Linux "dmesg" Command Line Options and Examples

print or control the kernel ring buffer

dmesg is used to examine or control the kernel ring buffer. The default action is to display all messages from the kernel ring buffer..

Usage:

dmesg [options]

dmesg --clear dmesg --read-clear [options] dmesg --console-level level dmesg --console-on dmesg --console-off

Command Line Options:

-C

Clear the ring buffer.


                            dmesg -C ...

-c

Clear the ring buffer after first printing its contents.


                            dmesg -c ...

-D

Disable the printing of messages to the console.


                            dmesg -D ...

-d

Display the timestamp and the time delta spent between messages. If used together with --notime then only the time deltawithout the timestamp is printed.


                            dmesg -d ...

-E

Enable printing messages to the console.


                            dmesg -E ...

-e

Display the local time and the delta in human-readable format. Be aware that conversion to the local time could be inaccurate(see -T for more details).


                            dmesg -e ...

-F

Read the syslog messages from the given file. Note that -F does not support messages in kmsg format. The old syslog format issupported only.


                            dmesg -F ...

-f

Restrict output to the given (comma-separated) list of facilities. For example:dmesg --facility=daemonwill print messages from system daemons only. For all supported facilities see the --help output.


                            dmesg -f ...

-H

Enable human-readable output. See also --color, --reltime and --nopager.


                            dmesg -H ...

-k

Print kernel messages.


                            dmesg -k ...

-L

Colorize the output. The optional argument when can be auto, never or always. If the when argument is omitted, it defaultsto auto. The colors can be disabled; for the current built-in default see the --help output. See also the COLORS sectionbelow.


                            dmesg -L ...

-l

Restrict output to the given (comma-separated) list of levels. For example:dmesg --level=err,warnwill print error and warning messages only. For all supported levels see the --help output.


                            dmesg -l ...

-n

Set the level at which printing of messages is done to the console. The level is a level number or abbreviation of the levelname. For all supported levels see the --help output.For example, -n 1 or -n alert prevents all messages, except emergency (panic) messages, from appearing on the console. Alllevels of messages are still written to /proc/kmsg, so syslogd(8) can still be used to control exactly where kernel messagesappear. When the -n option is used, dmesg will not print or clear the kernel ring buffer.


                            dmesg -n ...

-P

Do not pipe output into a pager. A pager is enabled by default for --human output.


                            dmesg -P ...

-r

Print the raw message buffer, i.e. do not strip the log-level prefixes.Note that the real raw format depends on the method how dmesg(1) reads kernel messages. The /dev/kmsg device uses a differentformat than syslog(2). For backward compatibility, dmesg(1) returns data always in the syslog(2) format. It is possible toread the real raw data from /dev/kmsg by, for example, the command 'dd if=/dev/kmsg iflag=nonblock'.


                            dmesg -r ...

-S

Force dmesg to use the syslog(2) kernel interface to read kernel messages. The default is to use /dev/kmsg rather than sys‐log(2) since kernel 3.5.0.


                            dmesg -S ...

-s

Use a buffer of size to query the kernel ring buffer. This is 16392 by default. (The default kernel syslog buffer size was4096 at first, 8192 since 1.3.54, 16384 since 2.1.113.) If you have set the kernel buffer to be larger than the default, thenthis option can be used to view the entire buffer.


                            dmesg -s ...

-T

Print human-readable timestamps.Be aware that the timestamp could be inaccurate! The time source used for the logs is not updated after system SUS‐PEND/RESUME.


                            dmesg -T ...

-t

Do not print kernel's timestamps.


                            dmesg -t ...

--time-format

Print timestamps using the given format, which can be ctime, reltime, delta or iso. The first three formats are aliases ofthe time-format-specific options. The iso format is a dmesg implementation of the ISO-8601 timestamp format. The purpose ofthis format is to make the comparing of timestamps between two systems, and any other parsing, easy. The definition of theiso timestamp is: YYYY-MM-DD<T>HH:MM:SS,<microseconds><-+><timezone offset from UTC>.The iso format has the same issue as ctime: the time may be inaccurate when a system is suspended and resumed.


                            dmesg --time-format ...

-u

Print userspace messages.


                            dmesg -u ...

-w

Wait for new messages. This feature is supported only on systems with a readable /dev/kmsg (since kernel 3.5.0).


                            dmesg -w ...

-x

Decode facility and level (priority) numbers to human-readable prefixes.


                            dmesg -x ...

-V

Display version information and exit.


                            dmesg -V ...

-h

Display help text and exit.COLORSImplicit coloring can be disabled by an empty file /etc/terminal-colors.d/dmesg.disable. See terminal-colors.d(5) for more detailsabout colorization configuration.The logical color names supported by dmesg are:subsys The message sub-system prefix (e.g. "ACPI:").time The message timestamp.timebreakThe message timestamp in short ctime format in --reltime or --human output.alert The text of the message with the alert log priority.crit The text of the message with the critical log priority.err The text of the message with the error log priority.warn The text of the message with the warning log priority.segfaultThe text of the message that inform about segmentation fault.


                            dmesg -h ...