amd64)
3. The Docker daemon created 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.
To try something more ambitious, you can run an Ubuntu container with:
$ docker run -it ubuntu bash
Share images, automate workflows, and more with a free Docker ID:
https://hub.docker.com/
For more examples and ideas, visit:
https://docs.docker.com/get-started/
List containers
List running containers
docker ps
# or
docker container ps
# or
docker container ls
# or
docker container list
List all containers
docker ps -a
# or use the other alias commands
List containers based on the hello-world image:
docker ps -a -f ancestor=hello-world
# or
docker container list --all --filter ancestor=hello-world
Delete containers
Delete a stopped container
docker rm containername
# or
docker container rm containername
Delete a running container:
docker rm -f containername
If the generated name of the container is “angry_shaw”
docker rm -f angry_shaw
Start a container with a name
docker run --name hello hello-world
Running the above command again results an error message since “hello” is already used for the previously started container. Run the following command to check the stopped containers:
docker ps -a
Or you can start the stopped container again by using its name:
docker start hello
The above command will display the name of the container. You need to start it in “attached” mode in order to see the output:
docker start -a hello
Delete the container named “hello”
docker rm hello
Start a container and delete it automatically when it stops
docker run --rm hello-world
Start an Ubuntu container
Start Ubuntu in foreground (“attached” mode)
docker run -it --name ubuntu-container ubuntu:20.04
Press Ctrl+P and then Ctrl+Q to detach from the container
or type exit and press enter to exit bash and stop the container.
Start Ubuntu in background (“detached” mode)
Linux distribution base Docker images usually don’t contain Systemd as LXD images
so these containers cannot run in background unless you pass -it
to get interactive terminal. It wouldn’t be necessary with a container
which has a process inside running in foreground continuously.
-it works with other containers too as long as
the containers command is “bash” or some other shell.
docker rm -f ubuntu-container
docker run -it -d --name ubuntu-container ubuntu:20.04
Note
Actually only -i or -t would be enough to keep the container
in the backgorund, but if you want to attach the container later, it requires
both of them. Of course, -d is always required.
Attach the container
You can attach the container and see the same as you could see
when you run a container without -d, in foreground.
You can even interact with the container’s main process so be careful
and don’t execute a command like exit, or you will stop the whole
container by stopping its main process.
docker attach ubuntu-container
Press Ctrl+P and then Ctrl+Q to quit without stopping the container.
The better way to “enter” the container is docker exec which
is similar to the way of LXD.
docker exec -it ubuntu-container
Now you can use the “exit” command to quit the container and leave it running.
Start Apache HTTPD webserver
Start the container in the foreground
docker run --name web httpd:2.4
There will be no prompt until you press “CTRL+C” to stop the container running in the foreground.
Note
When you change your terminal window it will send SIGWINCH signal to the container and shut down the server. Use it only for some quick test.
Start it in the background
docker rm web
docker run -d --name web httpd:2.4
Note
You don’t need to use -it and you should not use that either.
Running HTTPD server container with and interactive terminal
will send SIGWINCH signal to the container and shut down the HTTPD server
immediately when you try to attach it.
Even without -it, attaching the HTTPD server container
will shut down the server when you change the size of your terminal window.
Use docker logs instead.
Check container logs
docker logs shows the standard error and output of a container
without attaching it. Actually it will read and show the content of
the log file which was saved from the container’s output.
docker logs web
# or
docker container logs web
Watch the output (logs) continuously
docker logs -f web
# Press Ctrl-C to stop watching
Open the webpage using an IP address
Get the IP address:
CONTAINER_IP=$(docker container inspect web --format '{{.NetworkSettings.IPAddress}}')
You can test if the server is working using wget:
wget -qO- $CONTAINER_IP
Output:
<html><body><h1>It works!</h1></body></html>
Use port forwarding
Delete the container named “web” and forward the port 8080 from the host to the containers internal port 80:
docker rm -f web
docker run -d -p "8080:80" --name web httpd:2.4
Then you can access the page using the host’s IP address.
How we could enter a container in the past
Before docker exec was introduced, nsenter
was the only way to enter a container.
It does almost the same as docker exec
except it does not support Pseudo-TTY so some commands may not work.
CONTAINER_PID=$(docker container inspect --format '{{ .State.Pid }}' web)
sudo nsenter \
--target $CONTAINER_PID \
--mount \
--uts \
--ipc \
--net \
--pid \
--cgroup \
--wd \
env -i - $(sudo cat /proc/$CONTAINER_PID/environ | xargs -0) bash
As you can see, nsenter runs a process inside specific Linux namespaces.
Start Ubuntu virtual machine
There are multiple ways to run a virtual machine with Docker.
Using a parameter is not enough. You need to choose a different runtime.
The default is runc which runs containers.
One of the most popular and easiest runtime is Kata Containers.
Follow the instructions to install the latest stable version of the Kata runtime: Install Kata Containers
and configure Docker daemon to use it. An example /etc/docker/daemon.json
is the following:
{
"default-runtime": "runc",
"runtimes": {
"kata": {
"path": "/usr/bin/kata-runtime"
}
}
}
Now run
docker run -d -it --runtime kata --name ubuntu-vm ubuntu:20.04
It is still lightweight. You can run ps aux inside to see
there is no systemd or other process like that, however, run the
following command on the host machine and see it has only one CPU core:
docker exec -it ubuntu-vm cat /proc/cpuinfo