How to log in with kubectl exec when multiple containers are running in a Kubernetes Pod

I'm Teraoka, an infrastructure engineer.
This time, I'll be talking about Kubernetes.

As described in the URL below,

we will introduce how to run kubectl exec on a pod running multiple containers and

https://kubernetes.io/ja/docs/tasks/debug-application-cluster/get-shell-running-container/

Start the Pod

First, let's start a Pod according to the following manifest.
It is a simple Pod that uses a deployment controller to start a Pod running an Nginx container.

apiVersion: apps/v1 kind: Deployment metadata: name: example-web namespace: default spec: selector: matchLabels: app: example-web replicas: 1 template: metadata: labels: app: example-web spec: containers: - name: nginx image: nginx:latest imagePullPolicy: Always ports: - containerPort: 80

Reflect the manifest

$ Kubectl apply -f deployment_example_web.yaml

Check if the Pod is up and running

$ kubectl get pods NAME READY STATUS RESTARTS AGE example-web-59fccdb6d4-slrkv 1/1 Running 0 13s

It's up and running.
To log in to the container running in this Pod,

$ kubectl exec -it example-web-59fccdb6d4-slrkv bash root@example-web-59fccdb6d4-slrkv:/# 

Then you should be able to log in.
Next, let's edit the manifest file a bit.

apiVersion: apps/v1 kind: Deployment metadata: name: example-web namespace: default spec: selector: matchLabels: app: example-web replicas: 1 template: metadata: labels: app: example-web spec: containers: - name: nginx image: nginx:latest imagePullPolicy: Always ports: - containerPort: 80 - name: php-fpm image: php:7.4-fpm imagePullPolicy: Always ports: - containerPort: 9000

I also added a PHP-FPM container.
The commands to reflect this are the same as before, so I will omit them, but
when the Pod starts, the READY part will become 2/2.

$ kubectl get pods NAME READY STATUS RESTARTS AGE example-web-85cd8dbd8d-24rx4 2/2 Running 0 69s

Let's run the same kubectl exec as before

$ kubectl exec -it example-web-85cd8dbd8d-24rx4 bash Defaulting container name to nginx. Use 'kubectl describe pod/example-web-85cd8dbd8d-24rx4 -n default' to see all of the containers in this pod.

A message was displayed.
It meant that "The default container is named nginx, so check all container information with kubectl describe."
In other words, if you want to log in to a PHP-FPM container, you cannot use the above command.
You can log in by using the --container option as follows:

$ kubectl exec -it example-web-85cd8dbd8d-24rx4 --container php-fpm bash root@example-web-85cd8dbd8d-24rx4:/var/www/html# 

I was able to log in!

summary

It may seem obvious, but I'll leave it here as a reminder.
I need to study Kubernetes more...