DOCKER-3

 

                 DOCKER-3 (Basic Commands) 

What we will learn in this post,

·         Docker basic commands

·         Run & Pull Docker Images

·         Create container from image

·         See the running or exited containers

·         Remove container & images

·         Running a container

 

Let’s start…

# docker run ubuntu

What does it mean?

Docker will search the image locally if found then it will create a ubuntu container from that image, otherwise it will go to docker hub to pull the image and then loads up the container and then runs a command in that container. Though we did not provide any command or argument here, hence the container will booted, ran an empty command and then exited.

From below command we can see the images locally (we need to pull)

[root@centos7-i2 ~]# docker images

REPOSITORY   TAG       IMAGE ID   CREATED   SIZE

REPOSITORY: name of image

TAG: version of Image

IMAGE ID: Unique Random no for identification

CREATED: when it was created by actual owners/developers

SIZE: size of image

# docker ps = to show running containers

# docker ps -a = to show all containers

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

I do not have any image or container.

[root@centos7-i2 ~]# docker pull ubuntu

Using default tag: latest

latest: Pulling from library/ubuntu

7b1a6ab2e44d: Pull complete

Digest: sha256:626ffe58f6e7566e00254b638eb7e0f3b11d4da9675088f4781a50ae288f3322

Status: Downloaded newer image for ubuntu:latest

docker.io/library/ubuntu:latest

I used the command docker pull ubuntu, which fetched the latest image of ubuntu and saved locally.

[root@centos7-i2 ~]# docker images

REPOSITORY   TAG       IMAGE ID       CREATED        SIZE

ubuntu       latest    ba6acccedd29   35 hours ago   72.8MB

 

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

Now I have one image and no container, let’s create one.

[root@centos7-i2 ~]# docker run ubuntu

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND   CREATED          STATUS                     PORTS     NAMES

1426a0b641e3   ubuntu    "bash"    10 seconds ago   Exited (0) 7 seconds ago             modest_goldstine

How the container created? It’s created from the image we pulled before sometime.

Is this the only way? Let’s check.

First remove everything we pulled/created.

[root@centos7-i2 ~]# docker rm 1426a0b641e3

1426a0b641e3

[root@centos7-i2 ~]# docker rmi ubuntu

Untagged: ubuntu:latest

Untagged: ubuntu@sha256:626ffe58f6e7566e00254b638eb7e0f3b11d4da9675088f4781a50ae288f3322

Deleted: sha256:ba6acccedd2923aee4c2acc6a23780b14ed4b8a5fa4e14e252a23b846df9b6c1

Deleted: sha256:9f54eef412758095c8079ac465d494a2872e02e90bf1fb5f12a1641c0d1bb78b

***IMP*** You cannot remove running containers

Now we are cleaned,

[root@centos7-i2 ~]# docker images

REPOSITORY   TAG       IMAGE ID   CREATED   SIZE

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

Now do the “docker run” directly without pulling.

[root@centos7-i2 ~]# docker run ubuntu

Unable to find image 'ubuntu:latest' locally <= see, local is first preference

latest: Pulling from library/ubuntu

7b1a6ab2e44d: Pull complete

Digest: sha256:626ffe58f6e7566e00254b638eb7e0f3b11d4da9675088f4781a50ae288f3322

Status: Downloaded newer image for ubuntu:latest

[root@centos7-i2 ~]# docker images

REPOSITORY   TAG       IMAGE ID       CREATED        SIZE

ubuntu       latest    ba6acccedd29   35 hours ago   72.8MB

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND   CREATED          STATUS                      PORTS     NAMES

9b3bd4030a50   ubuntu    "bash"    18 seconds ago   Exited (0) 15 seconds ago             gracious_euclid

Both purpose solved, it pulled an image and created container from it also.

Now we understand difference between pull & run.

What is ps -a (list all containers) whereas ps only lists running containers

Let’s see how many images we have

[root@centos7-i2 ~]# docker images

REPOSITORY   TAG       IMAGE ID       CREATED       SIZE

ubuntu       latest    ba6acccedd29   8 days ago    72.8MB

centos       latest    5d0da3dc9764   5 weeks ago   231MB

Let’s check how many running containers we have,

[root@centos7-i2 ~]# docker ps

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

No running container, let’s see the status of all containers.

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND   CREATED      STATUS                    PORTS     NAMES

8ccf10715e1b   ubuntu    "bash"    3 days ago   Exited (127) 3 days ago             vigilant_swirles

0f761dc69b18   ubuntu    "bash"    3 days ago   Exited (0) 3 days ago               xenodochial_morse

9b3bd4030a50   ubuntu    "bash"    6 days ago   Exited (0) 6 days ago               gracious_euclid

All exited, they are of no use. Let’s remove all.

Removal one by one,

[root@centos7-i2 ~]# docker rm 8ccf10715e1b

8ccf10715e1b

Above command will remove selected container.

Below command will remove all containers in one go.

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND   CREATED      STATUS                  PORTS     NAMES

0f761dc69b18   ubuntu    "bash"    3 days ago   Exited (0) 3 days ago             xenodochial_morse

9b3bd4030a50   ubuntu    "bash"    6 days ago   Exited (0) 6 days ago             gracious_euclid

Removal in one go,

[root@centos7-i2 ~]# docker container prune

WARNING! This will remove all stopped containers.

Are you sure you want to continue? [y/N] y

Deleted Containers:

0f761dc69b181ee572fab853c3dd949ae0c223006202fcf24d6f0f12da12f543

9b3bd4030a504dc693f28e4b7566dc0c08fe0b7a1f64559c65b95a0769edb3b0

Total reclaimed space: 0B

All gone,

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

Let’s create containers, (there are 3 ways to run)

run

run with detach mode

run with interactive mode to get immediate login

 

Modes, -d & -it

-d for detach mode

-i for interactive mode

-t to assign terminal

By –it we can immediately login to container after creation

[root@centos7-i2 ~]# docker run centos

[root@centos7-i2 ~]# docker run -d centos

6319643e9bba5a080bdf294c9632e8bb68775bcb490e4e615c0affc2c1148a7d

[root@centos7-i2 ~]# docker run -it centos bash

[root@3b51b523cf0f /]# hostname

3b51b523cf0f

[root@3b51b523cf0f /]# ip a

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000

    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

    inet 127.0.0.1/8 scope host lo

       valid_lft forever preferred_lft forever

30: eth0@if31: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default

    link/ether 02:42:ac:11:00:02 brd ff:ff:ff:ff:ff:ff link-netnsid 0

    inet 172.17.0.2/16 brd 172.17.255.255 scope global eth0

       valid_lft forever preferred_lft forever

[root@3b51b523cf0f /]# exit

Exit

[root@centos7-i2 ~]# docker ps

CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

Strange!!! No running containers

[root@centos7-i2 ~]# docker ps -a

CONTAINER ID   IMAGE     COMMAND       CREATED              STATUS                      PORTS     NAMES

3b51b523cf0f   centos    "bash"        23 seconds ago       Exited (0) 10 seconds ago             funny_gould

6319643e9bba   centos    "/bin/bash"   44 seconds ago       Exited (0) 41 seconds ago             suspicious_sinoussi

0b4e93161d51   centos    "/bin/bash"   About a minute ago   Exited (0) 55 seconds ago             competent_murdock

 

Let’s try to start containers,

[root@centos7-i2 ~]# docker start funny_gould

funny_gould

[root@centos7-i2 ~]# docker ps

CONTAINER ID   IMAGE     COMMAND   CREATED         STATUS         PORTS     NAMES

3b51b523cf0f   centos    "bash"    7 minutes ago   Up 2 seconds             funny_gould

[root@centos7-i2 ~]# docker start suspicious_sinoussi

suspicious_sinoussi

[root@centos7-i2 ~]# docker ps

CONTAINER ID   IMAGE     COMMAND   CREATED         STATUS          PORTS     NAMES

3b51b523cf0f   centos    "bash"    7 minutes ago   Up 24 seconds             funny_gould

[root@centos7-i2 ~]# docker start competent_murdock

competent_murdock

[root@centos7-i2 ~]# docker ps

CONTAINER ID   IMAGE     COMMAND   CREATED         STATUS          PORTS     NAMES

3b51b523cf0f   centos    "bash"    8 minutes ago   Up 58 seconds             funny_gould

[root@centos7-i2 ~]#

Only one started, which one is this? Its container which run with –it option

You can identify via looking at COMMAND option in ps –a output.

Thanks to below authors for writing excellent explanation for

Why container exit after docker run

https://webkul.com/blog/docker-container-will-automatically-stop-run/

http://coddingbuddy.com/article/50776509/why-docker-container-exits-immediately

https://www.tutorialworks.com/why-containers-stop/#:~:text=When%20the%20process%20running%20inside,for%20the%20container%20to%20run.