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Monday, January 8, 2018

Working with Docker Container - Starting a container

Starting a container

Listing images

We can list the images available on the system running the Docker daemon. These images might have been pulled from the registry, imported through the docker command, or created through Docker files.
root@Docker:~# docker images
REPOSITORY                     TAG                 IMAGE ID               CREATED            SIZE
nskselvan/nsk                      latest              b2f0c17eed23        2 months ago        197MB
centos-latest                        latest              2083898799b1       2 months ago        197MB
wordpress                            latest              224b7eef6944        3 months ago        408MB
mysql                                   5.7                  b4e78b89bcf3        3 months ago        412MB
registry                                 2                    28525f9a6e46         3 months ago        33.2MB
localhost:5000/reg               latest              28525f9a6e46         3 months ago        33.2MB
centos                                  latest             196e0ce0c9fb          3 months ago        197MB
localhost:5000/centos-ka     latest             196e0ce0c9fb          3 months ago        197MB

For help with the docker images use --help

Once we have images, we can use them to start the containers. In this recipe, we will start a container with the fedora:latest image and see what all things happen behind the scene.
syntax  : docker run [ OPTIONS ]  IMAGE[:TAG]  [COMMAND]  [ARG...]

root@Docker:~# docker run -i -t --name=centos centos /bin/bash
[root@6d65e303381c /]# uptime
 04:07:13 up  1:02,  0 users,  load average: 0.02, 0.01, 0.00

The -i option starts the container in the interactive mode
The -t option allocates a pseudo-tty and attaches it to the standard input

So, with the preceding command, we start a container from the centos:latest image, attach pseudo-tty, name it centos, and run the /bin/bash command. If the name is not specified, then a random string will be assigned as the name.
Also, if the image is not available locally, then it will get downloaded from the registry first and then run. Docker will run the search and pull commands before running the run command.

Under the hood, Docker:

Will merge all the layers that make that image using UnionFS.
Allocates a unique ID to a container, which is referred to as Container ID.
Allocates a filesystem and mounts a read/write layer for the container. Any changes on this layer will be temporary and will be discarded if they are not committed.
Allocates a network/bridge interface.
Assigns an IP address to the container.
Executes the process specified by the user.
Also, with the default Docker configuration, it creates a directory with the container's ID inside /var/lib/docker/containers, which has the container's specific information such as hostname, configuration details, logs, and /etc/hosts.

To exit from the container, press Ctrl + D or type exit. It is similar to exiting from a shell but this will stop the container.
The run command creates and starts the container. With Docker 1.3 or later, it is possible to just create the container using the create command and run it later using the start command, as shown in the following example:

root@Docker:~# ID=$(docker create -t -i centos bash)
root@Docker:~# docker start -a -i $ID
[root@8813d555d0dc /]#

The container can be started in the background and then we can attach to it whenever needed. We need to use the -d option to start the container in the background:
root@Docker:~# docker run -d -i -t centos /bin/bash

The preceding command returns the container ID of the container to which we can attach later, as follows:
root@Docker:~# docker attach e3bfe67aa1759c5ccd0e8e061595b8214881673c4df599f9652a58369df7d948
[root@e3bfe67aa175 /]#

The --read-only option of the run command will mount the root filesystem in the read-only mode:
root@Docker:~# docker run --read-only -d -i -t centos /bin/bash

Note: this option just makes sure that we cannot modify anything on the root filesystem, but we are writing on volumes
For help with the docker run use --help

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