# Kubernetes Quick Reference

*kubectl, pods, deployments, services, configs, debugging*

> Source: Kubernetes Documentation (kubernetes.io/docs) · MIT

## kubectl Basics

### Cluster Info

```
kubectl cluster-info
kubectl get nodes
kubectl config current-context
kubectl config use-context my-cluster
```

### Essential Commands

| Command | Description |
|---------|-------------|
| `kubectl get <resource>` | List resources |
| `kubectl describe <resource> <name>` | Detailed resource info |
| `kubectl create -f file.yaml` | Create resource from file |
| `kubectl apply -f file.yaml` | Create or update resource |
| `kubectl delete -f file.yaml` | Delete resource from file |
| `kubectl edit <resource> <name>` | Edit resource in-place |
| `kubectl api-resources` | List all resource types |

### Output Formats

| Command | Description |
|---------|-------------|
| `-o wide` | Extra columns (IP, node) |
| `-o yaml` | Full YAML output |
| `-o json` | Full JSON output |
| `-o jsonpath='{.spec}'` | Extract specific fields |
| `--sort-by=.metadata.name` | Sort output by field |

## Pods

### Pod Operations

```
kubectl get pods
kubectl get pods -A                  # all namespaces
kubectl run nginx --image=nginx      # quick pod
kubectl delete pod nginx
```

### Pod YAML

```
apiVersion: v1
kind: Pod
metadata:
  name: myapp
  labels: { app: myapp }
spec:
  containers:
    - name: app
      image: nginx:1.27
      ports:
        - containerPort: 80
```

### Pod Status Values

| Command | Description |
|---------|-------------|
| `Running` | All containers started |
| `Pending` | Waiting for scheduling or image pull |
| `CrashLoopBackOff` | Container keeps crashing and restarting |
| `ImagePullBackOff` | Cannot pull container image |
| `Completed` | Ran to completion (Jobs) |

## Deployments

### Deployment YAML

```
apiVersion: apps/v1
kind: Deployment
metadata:
  name: web
spec:
  replicas: 3
  selector:
    matchLabels: { app: web }
  template:
    metadata:
      labels: { app: web }
    spec:
      containers:
        - name: web
          image: nginx:1.27
          ports:
            - containerPort: 80
```

### Deployment Commands

| Command | Description |
|---------|-------------|
| `kubectl get deploy` | List deployments |
| `kubectl scale deploy web --replicas=5` | Scale replicas |
| `kubectl set image deploy/web web=nginx:1.28` | Update image (rolling) |
| `kubectl rollout status deploy/web` | Watch rollout progress |
| `kubectl rollout undo deploy/web` | Rollback to previous revision |
| `kubectl rollout history deploy/web` | View revision history |

## Services

### Service Types

| Command | Description |
|---------|-------------|
| `ClusterIP` | Internal only (default) |
| `NodePort` | Expose on each node's IP at a static port |
| `LoadBalancer` | External load balancer (cloud) |
| `ExternalName` | DNS alias to external service |

### Service YAML

```
apiVersion: v1
kind: Service
metadata:
  name: web-svc
spec:
  type: ClusterIP
  selector: { app: web }
  ports:
    - port: 80
      targetPort: 80
```

### Quick Expose

```
kubectl expose deploy web --port=80 --type=ClusterIP
kubectl expose deploy web --port=80 --type=NodePort
kubectl get svc
```

## ConfigMaps & Secrets

### ConfigMap

```
kubectl create configmap app-cfg \
  --from-literal=DB_HOST=db.example.com \
  --from-file=config.ini
```

### Secret

```
kubectl create secret generic db-creds \
  --from-literal=username=admin \
  --from-literal=password=s3cret
```

### Using in Pods

```
# As environment variables
envFrom:
  - configMapRef: { name: app-cfg }
  - secretRef:    { name: db-creds }

# As volume mount
volumes:
  - name: cfg
    configMap: { name: app-cfg }
```

### Commands

| Command | Description |
|---------|-------------|
| `kubectl get cm` | List ConfigMaps |
| `kubectl get secret` | List Secrets |
| `kubectl describe cm app-cfg` | Show ConfigMap data |
| `kubectl get secret db-creds -o yaml` | Show Secret (base64-encoded) |

## Namespaces

### Namespace Commands

| Command | Description |
|---------|-------------|
| `kubectl get ns` | List namespaces |
| `kubectl create ns staging` | Create namespace |
| `kubectl delete ns staging` | Delete namespace and all resources |
| `kubectl get pods -n staging` | List pods in namespace |
| `kubectl get pods -A` | List pods across all namespaces |

### Set Default Namespace

```
kubectl config set-context --current \
  --namespace=staging
```

## Volumes

### PersistentVolumeClaim

```
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: data-pvc
spec:
  accessModes: [ReadWriteOnce]
  resources:
    requests: { storage: 10Gi }
```

### Mount in Pod

```
volumes:
  - name: data
    persistentVolumeClaim:
      claimName: data-pvc
containers:
  - volumeMounts:
      - name: data
        mountPath: /app/data
```

### Volume Types

| Command | Description |
|---------|-------------|
| `emptyDir` | Temp dir, deleted with pod |
| `hostPath` | Mount host filesystem path |
| `persistentVolumeClaim` | Persistent storage (PVC) |
| `configMap` | Mount ConfigMap as files |
| `secret` | Mount Secret as files |

## Ingress

### Ingress YAML

```
apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: web-ingress
spec:
  rules:
    - host: app.example.com
      http:
        paths:
          - path: /
            pathType: Prefix
            backend:
              service:
                name: web-svc
                port: { number: 80 }
```

### Ingress Notes

| Command | Description |
|---------|-------------|
| `Ingress Controller` | Required (nginx-ingress, traefik, etc.) |
| `pathType: Prefix` | Match URL prefix |
| `pathType: Exact` | Match exact URL path |
| `TLS` | Add `tls:` section with secret name |

## Debugging

### Diagnostic Commands

| Command | Description |
|---------|-------------|
| `kubectl logs <pod>` | Container stdout/stderr |
| `kubectl logs <pod> -c <ctr>` | Specific container logs |
| `kubectl logs <pod> --previous` | Logs from crashed container |
| `kubectl describe pod <pod>` | Events, conditions, status |
| `kubectl exec -it <pod> -- sh` | Shell into container |
| `kubectl port-forward <pod> 8080:80` | Forward local port to pod |
| `kubectl top pods` | CPU/memory usage (metrics-server) |
| `kubectl get events --sort-by=.lastTimestamp` | Cluster events timeline |

### Debug Pod

```
kubectl run debug --rm -it --image=busybox -- sh
# or attach ephemeral container
kubectl debug -it <pod> --image=busybox
```

## Common Patterns

### Labels & Selectors

```
kubectl get pods -l app=web
kubectl get pods -l 'env in (prod,staging)'
kubectl label pod myapp env=prod
```

### Resource Limits

```
resources:
  requests: { cpu: 100m, memory: 128Mi }
  limits:   { cpu: 500m, memory: 256Mi }
```

### Liveness & Readiness

```
livenessProbe:
  httpGet: { path: /healthz, port: 8080 }
  initialDelaySeconds: 5
  periodSeconds: 10
readinessProbe:
  httpGet: { path: /ready, port: 8080 }
```

### Quick Recipes

| Command | Description |
|---------|-------------|
| `Dry run` | `kubectl apply -f file.yaml --dry-run=client` |
| `Generate YAML` | `kubectl create deploy web --image=nginx --dry-run=client -o yaml` |
| `Watch` | `kubectl get pods -w` |
| `Copy files` | `kubectl cp file.txt pod:/tmp/` |
| `Restart deploy` | `kubectl rollout restart deploy/web` |