The dmesg command-line utility is used to view and control the kernel ring buffer in Linux and other Unix-like operating systems. It is useful for examining kernel boot messages and debugging hardware related issues. In this guide, we will show you the basics of the dmesg command.
In the Linux operating system the Linux kernel is the core that manages the system resources. The kernel writes various messages to the kernel ring buffer during the boot process, and when the system is running. These messages include various information about the operation of the system.
Using the dmesg Command
The basic syntax for the
dmesg command is as following:
If you run the
dmesg without any options it will writes all messages from the kernel ring buffer to the standard output:
In some of systems the root level users only can run the
dmesg command and in some of system it allows users to run
dmesg command. If the user have not permission then following error message will be displayed:
dmesg: read kernel buffer failed: Operation not permitted
You also can manage the restriction that the non-root user can use the
dmesg command or not by kernel parameter
kernel.dmesg_restrict. You should set it zero to remove the restrictions:
sudo sysctl -w kernel.dmesg_restrict=0
There will many lines in output so the last few lines are viewable on terminal. You can pipe the output with less utility:
dmesg --color=always | less
--color=always is used to preserve the colored output.
You also can use the grep command to filter the output. For example, to view only the USB related messages, you would type:
dmesg | grep -i usb
dmesg reads the messages generated by the kernel from the
/proc/kmsg virtual file. This file provides an interface to the kernel ring buffer and can be opened only by one process. If
syslog process is running on your system and you try to read the file with cat , or less, the command will hang.
syslog daemon dumps kernel messages to
/var/log/dmesg, so you can also use that log file:
Formating dmesg Output
As we seen previously the output of
dmesg have the lots of lines. The
dmesg command provides number of options that help to format and filter the output.
--human) is the commonly used option, which show the human readable output.
To print human-readable timestamps use the
[Fri Sep 4 10:20:04 2020] IPv6: ADDRCONF(NETDEV_CHANGE): wlp1s0: link becomes ready
The timestamps format can also be set using the –time-format option, which can be ctime, reltime, delta, notime, or iso. For example to use the delta format you would type:
You also can set the timestamps using the
--time-format option. To use delta format, type:
You also can use the two options simultaneously:
dmesg -H -T
To watch the output of the
dmesg command in real-time use the
Filtering dmesg Output
dmesg supports the following log facilities:
kern– kernel messages
user– user-level messages
daemon– system daemons
auth– security/authorization messages
syslog– internal syslogd messages
lpr– line printer subsystem
news– network news subsystem
--facility ) option you can limit the output to specific facilities. The option accepts one or more comma-separated facilities.
For example, to display only the kernel and system daemons messages you would use:
dmesg -f kern,daemon
Each log message is associated with a log level that shows the importance of the message. dmesg supports the following log levels:
emerg– system is unusable
alert– action must be taken immediately
crit– critical conditions
err– error conditions
warn– warning conditions
notice– normal but significant condition
debug– debug-level messages
--level ) option restricts the output to defined levels. The option accepts one or more comma-separated levels.
The following command displays only the error and critical messages:
dmesg -l err,crit
Clearing the Ring Buffer
--clear) allows you to clear the ring buffer:
sudo dmesg -C
Make sure to clear the ring buffer you should logged in as root or user with sudo privileges.
To print the buffer contents before clearing use the
sudo dmesg -c
If you want to save the current
dmesg logs in a file before clearing it, redirect the output to a file:
dmesg > dmesg_messages
dmesg command allows you to view and control the kernel ring buffer. Type
man dmesg in your terminal for information about all available
If you have any questions or feedback, feel free to leave a comment.