dmesg — check kernel messages

💬 "dmesg command"

dmesg displays kernel ring buffer messages. Disk errors, network link issues, and OOM kills are recorded here.

Because it is separate from user-space application logs, you should review it together when troubleshooting unexplained problems.

• Concept: Shows kernel messages in chronological order.
• Left: Kernel time in the format [ 12.345678] (seconds elapsed since boot).
• Contents: Drivers, disk I/O errors, OOM killer, link status, and more.
• -T: Converts to human-readable actual time.

Commonly used options

3 essential command combinations

• dmesg -T | tail -50 — Check recent kernel messages
• dmesg -T | grep -i -E 'oom|killed process' — Check whether OOM kills occurred
• dmesg -T | grep -i -E 'error|fail|i/o' — Find hardware and I/O errors

💬 "Show me an example of safely reverting this command"

Here are situations commonly encountered on live production servers, organized in order.

Step 1 · Check recent messages

View recent logs related to boot and devices.

dmesg -T | tail -50

Step 2 · Check for OOM

Check whether a process was killed due to insufficient memory.

dmesg -T | grep -i -E 'oom|killed process'

Step 3 · Check disk errors

Check for I/O errors.

dmesg -T | grep -i -E 'i/o error|medium error|ata'

How to use this in practice

1. Process suddenly dies

Situation — An app process terminates without warning.

dmesg -T | grep -i -E 'oom|killed process'

Result — If you see the message Out of memory: Killed process, the OOM killer terminated the process.

Point — Also check memory usage, and adjust swap or memory limits if needed.

2. Unexplained reboot

Situation — There is a record that the server rebooted in the early morning.

dmesg -T | head -30

Result — In the early boot messages, check hardware initialization and traces of the previous shutdown.

Point — When used together with journalctl --list-boots, you can identify the exact reboot time.

Important notes

• The ring buffer has a limited size, so old messages disappear. Use journal/rsyslog for permanent retention.
• Kernel time alone makes it hard to know the actual time, so use -T.
• Some distributions restrict dmesg access with kernel.dmesg_restrict.

Actual output

This is the result of checking for OOM kills and disk I/O errors.

!dmesg command output: kernel OOM and I/O errors

Live production server

Source (HTML): https://games.aidebatehub.com/programs/linux/logs/dmesg.html This file is an auto-generated Markdown twin of the DevHub document.