DMESG

DMESG(1)                        User Commands                       DMESG(1)

NAME

dmesg – print or control the kernel ring buffer

SYNOPSIS

dmesg [options]

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

DESCRIPTION

dmesg is used to examine or control the kernel ring buffer.

The default action is to display all messages from the kernel ring
buffer.

OPTIONS

The –clear, –read-clear, –console-on, –console-off, and
–console-level options are mutually exclusive.

-C, –clear
Clear the ring buffer.

-c, –read-clear
Clear the ring buffer after first printing its contents.

-D, –console-off
Disable the printing of messages to the console.

-d, –show-delta
Display the timestamp and the time delta spent between
messages.  If used together with –notime then only the time
delta without the timestamp is printed.

-E, –console-on
Enable printing messages to the console.

-e, –reltime
Display the local time and the delta in human-readable format.

-F, –file file
Read the messages from the given file.

-f, –facility list
Restrict output to the given (comma-separated) list of
facilities.  For example:

dmesg –facility=daemon

will print messages from system daemons only.  For all
supported facilities see the –help output.

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

-k, –kernel
Print kernel messages.

-L, –color[=when]
Colorize important messages (enabled by default).  The
optional argument when can be auto, never or always.  If the
when argument is omitted, it defaults to auto.

-l, –level list
Restrict output to the given (comma-separated) list of levels.
For example:

dmesg –level=err,warn

will print error and warning messages only.  For all supported
levels see the –help output.

-n, –console-level level
Set the level at which printing of messages is done to the
console.  The level is a level number or abbreviation of the
level name.  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.
All levels of messages are still written to /proc/kmsg, so
syslogd(8) can still be used to control exactly where kernel
messages appear.  When the -n option is used, dmesg will not
print or clear the kernel ring buffer.

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

-r, –raw
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
different format than syslog(2).  For backward compatibility,
dmesg(1) returns data always in the syslog(2) format.  It is
possible to read the real raw data from /dev/kmsg by, for
example, the command ‘dd if=/dev/kmsg iflag=nonblock’.

-S, –syslog
Force dmesg to use the syslog(2) kernel interface to read
kernel messages.  The default is to use /dev/kmsg rather than
syslog(2) since kernel 3.5.0.

-s, –buffer-size size
Use a buffer of size to query the kernel ring buffer.  This is
16392 by default.  (The default kernel syslog buffer size was
4096 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,
then this option can be used to view the entire buffer.

-T, –ctime
Print human-readable timestamps.

Be aware that the timestamp could be inaccurate!  The time
source used for the logs is not updated after system
SUSPEND/RESUME.

-t, –notime
Do not print kernel’s timestamps.

–time-format format
Print timestamps using the given format, which can be ctime,
reltime, delta or iso.  The first three formats are aliases of
the time-format-specific options.  The iso format is a dmesg
implementation of the ISO-8601 timestamp format.  The purpose
of this format is to make the comparing of timestamps between
two systems, and any other parsing, easy.  The definition of
the iso 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.

-u, –userspace
Print userspace messages.

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

-x, –decode
Decode facility and level (priority) numbers to human-readable
prefixes.

-V, –version
Display version information and exit.

-h, –help
Display help text and exit.

COLORS

Implicit coloring can be disabled by an empty file /etc/terminal-
colors.d/dmesg.disable.  See terminal-colors.d(5) for more details
about colorization configuration.

The logical color names supported by dmesg are:

subsys The message sub-system prefix (e.g. “ACPI:”).

time   The message timestamp.

timebreak
The 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.

segfault
The text of the message that inform about segmentation fault.

SEE ALSO

syslogd(8) terminal-colors.d(5)

AUTHORS

Karel Zak ⟨[email protected]

dmesg was originally written by Theodore Ts’o ⟨[email protected]

AVAILABILITY

The dmesg command is part of the util-linux package and is available
from Linux Kernel Archive
⟨ftp://ftp.kernel.org/pub/linux/utils/util-linux/⟩.

COLOPHON

This page is part of the util-linux (a random collection of Linux
utilities) project.  Information about the project can be found at
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩.  If you have a
bug report for this manual page, send it to
[email protected]  This page was obtained from the
project’s upstream Git repository
(git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git) on
2014-12-30.  If you discover any rendering problems in this HTML ver‐
sion of the page, or you believe there is a better or more up-to-date
source for the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original man‐
ual page), send a mail to [email protected]

util-linux                        July 2012                         DMESG(1)

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