Dmesg Kernel Messages - ryzendew/Linux-Tips-and-Tricks GitHub Wiki

DMESG Kernel Messages Troubleshooting for Beginners

Table of Contents

  1. :pencil: What is dmesg?
  2. :zap: Basic Commands
  3. :mag: Filtering Messages
  4. :bulb: Common Troubleshooting
  1. :link: Related Commands
  1. :keyboard: Quick Reference

:pencil: What is dmesg?

  • dmesg displays kernel ring buffer messages
  • Shows hardware detection, driver loading, and kernel events
  • Essential for troubleshooting hardware and boot issues
  • Usually requires root privileges or proper permissions

What dmesg can do:

  • Show kernel boot messages
  • Display hardware detection events
  • Show driver loading/unloading
  • Reveal kernel errors and warnings
  • Help troubleshoot hardware problems

:zap: Basic Commands

Show All Kernel Messages

sudo dmesg

What this does:

  • Shows all kernel messages from boot to now
  • Displays hardware detection, driver events, errors
  • Very long output (usually piped to less or grep)

Note: On some systems, you may need sudo to read kernel messages.

Show Human-Readable Timestamps

sudo dmesg -T

What this does:

  • Shows timestamps in human-readable format
  • Easier to correlate with system events
  • Shows when events occurred

Example output:

[Thu Nov 13 07:33:58 2025] Linux version 6.17.7-cachyos1.fc43.x86_64
[Thu Nov 13 07:33:58 2025] Command line: BOOT_IMAGE=/vmlinuz-6.17.7-cachyos1.fc43.x86_64

Show Only Errors

sudo dmesg -l err

What this does:

  • Shows only error-level messages
  • Filters out informational messages
  • Useful for finding problems

Priority levels:

  • emerg - Emergency
  • alert - Alert
  • crit - Critical
  • err - Error
  • warn - Warning
  • notice - Notice
  • info - Info
  • debug - Debug

Show Only Warnings and Above

sudo dmesg -l warn

What this does:

  • Shows warnings, errors, and critical messages
  • Less verbose than all messages
  • Good balance for troubleshooting

Show Last N Lines

sudo dmesg | tail -20

What this does:

  • Shows the last 20 kernel messages
  • Useful for recent events
  • Quick check of recent activity

Clear Kernel Buffer

sudo dmesg -C

What this does:

  • Clears the kernel ring buffer
  • Useful for testing (seeing only new messages)
  • Warning: You'll lose all previous messages

:mag: Filtering Messages

Search for Specific Text

sudo dmesg | grep -i error

What this does:

  • Searches for "error" in kernel messages
  • Case-insensitive with -i
  • Useful for finding specific issues

Common searches:

sudo dmesg | grep -i "usb"
sudo dmesg | grep -i "network"
sudo dmesg | grep -i "fail"

Show Messages Since Boot

sudo dmesg -w

What this does:

  • Shows new messages as they appear
  • Like tail -f for kernel messages
  • Useful for monitoring in real-time

Exit with Ctrl+C

:bulb: Common Troubleshooting

Check Hardware Detection

sudo dmesg | grep -i "usb\|pci\|sata"

Shows USB, PCI, and SATA device detection.

Find Driver Issues

sudo dmesg | grep -i "driver\|module"

Shows driver loading and module information.

Check for Errors

sudo dmesg -l err

Shows only error-level messages.

View Boot Messages

sudo dmesg | head -50

Shows early boot messages.

:link: Related Commands

journalctl for Kernel Messages

journalctl -k

What this does:

  • Shows kernel messages via systemd journal
  • More features than dmesg
  • Can filter by time, priority, etc.

See: Journalctl Troubleshooting Guide

:keyboard: Quick Reference

sudo dmesg                    # All messages
sudo dmesg -T                 # Human-readable timestamps
sudo dmesg -l err             # Errors only
sudo dmesg -l warn            # Warnings and above
sudo dmesg | tail -20         # Last 20 messages
sudo dmesg | grep -i error    # Search for errors
sudo dmesg -w                 # Watch for new messages
sudo dmesg -C                 # Clear buffer

For system logs, see the Journalctl Troubleshooting Guide.