How to Use the dmesg Linux Command

The dmesg command is a useful tool in Linux that lets you check what’s happening with your system’s kernel. It helps you see important messages from the kernel, which can be helpful for fixing problems and improving your system’s performance. These kernel messages come from the kernel modules that are loaded when your system starts up.

In this tutorial, we will demonstrate what the dmesg command is and practical examples of using this command in Linux.

What is dmesg in Linux?

The dmesg command displays kernel messages on Unix-like systems, which provide information stored in a special memory area known as the ring buffer. This ring buffer captures important events related to hardware, device driver initialization, and messages from kernel modules during system startup and operation. The basic syntax to use this command is shown below:

dmesg [options]

The common options for dmesg command are listed below, along with their functionalities:

• -C: It clears the ring buffer.
• -c: Print and clear the ring buffer.
• -H: It provides the human-readable output, making timestamps and information more understandable
• -T: Displays timestamps in a human-readable format for each message. 
• -l: Filters messages by their log level, such as errors, warnings, or info.
• -k: Displays only kernel-related messages, hiding user-space messages.
• -s: Specifies how much buffer to read for messages, such as 1024 bytes.
• -f: Filters messages by their facility, like kernel or user messages.
• -r: Shows the raw output without converting priority info into readable text.
• -x: Converts the facility and log level into readable text, like “kernel.info”.

You can learn more about the options available for the Linux dmesg command by typing man dmesg in the terminal. This command opens the manual page for dmesg, where you can see detailed information about its usage, options, and examples:

man dmesg

Practical Examples of Using dmesg Command in Linux

Here are a few examples of the dmesg command that demonstrate its basic usage in Linux:

Example 1: Displaying Kernel Messages Using dmesg

Let’s run the dmesg command with any optional flag to see its basic functionality in Linux:

sudo dmesg

The output shows that the dmesg command retrieves the system messages related to the Linux kernel.

Example 2: Navigating Through Logs Page by Page

The dmesg command displays a lot of information, which can be hard to read all at once. To make it easier, you can use commands like tail, head, or less with dmesg to view the logs one page at a time. For example, the below command uses the dmesg with less command to show the system messages page by page:

sudo dmesg | less

You can use the Down Arrow or Enter to scroll line by line in the less viewer, and press q to quit and return to the terminal.

Example 3: Adding Color to dmesg Output

If you want to add colors to make the messages easier to understand, you can colorize them with the -L option:

sudo dmesg -L

To disable the colored output, you can use the “–color=never” option, as follows:

sudo dmesg --color=never

Example 4: Displaying Kernal Messages in Human-Friendly Timestamps 

dmesg shows timestamps in seconds and nanoseconds. We can use the -H option with the dmesg command to see the timestamps in a human-friendly format with the date and time: 

sudo dmesg -H

Example 5: Viewing Kernel Messages in Standard Date and Time Format

You can use the -T (or –ctime) option with the dmesg command to enable human-readable timestamps. This option removes nanosecond details, making the timestamps simpler to read:

sudo dmesg -T

The output below shows that dmesg returns the kernel messages in standard date and time format, which is easily understandable:

Example 6: Displaying a Specific Number of Kernel Messages

We can use the dmesg command with the head command to show a limited/specific number of kernel messages from the top:

sudo dmesg | head -15

This command shows the first 15 kernel messages:

Similarly, we can use the tail command with dmesg to show the last N kernel messages:

sudo dmesg | tail -15

This command shows the last 15 kernel messages:

Example 7: Filtering Kernel Messages by Specific Log Level

Each message logged to the kernel ring buffer has a “level” that shows how important the message is. Here are the commonly used levels:

• emerg: System is unusable.
• alert: Immediate action needed.
• crit: Critical conditions.
• err: Errors.
• warn: Warnings.
• notice: Normal but important.
• info: General information.
• debug: Debugging details.

To filter messages by level, we can use the -l (level) option with the dmesg command, followed by the level name. For instance, to list all “warn” level messages, we can use the “dmesg -l warn” command to see the warning notifications:

sudo dmesg -l warn

Example 8: Filtering Disk Related Messages

To find the disk-related messages such as memory, RAM, hard disk, or USB drives, we can use the grep command with dmesg, as follows:

sudo dmesg | grep -i sda

This command uses grep with “sda” to check which hard disks the kernel has detected. The “sda” part refers to hard disk drives; this command will show any messages where “sda” appears:

Example 9: Displaying Facility and Level

The -x (decode) option adds the facility and level as prefixes to each line, making them easier to read. For instance, we use the -x option with the dmesg command to display each message’s facility and log level at the beginning of each line:

sudo dmesg -x

Example 10: Reading dmesg Log File

Each time our system boots, it saves the kernel ring buffer messages in the /var/log/dmesg file. We can use the dmesg command to view these messages. If we encounter issues with dmesg, we can open the /var/log/dmesg file in a text editor to view the messages directly. For this, first, access the /var/log directory using the Linux cd command:

cd /var/log

Now, use the Linux cat command to view the log file and then pipe it into grep to find a specific word or phrase in the log:

cat dmesg | grep amd

Conclusion

The dmesg command is a useful command line utility for checking kernel messages in Linux. It provides useful information about our system’s hardware and drivers, which helps us fix problems and improve performance. In this tutorial, we discussed several use cases of the dmesg command, such as filtering messages, showing timestamps, viewing logs in an easy-to-read way, etc. 

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