dmesg is used to view and control the kernel ring buffer. Its core functions include: 1. Display kernel messages; 2. Filter specific information; 3. Clear or control the buffer. By default, running dmesg will output all kernel messages, which is suitable for troubleshooting hardware problems and startup process analysis. You can view and use the --level parameter to filter error level information through the pipeline combined with less paging or use the --level parameter to filter error level information. Users can also use grep to filter such as USB, disk I/O errors by keywords, and add timestamps through the -T option to assist in log association. If necessary, use -dmesg -C to clear the message buffer, -D and -E to disable or enable console logging, but be careful to avoid affecting subsequent troubleshooting.
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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