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RHCSA - Manage Containers: Perform Basic Container Management such as Running, Starting, Stopping, & Listing Running Containers
Listing Containers
To list running containers you use the podman ps command and if you want to list all containers, whether they are running or stopped you use the podman ps -a command. You will use these commands in the upcoming exercises to validate the state of containers.
Running a Container
Running a container means creating and starting a new instance of a containerized application. This process involves specifying various parameters, such as which image to use, port mappings, environment variables, and more, using the podman run command.
Without specifying the -d (detach) option, the container will run in the foreground, which is usually not what we want. Also fo containers that you wish to serve content over the network you may need to specify the -p <localport>:<containerport> option.
Run an nginx container:
To run an nginx container:
podman run -d -p 8080:80 nginx
The containers ID will print to screen when running the command. For me the ID was:
01521c4608c6b12a45067256f8a38bba87e6a316c0bf4b95cb4cca42ada051ec
List details for our running container:
podman ps
The output will resemble the following, which confirms localhost 8080 points to container port 80. Also note that the container ID is shortened:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
01521c4608c6 docker.io/ubuntu/nginx:latest nginx -g daemon o... 3 minutes ago Up 3 minutes 0.0.0.0:8080->80/tcp recursing_kepler
Confirm the container is working as expected:
curl http://localhost:8080
The nginx welcome page should display:
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
html { color-scheme: light dark; }
body { width: 35em; margin: 0 auto;
font-family: Tahoma, Verdana, Arial, sans-serif; }
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>
<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>
<p><em>Thank you for using nginx.</em></p>
</body>
</html>
Stopping a Container
Stopping a container in halts the execution of all processes within the container, effectively pausing its operation. This action is initiated using the podman stop command followed by the container's name or ID. When a container is stopped, it gracefully terminates its running processes, allowing them to clean up and release any resources they were using. Stopping a container is useful when you want to temporarily halt its operation, save its state, or perform maintenance tasks.
Stop the running nginx container:
The nginx container in the previous exercise should still be running. To stop it, run the following, replacing the container ID I have used with the container ID you received hen running the podman container ls command:
podman stop 01521c4608c6
The container ID will print to screen when running this command:
01521c4608c6
Running podman ps will no longer display the container as it is not running. You need to include the -a option to display stopped containers:
podman ps -a
The output will show STATUS of Exited:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
01521c4608c6 docker.io/ubuntu/nginx:latest nginx -g daemon o... 14 minutes ago Exited (0) 2 minutes ago 0.0.0.0:8080->80/tcp recursing_kepler
Starting a Container
Starting a container refers to initializing a previously created container that is in a stopped state. When you start a container, it transitions from an inactive state to an active one, allowing it to execute its designated tasks and services. This operation uses the podman start command followed by the container's name or ID. Starting a container doesn't change its configuration; it retains the settings and data defined during its creation.
Start the stopped nginx container:
To start back up the nginx container you use the podman start command with the container ID as an argument:
podman start 01521c4608c6
The container ID will print to screen when running this command:
01521c4608c6
Confirm the container is running:
podman ps
The output will resemble:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
01521c4608c6 docker.io/ubuntu/nginx:latest nginx -g daemon o... 3 minutes ago Up 3 minutes 0.0.0.0:8080->80/tcp recursing_kepler
Rather than running a curl test, this time we will connect to the container using the podman exec -it command to run a shell process:
podman exec -it 01521c4608c6 /bin/sh
As the /bin/sh shell was chosen as the executable you get dropped into the container running that shell. From there you can run a limited set of commands, such as:
df -h
ps -ef
free -h
Use Ctrl+D to detach from the container.
Restarting Containers
Restarting a container in involves stopping the container if it's currently running and then immediately starting it again. This action is typically performed using the podman restart command followed by the container's name or ID. When a container is restarted, it goes through the process of stopping and then starting, which can be useful for applying configuration changes, refreshing the container's environment, or recovering from minor issues without having to recreate the container entirely.
Restart the nginx container:
To restart the nginx container you use the podman restart command with the container ID as an argument:
podman restart 01521c4608c6
The container ID will print to screen when running this command:
01521c4608c6
Confirm the container is running:
podman ps
The output will resemble:
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
01521c4608c6 docker.io/ubuntu/nginx:latest nginx -g daemon o... 24 minutes ago Up 34 seconds 0.0.0.0:8080->80/tcp recursing_kepler
This time, to check if the container restarted ok, we will use the podman top command which displays the processes running within the container:
podman top 01521c4608c6
The output should resemble:
USER PID PPID %CPU ELAPSED TTY TIME COMMAND
root 1 0 0.000 7m46.568465998s ? 0s nginx: master process nginx -g daemon off;
www-data 13 1 0.000 7m46.568578753s ? 0s nginx: worker process
www-data 14 1 0.000 7m46.568619465s ? 0s nginx: worker process
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