(RHCSA) Manage Containers

This is part of the independent and unofficial RHCSA Exam Study Guide series providing free 🤑 resources to prepare for the exam.

This post covers the objectives under the section:

“MANAGE CONTAINERS”

It contains two main parts:

• Resources: with links to learn and practice for each objective.
• Cheatsheet: containing some examples of commands and actions performed in each objective (when applied).

📚 Resources:

• 📌 What is a Container?

• 📌 Linux Container Internals: Lab 1: Container Architecture

• 📌 Containerization Explained

FIND AND RETRIEVE CONTAINER IMAGES FROM A REMOTE REGISTRY

• 📌 Chapter 3. Working with container images (items 3.1. to 3.5.)

INSPECT CONTAINER IMAGES

• 📌 3.6. Inspecting local images

• 📌 3.7. Inspecting remote images

• 📌 4.6. Inspecting containers from the host

PERFORM CONTAINER MANAGEMENT USING COMMANDS SUCH AS PODMAN AND SKOPEO

(Covered in previous and next items)

PERFORM BASIC CONTAINER MANAGEMENT SUCH AS RUNNING, STARTING, STOPPING, AND LISTING RUNNING CONTAINERS

• 📌 4.1. Podman run command

• 📌 4.4. Listing containers

• 📌 4.5. Starting containers

• 📌 4.12. Exporting and importing containers

• 📌 4.13. Stopping containers

• 📌 4.14. Removing containers

RUN A SERVICE INSIDE A CONTAINER

• 📌 4.2. Running commands in a container from the host

• 📌 4.3. Running commands inside the container

• 📌 4.9. Running a service as a daemon with a static IP

• 📌 4.10. Executing commands inside a running container

CONFIGURE A CONTAINER TO START AUTOMATICALLY AS A SYSTEMD SERVICE

• 📌 Chapter 10. Porting containers to systemd using Podman (items 10.1. to 10.4.)

ATTACH PERSISTENT STORAGE TO A CONTAINER

• 📌 4.7. Mounting directory on localhost to the container

• 📌 4.11. Sharing files between two containers

📑 Cheatsheet:

(Requires RHEL 8.3 or above, and container-tools module)

FIND AND RETRIEVE CONTAINER IMAGES FROM A REMOTE REGISTRY

• Configure (user-level) container registries

  • create $HOME/.config/containers/registries.conf ➡ file to override the system-wide settings, and configure user level container regisitries.

  [registries.search]
  registries = ['registry.access.redhat.com', 'registry.redhat.io', 'docker.io']
    
  [registries.insecure]
  registries = []
    
  [registries.block]
  registries = []
  

• podman search caddy ➡ searches the predefined container registries for container images containing the word ‘caddy’.

• podman login quay.io ➡ authenticates to ‘quay.io’ container registry.

• podman search quay.io/postgresql-10 ➡ searches for the particular image on the specified registry.

• podman search -f is-official --limit 3 --no-trunc alpine ➡ searches for a particular image on all repositories, and displays only official images, maximum of 3, and full description.

• podman pull registry.redhat.io/ubi8/ubi ➡ pull specified image from specified registry.

INSPECT CONTAINER IMAGES

• podman images ➡ lists images downloaded to the system or created on the system.

• podman inspect caddy ➡ inspect local image named ‘caddy’.

• skopeo inspect docker://registry.redhat.io/ubi8/ubi-init | less ➡ inspect remote image, using less pager.

PERFORM CONTAINER MANAGEMENT USING COMMANDS SUCH AS PODMAN AND SKOPEO

• podman tag docker.io/library/mariadb localstable ➡ add localstable tag to local image.

PERFORM BASIC CONTAINER MANAGEMENT SUCH AS RUNNING, STARTING, STOPPING, AND LISTING RUNNING CONTAINERS

• podman exec myubi ls -la ➡ execute ls -la command inside the myubi running container, and detach.

• podman attach myubi ➡ attach to myubi running container.

• podman stop myubi && podman rm myubi ➡ stop and remove myubi container.

• podman run --rm alpine ls /etc ➡ start container from alpine image, run the command ls /etc, exit and remove the container.

• podman run --name=myalpine -it alpine /bin/sh ➡ start container from alpine image, in interactive mode providing the /bin/sh shell, apply myalpine name to the container.

• podman run -d mysql ➡ start container from mysql image and detach the session, container keeps running.

• podman ps -a ➡ show running and stopped containers.

• podman start myubi ➡ start container named myubi detached.

• podman start -a -i myubi ➡ start container named myubi attached and interactive mode.

RUN A SERVICE INSIDE A CONTAINER

• item ➡ description.

CONFIGURE A CONTAINER TO START AUTOMATICALLY AS A SYSTEMD SERVICE

• podman create --name myhttpd docker.io/library/httpd ➡ create a container (do not start/run it).

• podman generate systemd --name myhttpd > ~/.config/systemd/user/container-myhttpd.service ➡ generate the container’s systemd unit file (create directory if needed). Inside the unit file, ExecStart run podman start command so, the container must exist in the host system.

• Auto-generate a systemd unit file that auto-generate a container:

  • Create a new user (i.e.: containeruser) and open a terminal session with it.

  • podman create --name myhttpd2 httpd ➡ create a container (do not start/run it).

  • podman generate systemd --new --files --name myhttp2 ➡ generate the container’s systemd unit file. Inside the unit file, ExecStart run podman run command so, the container will be created on start.

  • cp container-myhttpd2.service ~/.config/systemd/user/ ➡ install it as user service.

• Auto-starting containers using systemd

  • systemctl --user daemon-reload
  • systemctl --user --now enable container-myhttpd.service
  • loginctl enable-linger && loginctl show-user containeruser ➡ make the service autostart without the need for containeruser to log in.
  • verify with:
    • systemctl --user status container-myhttpd.service
    • systemctl --user | grep container
    • podman ps -a

ATTACH PERSISTENT STORAGE TO A CONTAINER

• podman run --name="log_test" -v /dev/log:/dev/log --rm ubi logger "LOG THIS" ➡ create container log_test, mount the host /dev/log directory inside the container, create log message from the container.

MORE EXAMPLES

• podman info ➡ display podman system information.

• podman inspect --format='{{.Config.ExposedPorts}}' myalpine ➡ inspect local container named myalpine displaying only the selected item from the JSON object.

• podman run -dp 8080:80 --name http-serv docker.io/library/httpd ➡ create a container from the image, name it as http-serv and expose it’s port n. 80 to port 8080 in the host system.

• podman run -it -e HISTSIZE -e SECRET="Mysecret" --name ubi8-vars ubi8 ➡ create container ubi8-vars from ubi8 image, in interactive mode, setting up the specified environment variables.

• podman port http-serv ➡ check the http-serv container port mapping status.

• podman unshare ls -la myshares ➡ shows myshares directory properties, UID, GID, etc, as it is viewed by a rootless container.

• podman unshare chown 1000:1000 -R myshares ➡ change user and group owner inside the ‘user namespace’ for myshares directory recursively.

• podman run -u 1000 -it -v /home/user1/myshares:/mnt/persistent:Z myubi /bin/bash ➡ start container from myubi image, in interactive mode (bash shell), as user 1000, mount myshares directory at /mnt/persistent with a private unshared label (SELinux).

  • <Ctrl> + p, <Ctrl> + q ➡ detach from the container’s interactive mode and go back to the host shell (container keeps running).

• podman volume create hostvolume ➡ create new volume.

• podmand volume inspect hostvolume ➡ display information about the volume.

• mntPoint=$(podman volume inspect hostvolume --format {{.Mountpoint}}) ➡ save the volume mountpoint to mntPoint varable for easier manipulation.

• podman run -it -v hostvolume:/mnt/sharedvol myubi /bin/bash ➡ start container form myubi image, interactive mode in a bash shell, mount the hostvolume created at /mnt/sharedvol directory.

🎉 Congratulations!

You achieved the end of the learning for the RHCSA Exam!

(practice, reinforce, excercise, etc, check notes at:)

Red Hat Certified System Administrator (RHCSA) Exam Study Guide

Useful links & references:

• Red Hat Enterprise Linux Documentation

• Enable Sysadmin (a blog from Red Hat)

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