Managing Docker Containers

Docker is OS-level virtualization software for developers and sysadmins, the beauty of docker is that it creates an individual tenant for every application that is deployed. A DockerFile is file is a script that is composed of a set of commands that will execute automatically for building new docker images. In this guide, we will make our own docker file. For the demo, we will keep it simple. We will be creating a centos based image.

# vi dockerfile

# Docker base on the latest CentOS 7

FROM centos:latest
MAINTAINER [email protected]

RUN useradd -ms /bin/bash user
USER user

Now we initiate the above script.

# docker build -t centos7/nonroot:v1 .

For launching an above docker script/file

# docker run -it centos7/nonroot:v1 /bin/bash

You will connect to docker container but you cannot enter as a root user for this we will issue a command to connect to docker container

#  docker ps -a

CONTAINER ID     IMAGE               COMMAND         CREATED         STATUS               PORTS   NAMES
0b45a206377e   centos7/nonroot:v1  "/bin/bash"   2 hours ago     Exited (1) 2 hours ago    pedantic_banach
696797d4e72b   centos7/nonroot:v1 "/bin/bash"    2 hours ago     Exited (0) 2 hours ago    stupefied_leakey
c54689519958    ubuntu:xenial     "/bin/bash"    16 hours ago    Up 16 hours               thirsty_gates
0829a7b03aee    hello-world       "/hello"       11 days ago     Exited (0) 11 days ago    adoring_mirzakhani

After noting the name of container we will start it.

# docker start pedantic_banach

Now we know the container name, we will issue below command for user to attach as a root

# docker exec -u 0 -it pedantic_banach /bin/bash

How to expose a port on docker

For this we will pull nginx image from docker hub.

# docker pull nginx

Now for checking images

# docker images
REPOSITORY              TAG                 IMAGE ID            CREATED             SIZE
centos7/nonroot         v1                  1b7fc6bf7fed        13 hours ago        196.9 MB
docker.io/ubuntu        xenial              2d696327ab2e        2 days ago          121.6 MB
docker.io/centos        latest              196e0ce0c9fb        6 days ago          196.6 MB
docker.io/nginx         latest              da5939581ac8        7 days ago          108.3 MB
docker.io/hello-world   latest              1815c82652c0        3 months ago        1.84 kB

For running a container from image use the below command.

# docker run -d nginx:latest

For confirming that nginx container has been started use the below command.

# docker ps -a

CONTAINER ID    IMAGE          COMMAND       CREATED        STATUS                 PORTS               NAMES
23d81c345e00   nginx:latest   "nginx -g 'daemon off"   6 seconds ago   Up 5 seconds    80/tcp   sharp_torvalds
0b45a206377e  centos7/nonroot:v1   "/bin/bash"        13 hours ago     Up 10 hours              pedantic_banach
696797d4e72b   centos7/nonroot:v1   "/bin/bash"       13 hours ago     Exited (0) 13 hours ago  stupefied_leakey
c54689519958   ubuntu:xenial         "/bin/bash"      26 hours ago     Uphours                  thirsty_gates
0829a7b03aee    hello-world      "/hello"             11 days ago     Exited (0) 11 days ago    adoring_mirzakhan

To find out the IP Address of the container

# docker inspect sharp_torvalds | grep IPAddress
"SecondaryIPAddresses": null,
"IPAddress": "172.17.0.4",
"IPAddress": "172.17.0.4",

Note the IP address of docker container. We will test the nginx application on elink browser which is a utility of linux core version.

# yum install elinks -y
# elinks http://172.17.0.4

This confirms that Nginx is running fine, but we are accessing it from the container on base machine (centos 7). In the next step, we will expose the port of Nginx to base machine, so when we put in “elinks http://localhost” on base machine, it will show us the Nginx default page. First, we will stop the nginx container, and then we will expose the port.

# docker stop sharp_torvalds
# docker run -d -p 8080:80 nginx:latest

Note: “-p” denotes the port number

Now we have exposed the port of nginx to the base machine, but when we run the container again it will change the name of the container, so for confirming container name we use

# docker ps -a

CONTAINER ID        IMAGE                COMMAND                  CREATED             STATUS                   PORTS                  NAMES
20dddf7697ce        nginx:latest         "nginx -g 'daemon off"   4 seconds ago       Up 3 seconds               0.0.0.0:8080->80/tcp   goofy_shirley
23d81c345e00        nginx:latest         "nginx -g 'daemon off"   23 minutes ago      Exited (0) 6 minutes ago                          sharp_torvalds
0b45a206377e        centos7/nonroot:v1   "/bin/bash"              13 hours ago        Up 11 hours                                       pedantic_banach
696797d4e72b        centos7/nonroot:v1   "/bin/bash"              13 hours ago        Exited (0) 13 hours ago                           stupefied_leakey
c54689519958        ubuntu:xenial        "/bin/bash"              27 hours ago        Up 27 hours                                       thirsty_gates
0829a7b03aee        hello-world          "/hello"                 11 days ago         Exited (0) 11 days ago                            adoring_mirzakhani

Now for confirming that nginx is accessible.

# elinks http://localhost:8080

This is for basic container management.