Linux tools: Getting the message out with dmesg (2024)

Posted: January 22, 2020 | | by Ken Hess (Sudoer alumni)

Linux tools: Getting the message out with dmesg (1)

The dmesg command is one of those easily forgotten troubleshooting tools that should stay at the top of your sysadmin arsenal. It contains so much information about your system that dmesg should be the first place you look when something goes wrong. The output from dmesg is long, as you can see for yourself if you type it in at a command prompt becauseit reports information from all aspects of your system when there are no error messages—other than those that might appear at boot.

Formally, dmesg prints or controls the kernel ring buffer.The default action is to display all messages from this buffer.

For future reference and comparison, the best time to look at dmesgis just after boot. I usually send dmesg information to a text fileusing a command such as the following:

$ dmesg > dmesg.`date +%m.%d.%Y`.txt

This command creates a text file nameddmesg.12.11.2019.txt. Between the file creation date and the next boot, you can check new messages generated by the kernel.

Possible post-boot messages include system errors, device errors, and information about anyUSB device thatsomeone might plugin.For example, the following dmesg information appeared after inserting a USB drive:

[ 9189.631808] usb 1-1: new full-speed USB device number 2 using ohci-pci[ 9189.909896] ohci-pci 0000:00:06.0: frame counter not updating; disabled[ 9189.909958] ohci-pci 0000:00:06.0: HC died; cleaning up[ 9194.910072] usb usb1-port1: attempt power cycle

To see a full listing of USB-related messages, issue the dmesg command and grep for usb:

$ dmesg |grep -i usb[ 0.052580] ACPI: bus type USB registered[ 0.052594] usbcore: registered new interface driver usbfs[ 0.052598] usbcore: registered new interface driver hub[ 0.052605] usbcore: registered new device driver usb[ 0.414901] ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver[ 0.414907] ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver[ 0.415398] ohci-pci 0000:00:06.0: new USB bus registered, assigned bus number 1[ 0.468262] usb usb1: New USB device found, idVendor=1d6b, idProduct=0001, bcdDevice= 4.18[ 0.468264] usb usb1: New USB device strings: Mfr=3, Product=2, SerialNumber=1[ 0.468266] usb usb1: Product: OHCI PCI host controller[ 0.468268] usb usb1: Manufacturer: Linux 4.18.0-80.el8.x86_64 ohci_hcd[ 0.468269] usb usb1: SerialNumber: 0000:00:06.0[ 0.468454] hub 1-0:1.0: USB hub found[ 0.468842] uhci_hcd: USB Universal Host Controller Interface driver[ 0.468885] usbcore: registered new interface driver usbserial_generic[ 0.468889] usbserial: USB Serial support registered for generic[ 0.470765] usbcore: registered new interface driver usbhid[ 0.470765] usbhid: USB HID core driver[ 9189.631808] usb 1-1: new full-speed USB device number 2 using ohci-pci[ 9194.910072] usb usb1-port1: attempt power cycle

As you can see, I used thegrepcommand with the -i option so it ignores case and I will see everything associated with USB devices regardless of that (Usb, usb, or USB). This practiceis good for any subsystem or filter that you want to use. Always ignore your filter's case.

For example, to see all disk (block devices) attached to your system, use the following command:

$ dmesg |grep -i sd[ 0.000000] Command line: BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-80.el8.x86_64 root=UUID=f695f641-e489-4674-afd8-8f354533811d ro crashkernel=auto rhgb quiet[ 0.000000] ACPI: RSDP 0x00000000000E0000 000024 (v02 VBOX )[ 0.000000] ACPI: XSDT 0x000000003FFF0030 00003C (v01 VBOX VBOXXSDT 00000001 ASL 00000061)[ 0.000000] ACPI: DSDT 0x000000003FFF0470 0022EA (v02 VBOX VBOXBIOS 00000002 INTL 20100528)[ 0.000000] ACPI: SSDT 0x000000003FFF02A0 0001CC (v01 VBOX VBOXCPUT 00000002 INTL 20100528)[ 0.000000] Kernel command line: BOOT_IMAGE=(hd0,msdos1)/vmlinuz-4.18.0-80.el8.x86_64 root=UUID=f695f641-e489-4674-afd8-8f354533811d ro crashkernel=auto rhgb quiet[ 1.545750] sd 2:0:0:0: [sda] 33554432 512-byte logical blocks: (17.2 GB/16.0 GiB)[ 1.545756] sd 2:0:0:0: [sda] Write Protect is off[ 1.545757] sd 2:0:0:0: [sda] Mode Sense: 00 3a 00 00[ 1.545764] sd 2:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA[ 1.546960] sda: sda1 sda2[ 1.547316] sd 2:0:0:0: [sda] Attached SCSI disk[ 1.975545] XFS (sda2): Mounting V5 Filesystem[ 2.092251] XFS (sda2): Starting recovery (logdev: internal)[ 2.137813] XFS (sda2): Ending recovery (logdev: internal)[ 3.993219] sd 2:0:0:0: Attached scsi generic sg1 type 0[ 5.909006] XFS (sda1): Mounting V5 Filesystem[ 5.959833] XFS (sda1): Starting recovery (logdev: internal)[ 5.962287] XFS (sda1): Ending recovery (logdev: internal)

The results show more than just a list of block devices, so you can further filter your results by specifying the filesystem, XFS in this case:

$ dmesg |grep -i xfs[ 1.965741] SGI XFS with ACLs, security attributes, no debug enabled[ 1.975545] XFS (sda2): Mounting V5 Filesystem[ 2.092251] XFS (sda2): Starting recovery (logdev: internal)[ 2.137813] XFS (sda2): Ending recovery (logdev: internal)[ 5.909006] XFS (sda1): Mounting V5 Filesystem[ 5.959833] XFS (sda1): Starting recovery (logdev: internal)[ 5.962287] XFS (sda1): Ending recovery (logdev: internal)

The use of the -i option here is superfluous but I always include it just in case there are any results that I wouldn't see otherwise. I use the up arrow key to replay my last command (amost excellent Bash feature) and just backspace over the last thing I searched for and replace it with my new keyword, so once I enter the command, I never have to bother with anything except what I'm searching for. No harm is done either way.

If you'd like to see if your remote system is equipped with a CD/DVD drive, try this command:

$ dmesg |grep -i cd[ 0.000000] clocksource: kvm-clock: mask: 0xffffffffffffffff max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns[ 0.414901] ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver[ 0.414907] ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver[ 0.468262] usb usb1: New USB device found, idVendor=1d6b, idProduct=0001, bcdDevice= 4.18[ 0.468268] usb usb1: Manufacturer: Linux 4.18.0-80.el8.x86_64 ohci_hcd[ 0.468842] uhci_hcd: USB Universal Host Controller Interface driver[ 1.328589] ata2.00: ATAPI: VBOX CD-ROM, 1.0, max UDMA/133[ 1.329773] scsi 1:0:0:0: CD-ROM VBOX CD-ROM 1.0 PQ: 0 ANSI: 5[ 1.577662] cdrom: Uniform CD-ROM driver Revision: 3.20[ 1.578616] sr 1:0:0:0: Attached scsi CD-ROM sr0

The last four lines display information about the CD-ROM drive. Although the CD-ROM drive is virtual on this system, if the virtual machine's complement of hardware includes it the drivecan load ISO image files as if they were abootable image on physical media.

The dmesgcommand isn'tbig and flashy. Itdoesn't do a lot of things or have a long list of options. Instead, it is elegant in its simplicity and as practical as that your pocket protector. Rather than as an afterthought, you should get into the habit of running dmesg on a regular basis on your systems. And, when something goes wrong, run it again to find out what the kernel knows about the problem. You might save yourself some grief anda few troubleshooting steps. You also might also look like ahero to your coworkers and management for finding the problem so quickly. Remember, time is money and you're trying to save it and the day.

Want to try out Red Hat Enterprise Linux? Download it now for free.

Topics: Linux

Linux tools: Getting the message out with dmesg (2024)

FAQs

How to check dmesg for errors? ›

To filter dmesg output and view only error messages, you can use the -l (level) option with the “err” level. For example: sudo dmesg -l err .

Find Out More ›

What is the output of dmesg in Linux? ›

dmesg (diagnostic messages) is a command on most Unix-like operating systems that prints the message buffer of the kernel. The output includes messages produced by the device drivers.

Show Me More ›

What information is displayed by dmesg? ›

The dmesg command is a Linux utility that displays kernel-related messages retrieved from the kernel ring buffer. The ring buffer stores information about hardware, device driver initialization, and messages from kernel modules that take place during system startup.

See Details ›

How to print in dmesg? ›

The -W option of dmesg is for waiting and printing only new messages. Otherwise, dmesg just prints all the messages in the kernel buffer and exits. If the -W option isn't available, we can use the -w option instead. It prints all the messages in the kernel buffer and waits for new ones.

Show Me More ›

How do I check error messages in Linux? ›

To view Linux logs you have to type the command cd/var/log and then by typing Is to see the logs under the directory. syslog is one of the important logs to view every log except auth-related messages. To view everything under syslog issue the var/log/syslog command in Linux, but looking for all issues will take time.

Get More Info ›

How to troubleshoot dmesg in Linux? ›

Here are some examples of using the dmesg command for troubleshooting and collecting information in Linux systems are:

Checking the dmesg Command Version. ...
Colorizing the Output using the dmesg Command. ...
Display dmesg Output with Timestamps in Local Time Zone. ...
Filter dmesg Output. ...
Making dmesg using Syslog.

More items...

Jul 4, 2023

Show Me More ›

How to output a message in Linux? ›

To send a text message to the standard output stream, use the echo command with a string argument enclosed in double quotes. For example, echo “Hello, world!” will write the string “Hello, world!” followed by a newline character to the standard output.

Get More Info ›

How to see kernel messages in Linux? ›

You can also view this log using the dmesg command. Another log you can use to view kernel information is the /var/log/kern. log file, this logs the kernel information and events on your system, it also logs dmesg output.

Discover More ›

How to use dmesg command? ›

To use it, simply type dmesg in your terminal. For more detailed output, you can use dmesg -H for a human-readable format. This command will display the kernel ring buffer in a human-readable format, making it easier to understand the system messages.

How to read dmesg timestamps? ›

The timestamps at the beginning of each message within brackets show the time in seconds since the kernel started. For example, the timestamp [ 19.303922] in the last message means that the input handler for rfkill was disabled about 19.3 seconds after the system started.

Discover More ›

How to understand dmesg logs? ›

The 'dmesg' command provides real-time access to the kernel ring buffer, making it ideal for troubleshooting current issues. The '/var/log/dmesg' file provides a snapshot of the kernel messages at boot time, which can be useful for diagnosing boot issues.

Keep Reading ›

What is the difference between syslog and dmesg? ›

The general convention is: syslog: Everything. messages: General events, no debug stuff, excludes some errors. dmesg: Kernel messages, reset on every boot.

How to view printk messages? ›

The usual way to read it is using dmesg . The log level specifies the importance of a message. The kernel decides whether to show the message immediately (printing it to the current console) depending on its log level and the current console_loglevel (a kernel variable).

Tell Me More ›

What is the command to print the messages generated by the kernel? ›

dmesg command also called as “driver message” or “display message” is used to examine the kernel ring buffer and print the message buffer of kernel. The output of this command contains the messages produced by the device drivers.

Which command can you use to generate log messages? ›

The logger command provides an interface to the syslog subroutine, which writes entries to the system log. A Message variable can be specified on the command line, which is logged immediately, or a File variable is read and each line of the File variable is logged.