Retrieving Kubernetes Logs

Name: retrieving-k8s-logs
Rating: 92
Author: rio

Patterns for retrieving container logs effectively.

Basic Log Commands

bash

# Current container logs
kubectl logs <pod> -n <ns>

# Last N lines
kubectl logs <pod> -n <ns> --tail=100

# Logs since time
kubectl logs <pod> -n <ns> --since=1h
kubectl logs <pod> -n <ns> --since=30m

# With timestamps
kubectl logs <pod> -n <ns> --timestamps

Multi-Container Pods

bash

# List containers in pod
kubectl get pod <pod> -n <ns> -o jsonpath='{.spec.containers[*].name}'

# Logs from specific container
kubectl logs <pod> -n <ns> -c <container>

# Logs from all containers
kubectl logs <pod> -n <ns> --all-containers

Previous Container (After Crash)

bash

# Logs from previous container instance (after restart/crash)
kubectl logs <pod> -n <ns> --previous

# Previous logs from specific container
kubectl logs <pod> -n <ns> -c <container> --previous

Use --previous when:

•Pod is in CrashLoopBackOff
•Container restarted and you need pre-crash logs
•Current container logs don't show the error

Init Container Logs

bash

# List init containers
kubectl get pod <pod> -n <ns> -o jsonpath='{.spec.initContainers[*].name}'

# Get init container logs
kubectl logs <pod> -n <ns> -c <init-container-name>

Label-Based Log Aggregation

bash

# Logs from all pods with label
kubectl logs -l app=<app-name> -n <ns>

# With tail limit per pod
kubectl logs -l app=<app-name> -n <ns> --tail=50

# All containers in labeled pods
kubectl logs -l app=<app-name> -n <ns> --all-containers

Searching Logs

bash

# Grep for pattern
kubectl logs <pod> -n <ns> | grep -i error

# Grep with context
kubectl logs <pod> -n <ns> | grep -i -A5 -B5 "exception"

# Count occurrences
kubectl logs <pod> -n <ns> | grep -c "error"

Log Patterns for Debugging

Application Crash

bash

# Get last 200 lines before crash
kubectl logs <pod> -n <ns> --previous --tail=200

# Look for error/exception
kubectl logs <pod> -n <ns> --previous | grep -i -E "error|exception|fatal|panic"

Startup Issues

bash

# First 100 lines (startup)
kubectl logs <pod> -n <ns> | head -100

# Check init containers first
kubectl logs <pod> -n <ns> -c <init-container>

Intermittent Issues

bash

# Logs from last 2 hours with timestamps
kubectl logs <pod> -n <ns> --since=2h --timestamps

# Tail and follow (for live debugging)
# Note: This blocks, use Ctrl+C to stop
kubectl logs <pod> -n <ns> -f --tail=50

Memory Issues (OOM)

bash

# Check for memory-related messages before OOM
kubectl logs <pod> -n <ns> --previous | grep -i -E "memory|heap|oom|gc"

Log Output Formats

bash

# Raw logs (default)
kubectl logs <pod> -n <ns>

# With pod name prefix (useful for multi-pod)
kubectl logs -l app=<app> -n <ns> --prefix

# Limit bytes (for huge logs)
kubectl logs <pod> -n <ns> --limit-bytes=50000

Quick Debugging Pattern

bash

# 1. Check current logs for errors
kubectl logs <pod> -n <ns> --tail=100 | grep -i error

# 2. If pod crashed, check previous logs
kubectl logs <pod> -n <ns> --previous --tail=200

# 3. If init container failed
kubectl get pod <pod> -n <ns> -o jsonpath='{.spec.initContainers[*].name}'
kubectl logs <pod> -n <ns> -c <init-container>

# 4. Check all containers in pod
kubectl logs <pod> -n <ns> --all-containers --tail=50

Notes

•--previous only works if container restarted (keeps one previous log)
•Logs are ephemeral; they're lost when pod is deleted
•For persistent logs, use a logging stack (EFK, Loki, etc.)
•Load debugging-k8s-pods for container state analysis