Podman Guide #

A pod is a group of containers that share the same name space, ports, and network connection. Usually, containers within one pod can communicate directly with each other. Each pod contains an infrastructure container (INFRA), whose purpose is to hold the name space. INFRA also enables Podman to add other containers to the pod. Port bindings, cgroup-parent values, and kernel name spaces are all assigned to the infrastructure container. Therefore, later changes of these values are not possible.

Figure 1: Pods architecture #

 

Each container in a pod has its own instance of a monitoring program. The monitoring program watches the container's process and if the container dies, the monitoring program saves its exit code. The program also holds open the tty interface for the particular container. The monitoring program enables you to run containers in the detached mode when Podman exits, because this program continues to run and enables you to attach tty later.

After you have pulled your container image, you can create containers based on it. You can run an instance of the image using the podman run command. The command syntax is as follows:

# podman run [OPTIONS] IMAGE [CONTAINER_NAME]

IMAGE is specified in format transport:path. If transport is omitted, the default docker is used. The path can reference to a specific image registry. If omitted, Podman searches for the image in registries defined in the /etc/containers/registries.conf file. An example that runs a container called sles15 based on the sle15 image follows:

# podman run registry.opensuse.org/suse/templates/images/sle-15-sp4/base/images/suse/sle15 sles15

Below is a list of frequently used options. For a complete list of available options, run the command: podman run --help.

If the podman run command finished successfully, a new container has been started. You can stop the container by running:

# podman stop [OPTIONS] CONTAINER

You can specify a single container name or ID or a space-separated list of containers. The command takes the following options:

To view all possible options of the podman stop command, run the following:

# podman stop --help

To start already created but stopped containers, use the podman start command. The command syntax is as follows:

# podman start [OPTIONS] CONTAINER

CONTAINER can be a container name or a container ID.

For a complete list of possible options of podman start, run the command:

# podman start --help

You can run a new container with specific attributes that are not part of the original image. To save the container with these attributes as a new image, you can use the podman commit command:

# podman commit [OPTIONS] CONTAINER IMAGE

CONTAINER is a container name or a container ID. IMAGE is the new image name. If the image name does not start with a registry name, the value localhost is used.

When using Cockpit, you can perform the commit operation directly from a container's Details, by clicking Commit. A dialog box opens. Specify all required details as shown below and click Commit:

Figure 2: Committing a container in Cockpit #

 

Podman enables you to list all running containers using the podman ps command. The generic syntax of the command is as follows:

# podman  ps [OPTIONS]

Command options can change the displayed information. For example, using the --all option will output all containers created by Podman (not only the running containers).

For a complete list of podman ps options, run:

# podman ps --help

To remove one or more unused container from the host, use the podman rm command as follows:

# podman rm [OPTIONS] CONTAINER

CONTAINER can be a container name or a container ID.

The command does not remove the specified container if the container is running. To remove a running container, use the -f option.

For a complete list of podman rm options, run:

# podman rm --help

Note: Deleting all stopped containers

You can delete all stopped containers from your host with a single command:

# podman container prune

Make sure that each stopped container is intended to be removed before you run the command, otherwise you might remove containers that are still in use and were stopped only temporarily.

The command podman pod create is used to create a pod. The syntax of the command is as follows:

# podman pod create [OPTIONS]

The command outputs the pod ID. By default, the pods are created without being started. You can start a pod by running a container in the pod, or by starting the pod as described in Section 6.3, “Starting/stopping/restarting pods”.

Note: Default pod names

If you do not specify a pod name with the --name option, Podman will assign a default name for the pod.

For a complete list of possible options, run the following command:

# podman pod create --help

You can list all pods by running the command:

# podman pod list

The output looks as follows:

POD ID        NAME               STATUS   CREATED       # OF CONTAINERS  INFRA ID
30fba506fecb  upbeat_mcclintock  Created  19 hours ago  1                4324f40c9651
976a83b4d88b  nervous_feynman    Running  19 hours ago  2                daa5732ecd02

As each pod includes the INFRA container, the number of containers in a pod is always larger than zero.

After a pod is created, you must start it, as it is not in the state running by default. In the commands below, POD can be a pod name or a pod ID.

To start a pod, run the command:

# podman pod start [OPTIONS] POD

For a complete list of possible options, run:

# podman pod start --help

To stop a pod, use the podman pod stop as follows:

# podman pod stop POD

To restart a pod, use the podman pod restart command as follows:

# podman pod restart POD

To add a new container to a pod, use the podman run command with the option --pod. A general syntax of the command follows:

# podman run [OPTIONS] --pod POD_NAME IMAGE

For details about the podman run command, refer to Section 5.1, “Running containers”.

Note: Only new containers can be added to a pod

The podman start command does not allow for starting a container in a pod if the container was not added to the pod during the container's initial running.

You cannot remove a container from a pod and keep the container running, because the container itself is removed from the host.

Other actions like start, restart, and stop can be performed on specific containers without affecting the status of the pod.

There are two ways to remove pods. You can use the podman pod rm command to remove one or more pods. Alternatively, you can remove all stopped pods using the podman pod prune command.

To remove a pod or several pods, run the podman pod rm command as follows:

# podman pod rm POD

POD can be a pod name or a pod ID.

To remove all currently stopped pods, use the podman pod prune command. Make sure that all stopped pods are intended to be removed before you run the podman pod prune command, otherwise you might remove pods that are still in use.

To view all containers in all pods, use the following command:

# podman ps -a --pod

The output of the command will be similar to the following one:

CONTAINER ID  IMAGE                       COMMAND    CREATED       STATUS                 [...]
4324f40c9651  k8s.gcr.io/pause:3.2                   21 hours ago  Created
daa5732ecd02  k8s.gcr.io/pause:3.2                   22 hours ago  Up 3 hours ago
e5c8e360c54b  localhost/test:latest       /bin/bash  3 days ago    Exited (137) 3 days ago
82dad15828f7  localhost/opensuse/toolbox  /bin/bash  3 days ago    Exited (137) 3 days ago
1a23da456b6f  docker.io/i386/ubuntu       /bin/bash  4 days ago    Exited (0) 6 hours ago
df890193f651  localhost/opensuse/toolbox  /bin/bash  4 days ago    Created

The first two records are the INFRA containers of each pod, based on the k8s.gcr.io/pause:3.2 image. Other containers in the output are stand-alone containers that do not belong to any pod.