Docker CLI working

Docker CLI operates as a client/server to communicate with the Docker engine upon creation of operations of a container. But it will raise issues when you have to start the Docker daemon before accessing Docekr CLI live. After it CLI sends API call to the Docker Engine to launch Open Contianer Initiative (OCI) runtime.

Difference between Docker and Podman

One of the main difference between Docker and Podman is that there is no daemon in the Podman. Upon applying any cgroup constraints you apply on the podman command containers will recieve those constraints directly. Moreover, advanced features of systemd can be utilized using this model.

Libpod

Libpod provides the library for those applications which are looking to use the Container Pod concept. It is also refereed as the Home of Podman.
So now lets proceed further

Step 1: Managing Container Images

Type the below command to download the container Images using Podman

$ podman pull ubuntu
$ podman pull centos
$ podman pull centos:8

Run the below command to list the downloaded images

$ podman  images
 REPOSITORY                 TAG      IMAGE ID       CREATED        SIZE
 docker.io/library/ubuntu   latest   3556258649b2   2 weeks ago    66.6 MB
 docker.io/library/alpine   latest   b7b28af77ffe   3 weeks ago    5.85 MB
 docker.io/library/debian   latest   00bf7fdd8baf   4 weeks ago    119 MB
 docker.io/library/centos   latest   9f38484d220f   4 months ago   209 MB

Follow the below syntax to remove the images

$ podman rmi <Type your Container Image ID here>
$ podman rmi 00bf7fdd8baf
 00bf7fdd8baf2ba6f0918e1f48415b2a4a1a616806e7cf32527a749dd2ce4b2c
 $ podman rmi docker.io/library/ubuntu
 3556258649b2ef23a41812be17377d32f568ed9f45150a26466d2ea26d926c32

Tagging Image

Check out the tag by running podman images command

podman tag  <Image ID> webserver

Step 2: Running Containers with Podman

Let’s print a message using simple container

[sabi@localhost ~]$ podman run --rm centos /bin /echo/ "Osradar"
Osradar

Use -d option to run the container in background

podman run -dt -p 8080:8080/tcp -e HTTPD_VAR_RUN=/var/run/httpd -e HTTPD_MAIN_CONF_D_PATH=/etc/httpd/conf.d \

                  -e HTTPD_MAIN_CONF_PATH=/etc/httpd/conf \

                  -e HTTPD_CONTAINER_SCRIPTS_PATH=/usr/share/container-scripts/httpd/ \

                  registry.fedoraproject.org/f29/httpd /usr/bin/run-httpd

It will return the contianer ID:

Trying to pull registry.fedoraproject.org/f29/httpd…Getting image source signatures
 Copying blob d77ff9f653ce done
 Copying blob aaf5ad2e1aa3 done
 Copying blob 7692efc5f81c done
 Copying config 25c76f9dcd done
 Writing manifest to image destination
 Storing signatures
 d2cdf0efb0ddc6e2ae52a5a0bdadababa6ee6cc2e1e49145c92a0474b089b664

To List the running containers, use the below command

$ podman ps
 CONTAINER ID  IMAGE                                        COMMAND               CREATED        STATUS            PORTS                   NAMES
 d2cdf0efb0dd  registry.fedoraproject.org/f29/httpd:latest  container-entrypo…  4 minutes ago  Up 4 minutes ago  0.0.0.0:8080->8080/tcp  cranky_borg
 To include stopped / exited containers, use:
 $ podman ps --all

For accessing a container shell use the option -it

podman run -it ubuntu bash
 root@d273c12899cd:/#
 root@d273c12899cd:/# apt update
  Get:1 http://archive.ubuntu.com/ubuntu bionic InRelease [242 kB]
  Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]          
  Get:3 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages [717 kB]
  Get:4 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]        
  Get:5 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages [535 kB]
  Get:6 http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]

Step 3: Searching in Podman

podman search httpd

Inspect Containers

It will display the some basic info on containers & images identified by name or ID

podman inspect <image id>

For help run the below command

podman inspect --help

Step 4: Removing Containers

To remove the containers, list all running containers.

podman ps -a

Now type the command podman rm with the ID of container image to remove it.

podman rm <image id>

For removing all containers, type the following command.

Note it will remove all containers running and stopped.

podman rm $(podman ps -a -q)

Step 5: View the container’s logs

Here we will view the containers logs in different ways

View by latest category

podman logs --latest

View by mentioning container ID:

podman p
podman logs <image id>

For attaining real-time, use the below command

podman logs -f 
podman logs --follow=true --since 10m Container ID

You can also specify the number of lines in logs:

podman logs --tail 10 

Step 6: View the container’s pids

Type podman top to view container pids.

podman top <image id>

Output:

$ podman top d2cdf0efb0dd
 USER      PID   PPID   %CPU    ELAPSED            TTY     TIME   COMMAND
 default   1     0      0.000   29m22.496484247s   pts/0   0s     httpd -D FOREGROUND 
 default   22    1      0.000   29m21.496767511s   pts/0   0s     /usr/bin/coreutils --coreutils-prog-shebang=cat /usr/bin/cat 
 default   23    1      0.000   29m21.496866314s   pts/0   0s     /usr/bin/coreutils --coreutils-prog-shebang=cat /usr/bin/cat 
 default   24    1      0.000   29m21.497020539s   pts/0   0s     /usr/bin/coreutils --coreutils-prog-shebang=cat /usr/bin/cat 
 default   25    1      0.000   29m21.497127237s   pts/0   0s     /usr/bin/coreutils --coreutils-prog-shebang=cat /usr/bin/cat 
 default   26    1      0.000   29m21.49723933s    pts/0   0s     httpd -D FOREGROUND 
 default   27    1      0.000   29m21.497361006s   pts/0   0s     httpd -D FOREGROUND 
 default   28    1      0.000   29m21.497459891s   pts/0   0s     httpd -D FOREGROUND 
 default   29    1      0.000   29m21.497552695s   pts/0   0s     httpd -D FOREGROUND 

Step 7: Checkpointing the container

Checkpointing a container stops the container while writing the state of all processes in the container to disk. To do this you’ll requires CRIU 3.11 or later installed on the system.

See CRIU Installation guide here.

podman container checkpoint <container id>

Example:

podman container checkpoint d1hf0dbf0dd

It can later be restored and continue running exactly at the same point in time as the checkpoint.

podman container restore <container id>

Step 8: Migrate Container

Migrating the container contains 3 steps.

In first step, checkpoint the container on the source system.

podman container checkpoint <container id> -e /tmp/checkpoint.tar.gz

In Second Step copy it to the destination server.

scp /tmp/checkpoint.tar.gz :/tmp

Last step involve the restoration on the destination system.

Step 9: Manage Container pods with Podman

Podman pods are similar to the Kubernetes concept of Pods.

You can expose most of the interaction with pods through the podman pod command.

podman pod --help

Create a pod called web

podman pod create --name web

Below will creates a pod without the extra attributes available on:

podman pod create --help

Make sure pod is created

podman pod list

By default, the created pod will have a container called infra. The infra container holds the namespace associated with the pod to allow podman to connect to the containers to the pod. It is in sleep mode by default.

podman ps -a --pod

After creating pod, we can add a container to it.

podman run -dt --pod web alpine:latest top

You’ll see that the pod has two containers.

podman ps -a --pod
CONTAINER ID  IMAGE                            COMMAND  CREATED             STATUS                 PORTS  NAMES               POD
 36ccace2d653  docker.io/library/alpine:latest  top      About a minute ago  Up About a minute ago         zen_hugle           0f565b11e9cb
 44cca777d12f  k8s.gcr.io/pause:3.1                      28 minutes ago      Up 28 minutes ago             0f565b11e9cb-infra  0f565b11e9cb

You can also expose a port to the created container with root privileges.

sudo podman pod create -p 8080:80 --name web1
sudo podman run -dt --pod web1 -p  8080 nginx:latest

You need to publish as many port as will be used during the pod creation. This can't be changed after the creation.

So this is how we can run Docker Container using podman and libpod.

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