dmesg Command In Linux Kernel

Introduction To ‘dmseg’ Command In Linux Kernel

The “dmesg” command is called “d

river message” or “display message”. It is used to test or diagnose the kernel ring buffer and print the message buffer of the kernel. The kernel keeps all the kernel logs in a ring buffer. This buffer is only accessible by the kernel buffer. It is nothing like a printf() message that will directly display on the console. So these kernel logs will be stored in a kernel buffer. If you want to check the logs, then you need to use the command “dmesg“.

Basically, you would have thought about why we need all these methods. We can display the messages directly like <printf()> on the console, then why we will keep it in a buffer. Even if it will also use some kernel memory space. This is required to avoid the boot logs being getting lost until the <syslog> daemon starts. So that it will collect them and store them in “/var/log/dmesg/“.

What Is dmseg Command?

The dmesg command is a utility in Linux that allows users to view the kernel’s message buffer. The kernel message buffer is a temporary storage area for messages generated by the kernel, such as information about hardware devices, system errors, and other important events.

The dmesg command is used to display the contents of the kernel message buffer, either to the console or to a file. It is often used to troubleshoot problems with the system or to view information about the hardware and software configuration of the system.

To use the dmesg command, you can simply type dmesg at the command prompt and press Enter. This will display the contents of the kernel message buffer to the console. You can also use options with the dmesg command to control how the output is displayed, such as specifying a different level of verbosity or directing the output to a file.

For example, the following command will display the contents of the kernel message buffer in verbose mode:

dmesg -v

And this command will save the contents of the kernel message buffer to a file called dmesg.log:

dmesg > dmesg.log

The dmesg command is a useful tool for troubleshooting problems with the system and for learning more about the hardware and software configuration of the system.

Need Of dmesg Command In Linux Kernel

If there is any bug or issue in any kernel module, then how you will get to know the cause. Because these logs will be stored, if the kernel fails to load. So you need to reload the kernel again. So to prevent this it should log in a ring buffer so that we can access it any time even the kernel load fails at bootup time. To get these logs from the Kernel ring buffer, Linux provides a command called “dmesg“. When the kernel boots there is no root file system. That means it does not have any capability to write any file.

We can say the the function of “dmesg” command is to control or print the kernel ring buffer message on the console. The default value is to print the “dmesg” command.

‘dmseg’ Command Control Functions

The dmesg command provides several options and control functions that can be used to modify its behavior and output. Some of the most common options and control functions are:

• -c: This option clears the kernel message buffer after the messages have been displayed. This can be useful if you want to view the most recent messages and do not want to see old messages from previous boots.
• -n level: This option sets the level of verbosity for the output. The level parameter can be a number from 0 to 7, with higher numbers indicating more verbose output.
• -s size: This option sets the maximum size of the kernel message buffer. The size parameter is specified in bytes.
• -T: This option displays the timestamp of each message in the output.
• -t: This option displays the timestamp of each message in human-readable format, rather than in raw seconds since the epoch.
• -v: This option displays the messages in verbose mode, which provides more detailed information about each message.

These options and control functions can be combined and used in various ways to customize the output of the dmesg command and to suit the specific needs of a given situation. For example, to display the most recent messages in verbose mode with timestamps, you could use the following command:

dmesg -c -v -T

You can also use the grep command to filter the output of dmesg based on specific keywords or patterns. For example, to search for all messages containing the word “error”, you could use the following command:

dmesg | grep error

By using these options and control functions, you can more easily find and analyse the information you need from the kernel message buffer.

Use Of ‘dmseg’ In Kernel Ring Buffer Clear

/*Small "c" clears the ring buffer after prints on the console*/
$dmesg -c 
/*Capital "C" clears the ring buffer without prints it on the console*/
$dmesg -C

Kernel Message Print without Timestamp

/*how to print kernel message without timestamp*/
$dmesg -t