DMESG命令是用什麼？

dmesg用於查看和控制內核環形緩衝區，其核心功能包括：1. 顯示內核消息；2. 過濾特定信息；3. 清除或控制緩衝區。默認運行dmesg將輸出所有內核消息，適用於排查硬件問題及啟動過程分析，可通過管道結合less分頁查看或使用--level參數篩選錯誤級別信息。用戶還可利用grep按關鍵詞過濾如USB、磁盤I/O錯誤等，並通過-T選項添加時間戳以輔助日誌關聯。在必要時，可使用-dmesg -C清除消息緩衝區，-D和-E禁用或啟用控制台日誌記錄，但需謹慎操作以免影響後續故障診斷。

The dmesg command is used to view and control the kernel ring buffer, which stores messages generated by the Linux kernel. These messages include system startup information, hardware detection, driver loading, and various runtime events — both normal and error-related.

Viewing Kernel Messages

By default, running dmesg without any arguments will print all the messages from the kernel ring buffer to the terminal. This can be especially useful when troubleshooting hardware issues or understanding what happened during boot-up.

For example:

 dmesg

You'll see a long list of messages that might look cryptic at first, but they contain valuable details like detected devices, memory allocation, and errors.

If the output is too much to read at once, you can pipe it into a pager:

 dmesg | less

Or filter specific types of messages, such as those related to errors or warnings:

 dmesg --level=err,warn

This helps narrow down problems quickly without scanning through thousands of lines.

Filtering Output for Specific Information

Since the full output of dmesg can be overwhelming, filtering is key. You can use tools like grep to search for specific keywords such as disk names ( sda , nvme0n1 ), drivers, or error codes.

Examples:

• Check for USB-related messages:

   dmesg | grep usb

• Look for disk I/O errors:

   dmesg | grep -i "i/o"

• See if a particular module loaded correctly:

   dmesg | grep nouveau

These filters help isolate relevant logs, especially when debugging hardware or driver issues.

Also, timestamps are often useful. Adding the -T option shows human-readable timestamps next to each message:

 dmesg -T

This makes it easier to correlate kernel events with other logs or user-reported issues.

Clearing or Controlling the Ring Buffer (Use with Caution)

Although not commonly needed, dmesg allows you to clear the kernel log buffer using:

 dmesg -C

This clears all existing messages. It's usually only done in specific diagnostic scenarios or automated testing.

There's also an option to disable and re-enable the logging of messages to the console:

• Disable: dmesg -D
• Enable: dmesg -E

These options are rarely used outside of server maintenance or embedded systems work.

⚠️ Keep in mind: clearing logs can make troubleshooting harder later, so do this only when necessary and understand the consequences.

That's basically how most users interact with dmesg . It's a simple tool but incredibly powerful when you're trying to figure out what the kernel knows — especially when things aren't working quite right.

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