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Docker containers
Launch a container
We launch a container by running, e.g
docker run hello-world
Docker looks for the hello-world image in order to start the container. If it cannot find it, it will fetch the image from DockerHub.
(docker run assumes you are saying "run image in container" but you can specify other Docker objects such as networks.)
The docker run command is actually a composite of the following two commands:
docker create --name [my_container_name] hello-world
docker start [my_container]
Here is an applied example:
docker run --name my-container debian /bin/echo "Hello, world"
This creates and runs a container from the Debian image and executes /bin/echo inside of it. Once you have run the above line, if the process is successful it will exit. The container will stop running but it will remain in memory. Nothing will be output (no "Hello, world"), because when you run it, you are outside of the container.
Launch a container as a daemon
You can also run a container as a daemon. In this mode, the container will run in the background and detach from the console. For example:
docker run -d debian /bin/sh -c /bin/sh -c "while true; do echo 'Hello!'; sleep 1; done"
Inside the container, this will echo "Hello!" every second in an infinite loop whilst the daemon is active.
The above command will not actually output anything. Instead it wil ouput the container ID, e.g:
2749d796cbd64e9cf57307329e792587c39d8244f2377e62d78f3f3f77eecdb4
You can use this to access the log for the container. When you do so, you will then see the output:
docker log 2749
hello
hello
hello
...
(We could also use the container name to reference the container, if we launched it with the --name param.)
Stopping a container
docker stop 2749
There will be a delay because it shuts down gracefully. It sends a SIGINT to the process in the container with PID 1 (i.e the root or parent process for the container).
stop will keep the container in memory. This means you can still refer back to the logs and that it can be restarted.
Instead of stop, if you were to use:
docker rm 2749
The container will be stopped and deleted. The logs are deleted and the container cannot be recovered.
We can also tell Docker to immediately remove a container after it exits:
docker run --rm [image]