A Pod would be the smallest deployable unit one can create and manage inside Kubernetes(K8s). In practice, rarely you create a Pod directly.
Instead, one would create a Deployment, a StatefulSet, a DaemonSet, a Job, or a CronJob. These are higher-level constructs that would create
Pods for you.
Here I'm covering common tasks that one would do with a Pod in daily work, like getting logs, opening a shell inside a container, debugging a Pod, etc.
If you don't have access to other K8s clusters where you can execute these examples, go to my quick intro to k3d. I assume that you
already know what kubectl is, and you have it installed.
All these examples are run locally with kind. To add resources to the cluster run:
#!bin/sh
#
kubectl create ns apps
kubectl create ns dbs
kubectl create ns workers
kubectl create ns kafka
kubectl create deployment -n apps nginx --image nginx:stable-alpine
kubectl create deployment -n workers nginx --image nginx:stablex-alpine
kubectl create deployment -n dbs postgres --image postgres:15.2-alpine
Find/List/Filter pods
You are looking for a Pod. All you know is an application name that is running in a Pod. How do you find it?
As already mentioned, Pods are usually created by higher-level constructs. And usually, they are named by an application that they are running. If
that is the case pods will include the application name in their name. One way would be:
> kubectl get pods -A | grep <application-name>
For example:
❯ kubectl get pods -A | grep nginx
apps nginx-9dc8cc55b-9lcpg 1/1 Running 0 2m52s
workers nginx-7f685bb9bb-4wh6p 0/1 ImagePullBackOff 0 100s
Command kubectl get pods would search for Pods in the current namespace. If you want to search in all namespaces use -A flag. The output needs
to be filtered by grep to find the Pod that you are looking for. In this case, Pods are running in apps and workers namespaces. To see the
default namespace use kubectl config get-contexts command:
❯ kubectl config get-contexts
CURRENT NAME CLUSTER AUTHINFO NAMESPACE
cloud-run-dev-internal cloud-run-dev-internal cloud-run-dev-internal default
dev dev bgates default
* kind-kind-test-cluster kind-kind-test-cluster kind-kind-test-cluster
If you follow the best practices for labeling resources
you can get search for Pods by labels. In these examples I'm using the app label:
> kubectl get pods -l app=<app-name>
For example:
❯ kubectl get pods -A -l app=nginx
NAMESPACE NAME READY STATUS RESTARTS AGE
apps nginx-9dc8cc55b-9lcpg 1/1 Running 0 19m
workers nginx-7f685bb9bb-4wh6p 0/1 ImagePullBackOff 0 18m
Of course, Pods can have different labels that can be used for searching.
Get additional informations about a Pod/Debug Pods
To learn more about a Pod you can get its manifest with kubectl get pod <pod-name> -o yaml command. I find this rather unpractical so I use
kubectl describe pod <pod-name> command. It gives me more information about a Pod in a human-readable format:
❯ kubectl describe pod -n apps nginx-9dc8cc55b-9lcpg
Name: nginx-9dc8cc55b-9lcpg
Namespace: apps
Priority: 0
Service Account: default
Node: kind-test-cluster-worker/172.18.0.2
Start Time: Sat, 25 Feb 2023 18:29:08 +0100
Labels: app=nginx
pod-template-hash=9dc8cc55b
Annotations: <none>
Status: Running
IP: 10.244.1.3
IPs:
IP: 10.244.1.3
Controlled By: ReplicaSet/nginx-9dc8cc55b
Containers:
nginx:
Container ID: containerd://4a09db88bcc07dcaab7f5cef4277cb7695cdeff63524e0b037cde8cb1f03eaa2
Image: nginx:stable-alpine
[...snip...]
Node-Selectors: <none>
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 24m default-scheduler Successfully assigned apps/nginx-9dc8cc55b-9lcpg to kind-test-cluster-worker
Normal Pulled 24m kubelet Container image "nginx:stable-alpine" already present on machine
Normal Created 24m kubelet Created container nginx
Normal Started 24m kubelet Started container nginx
Sometimes your pods can not start. So you do not have logs to check. Usually, these are cases when Pods are in CrashLoopBackOff or
ImagePullBackOff state. Then describe a Pod and look under the Events section. It will give you information about what is going on.
For example:
❯ kubectl get pods -A -l app=nginx
NAMESPACE NAME READY STATUS RESTARTS AGE
apps nginx-9dc8cc55b-9lcpg 1/1 Running 0 43m
workers nginx-7f685bb9bb-4wh6p 0/1 ImagePullBackOff 0 42m
Why is nginx-7f685bb9bb-4wh6p in ImagePullBackOff state? Let's check:
❯ kubectl describe pods -n workers nginx-7f685bb9bb-4wh6p
Name: nginx-7f685bb9bb-4wh6p
Namespace: workers
Priority: 0
Service Account: default
[...snip...]
Tolerations: node.kubernetes.io/not-ready:NoExecute op=Exists for 300s
node.kubernetes.io/unreachable:NoExecute op=Exists for 300s
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal Scheduled 42m default-scheduler Successfully assigned workers/nginx-7f685bb9bb-4wh6p to kind-test-cluster-worker2
Normal Pulling 41m (x4 over 42m) kubelet Pulling image "nginx:stablex-alpine"
Warning Failed 41m (x4 over 42m) kubelet Failed to pull image "nginx:stablex-alpine": rpc error: code = NotFound desc = failed to pull and unpack image "docker.io/library/nginx:stablex-alpine": failed to resolve reference "docker.io/library/nginx:stablex-alpine": docker.io/library/nginx:stablex-alpine: not found
Warning Failed 41m (x4 over 42m) kubelet Error: ErrImagePull
Warning Failed 40m (x6 over 42m) kubelet Error: ImagePullBackOff
Normal BackOff 2m40s (x172 over 42m) kubelet Back-off pulling image "nginx:stablex-alpine"
As you can see the image nginx:stablex-alpine does not exist. That is why the Pod can not start.
Get logs
To get logs from a Pod you can use kubectl logs <pod-name> command. It will give you logs from all containers in Pod. If you
want to get logs from a specific container inside a Pod you can use -c <container-name> flag. For example:
❯ kubectl logs -n apps nginx-9dc8cc55b-9lcpg -c nginx
/docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration
/docker-entrypoint.sh: Looking for shell scripts in /docker-entrypoint.d/
/docker-entrypoint.sh: Launching /docker-entrypoint.d/10-listen-on-ipv6-by-default.sh
10-listen-on-ipv6-by-default.sh: info: Getting the checksum of /etc/nginx/conf.d/default.conf
10-listen-on-ipv6-by-default.sh: info: Enabled listen on IPv6 in /etc/nginx/conf.d/default.conf
/docker-entrypoint.sh: Launching /docker-entrypoint.d/20-envsubst-on-templates.sh
/docker-entrypoint.sh: Launching /docker-entrypoint.d/30-tune-worker-processes.sh
/docker-entrypoint.sh: Configuration complete; ready for start up
2023/02/25 17:29:09 [notice] 1#1: using the "epoll" event method
2023/02/25 17:29:09 [notice] 1#1: nginx/1.22.1
2023/02/25 17:29:09 [notice] 1#1: built by gcc 11.2.1 20220219 (Alpine 11.2.1_git20220219)
2023/02/25 17:29:09 [notice] 1#1: OS: Linux 5.15.49-linuxkit
2023/02/25 17:29:09 [notice] 1#1: getrlimit(RLIMIT_NOFILE): 1048576:1048576
2023/02/25 17:29:09 [notice] 1#1: start worker processes
2023/02/25 17:29:09 [notice] 1#1: start worker process 33
2023/02/25 17:29:09 [notice] 1#1: start worker process 34
2023/02/25 17:29:09 [notice] 1#1: start worker process 35
2023/02/25 17:29:09 [notice] 1#1: start worker process 36
It will output logs on stdout. If you do not want to repeat this command all the time you can use -f flag to follow logs. Then
when ever new logs are added to the Pod you will see them in your console.
Execute a command in a Pod/Open shell in a Pod
Sometimes you need to execute a command in a Pod. Like to see all environment variables, so you do something like kubectl exec <pod-name> -- <command>:
❯ kubectl exec -n apps nginx-9dc8cc55b-9lcpg -- env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=nginx-9dc8cc55b-9lcpg
NGINX_VERSION=1.22.1
NJS_VERSION=0.7.7
PKG_RELEASE=1
KUBERNETES_PORT_443_TCP_ADDR=10.96.0.1
KUBERNETES_SERVICE_HOST=10.96.0.1
KUBERNETES_SERVICE_PORT=443
KUBERNETES_SERVICE_PORT_HTTPS=443
KUBERNETES_PORT=tcp://10.96.0.1:443
KUBERNETES_PORT_443_TCP=tcp://10.96.0.1:443
KUBERNETES_PORT_443_TCP_PROTO=tcp
KUBERNETES_PORT_443_TCP_PORT=443
HOME=/root
But if you want to open a shell in a Pod and interact with it you can use kubectl exec -it <pod-name> -- <command> command. For example:
❯ kubectl exec -n apps nginx-9dc8cc55b-9lcpg -it -- sh
/ #
Close shell with exit command or Ctrl+D.
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