dmesg Command in Linux
The Linux dmesg command displays or controls the kernel ring buffer. The kernel ring buffer stores log messages generated by various kernel parts such as the device drivers.
The device drivers manage various hardware components such as graphics cards, storage devices, or network adapters. These log messages play a vital role in troubleshooting or debugging the system anomalies.
It is important to note that the ring buffer only has a fixed amount of memory. Once it is filled, it overwrites the oldest messages.
Table of Contents
Here is a comprehensive guide to the options available with the dmesg command −
Syntax of dmesg Command
The syntax for using the Linux dmesg command is as follows −
dmesg [options]
The [options] field is used to print log messages in a specific format.
dmesg Command Options
The commonly used options for the dmesg command are listed in the table below −
| Flags | Options | Description |
|---|---|---|
| -C | --clear | It clears the ring buffer |
| -c | --read-clear | It displays the content of the buffer and then clears it |
| -D | --console-off | It disables printing the messages to the console |
| -d | --show-delta | It shows the timestamp and delta time spent between messages |
| -E | --console-on | It enables the printing the messages to the console |
| -e | --reltime | It displays the local time and delta in human-readable format (time conversion can be inaccurate) |
| -F file | --file file | It reads the system log messages from the specified file (it does not support kmsg format) |
| -f list | --facility list | To list specific facilities from the output (e.g. facility=daemon) |
| -H | --human | It displays the output in human-readable format |
| -J | --json | It displays the output in JSON format |
| -K file | --kmsg-file file | It reads the /dev/kmsg messages from a file |
| -L when | --color=when | It colorizes the output (auto, never, always) |
| -l list | --level list | To list specific levels from the output e.g. facility=err, warn) |
| -n level | --console-level | To print messages of the specific level only |
| --noescape | To disable escaping the unprintable or unsafe characters | |
| -P | --no-pager | It disables the pagination of the output |
| -r | --raw | It prints the raw output without stripping the log-level prefixes (unprintable data still escapes) |
| -S | --syslog | It forces the command to read the syslog instead of /dev/kmsg |
| -T | --ctime | It prints the human-readable timestamps (which can be inaccurate) |
| --since time | It displays the messages since the specified time | |
| --until time | It displays the messages till the specified time | |
| -t | --no-time | It does not display the timestamp with kernel messages |
| --time-format | It prints the timestamps according to the given format | |
| -u | --userspace | It displays the user space messages |
| -w | --follow | It waits for the new messages (from kernel version 3.5.0 and above) |
| -W | --follow-new | It waits and displays only new messages |
| -x | --decode | It decodes facility and level numbers to human-readable prefixes |
| --help | It prints help about command | |
| --version | It prints the command version |
Examples of dmesg Command in Linux
This section demonstrates the usage of the Linux dmesg command in Linux using examples −
- Printing all the Kernel Messages
- Printing the dmesg Command Output in Scrollable Format
- Printing dmesg Command Output in Human Readable Format
- Printing the dmesg Message Timestamps in Human Readable Format
- Printing the dmesg Command Output in JSON Format
- Printing the dmesg Command Output using Log Level
- Printing dmesg Command Output using Facilities
- Printing the dmesg Command Output with both Facility and Level
- Extracting the Specific Information from the dmesg Command Output
- Viewing the Live dmesg Command Output
Printing all the Kernel Messages
To print all the messages from the kernel ring buffer, execute the dmesg command with sudo privileges −
sudo dmesg
Printing the dmesg Command Output in Scrollable Format
By default, the dmesg command prints all the messages in one output. It sometimes becomes hard to read the messages. To make the output scrollable output can be piped to the less command.
sudo dmesg | less
Printing dmesg Command Output in Human Readable Format
By default, the timestamps of the dmesg command are displayed in seconds or nanoseconds since the kernel started. To display them in human-readable format, use the -H flag or --human option.
sudo dmesg -H
The output shows the timestamp in the human-readable format. Note that the output is also scrollable.
Printing the dmesg Message Timestamps in Human Readable Format
To print the message timestamps in human-readable format use the -T or --ctime option −
sudo dmesg -T
Printing the dmesg Command Output in JSON Format
To display the output in JSON format -J or --json options are used. The JSON format is a key-value pair format. Each message is represented by 3 keys: pri, time, and msg, with their respective values.
sudo dmesg -J
Printing the dmesg Command Output using Log Level
Each dmesg message is stored in the buffer with a specific log level. The log levels play a key role in troubleshooting especially finding the level of attention required to fix an issue. These levels are listed below −
| Level | Numerical Value | Description |
|---|---|---|
| emerg | 0 | It indicates the emergency state which means that the system is unstable (for example: hardware failure) |
| alert | 1 | It indicates the alerts, which means an immediate action is required (for example: data corruption) |
| crit | 2 | It indicates the critical state that can affect the system (for example: out of memory errors) |
| err | 3 | It indicates the error state that could disrupt normal operations (for example: connection failures) |
| warn | 4 | It indicates the potential issue but does not require immediate action (for example: low storage) |
| notice | 5 | It indicates normal operation but is significant enough to require attention (example: user logged in) |
| info | 6 | It shows the general information message about normal operations |
| debug | 7 | It provides the detailed information for debugging |
The numerical values 0-7 are used in syslog or similar logging systems to manage the logs based on severity.
To print messages of a specified level, use the -l or --level option with the level name −
sudo dmesg -l warn
Or,
sudo dmesg --level=warn
To display messages of the different levels, use −
sudo dmesg -l warn,debug
Multiple levels can be mentioned separating them by comma (,).
Printing dmesg Command Output using Facilities
The log messages are categorized into groups called facilities. The facilities are listed below −
| Facility | Description |
|---|---|
| kern | It shows the logs generated by the kernel |
| user | It shows the logs generated by the user |
| It shows the logs generated by the mail system | |
| daemon | It shows the logs generated by the system daemon |
| auth | It shows security and authorization-related logs |
| syslog | It shows the logs generated by the internal syslog |
| lpr | It shows line printer-related logs |
| news | It shows the logs generated by the network news subsystem |
To display the message of a facility, use the -f or --facility option −
sudo dmesg -f kern
Or,
sudo dmesg --facility=kern
To display messages from multiple facilities, use −
sudo dmesg -f kern,daemon
Printing the dmesg Command Output with both Facility and Level
To print the facility and level with each log message, use -x flag or --decode option −
sudo dmesg -x
Extracting the Specific Information from the dmesg Command Output
It can be piped to the grep command to print specific information from the dmesg command output. For instance, to extract information about the specific device, mention the device name with the grep command −
sudo dmesg | grep -i vda
The -i option is used to make the search case insensitive.
Viewing the Live dmesg Command Output
To print the live into the console, use -w or --follow option −
sudo dmesg -w
Or,
sudo dmesg --follow
It prints the messages as they are logged. Note that it also prints the previous messages. To live-print the new messages only, use -W −
sudo dmesg -W
To exit the wait, press Ctrl + C.
Conclusion
The dmesg command is a powerful tool to debug system-related issues. It prints the log messages stored in the kernel ring buffer. This tool also allows manipulation of the output to extract the required information only.
This tutorial covered the dmesg command, its syntax, and its usage in Linux through various examples.