| .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu) |
| .\" May be distributed under the GNU General Public License |
| .TH DMESG "1" "July 2012" "util-linux" "User Commands" |
| .SH NAME |
| dmesg \- print or control the kernel ring buffer |
| .SH SYNOPSIS |
| .B dmesg |
| [options] |
| .sp |
| .B dmesg \-\-clear |
| .br |
| .BR "dmesg \-\-read\-clear " [options] |
| .br |
| .BI "dmesg \-\-console\-level " level |
| .br |
| .B dmesg \-\-console\-on |
| .br |
| .B dmesg \-\-console\-off |
| .SH DESCRIPTION |
| .B dmesg |
| is used to examine or control the kernel ring buffer. |
| .PP |
| The default action is to display all messages from the kernel ring buffer. |
| .SH OPTIONS |
| The |
| .BR \-\-clear , |
| .BR \-\-read\-clear , |
| .BR \-\-console\-on , |
| .BR \-\-console\-off , |
| and |
| .B \-\-console\-level |
| options are mutually exclusive. |
| .PP |
| .IP "\fB\-C\fR, \fB\-\-clear\fR" |
| Clear the ring buffer. |
| .IP "\fB\-c\fR, \fB\-\-read\-clear\fR" |
| Clear the ring buffer after first printing its contents. |
| .IP "\fB\-D\fR, \fB\-\-console\-off\fR" |
| Disable the printing of messages to the console. |
| .IP "\fB\-d\fR, \fB\-\-show\-delta\fR" |
| Display the timestamp and the time delta spent between messages. If used |
| together with |
| .B \-\-notime |
| then only the time delta without the timestamp is printed. |
| .IP "\fB\-E\fR, \fB\-\-console\-on\fR" |
| Enable printing messages to the console. |
| .IP "\fB\-e\fR, \fB\-\-reltime\fR" |
| Display the local time and the delta in human-readable format. Be aware that |
| conversion to the local time could be inaccurate (see \fB\-T\fR for more |
| details). |
| .IP "\fB\-F\fR, \fB\-\-file \fIfile\fR" |
| Read the messages from the given |
| .IR file . |
| .IP "\fB\-f\fR, \fB\-\-facility \fIlist\fR" |
| Restrict output to the given (comma-separated) |
| .I list |
| of facilities. For example: |
| .PP |
| .RS 14 |
| .B dmesg \-\-facility=daemon |
| .RE |
| .IP |
| will print messages from system daemons only. For all supported facilities |
| see the |
| .B \-\-help |
| output. |
| .IP "\fB\-H\fR, \fB\-\-human\fR" |
| Enable human-readable output. See also \fB\-\-color\fR, \fB\-\-reltime\fR |
| and \fB\-\-nopager\fR. |
| .IP "\fB\-k\fR, \fB\-\-kernel\fR" |
| Print kernel messages. |
| .IP "\fB\-L\fR, \fB\-\-color\fR[=\fIwhen\fR]" |
| Colorize the output. The optional argument \fIwhen\fP |
| can be \fBauto\fR, \fBnever\fR or \fBalways\fR. If the \fIwhen\fR argument is omitted, |
| it defaults to \fBauto\fR. The colors can be disabled; for the current built-in default |
| see the \fB\-\-help\fR output. See also the \fBCOLORS\fR section below. |
| .IP "\fB\-l\fR, \fB\-\-level \fIlist\fR" |
| Restrict output to the given (comma-separated) |
| .I list |
| of levels. For example: |
| .PP |
| .RS 14 |
| .B dmesg \-\-level=err,warn |
| .RE |
| .IP |
| will print error and warning messages only. For all supported levels see the |
| .B \-\-help |
| output. |
| .IP "\fB\-n\fR, \fB\-\-console\-level \fIlevel\fR |
| Set the |
| .I level |
| at which printing of messages is done to the console. The |
| .I level |
| is a level number or abbreviation of the level name. For all supported |
| levels see the |
| .B \-\-help |
| output. |
| .sp |
| For example, |
| .B \-n 1 |
| or |
| .B \-n alert |
| prevents all messages, except emergency (panic) messages, from appearing on |
| the console. All levels of messages are still written to |
| .IR /proc/kmsg , |
| so |
| .BR syslogd (8) |
| can still be used to control exactly where kernel messages appear. When the |
| .B \-n |
| option is used, |
| .B dmesg |
| will |
| .I not |
| print or clear the kernel ring buffer. |
| .IP "\fB\-P\fR, \fB\-\-nopager\fR" |
| Do not pipe output into a pager. A pager is enabled by default for \fB\-\-human\fR output. |
| .IP "\fB\-r\fR, \fB\-\-raw\fR" |
| 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 |
| .BR dmesg (1) |
| reads kernel messages. The /dev/kmsg device uses a different format than |
| .BR syslog (2) . |
| For backward compatibility, |
| .BR dmesg (1) |
| returns data always in the |
| .BR 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'. |
| .IP "\fB\-S\fR, \fB\-\-syslog\fR" |
| Force \fBdmesg\fR to use the |
| .BR syslog (2) |
| kernel interface to read kernel messages. The default is to use /dev/kmsg rather |
| than |
| .BR syslog (2) |
| since kernel 3.5.0. |
| .IP "\fB\-s\fR, \fB\-\-buffer\-size \fIsize\fR |
| Use a buffer of |
| .I 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. |
| .IP "\fB\-T\fR, \fB\-\-ctime\fR" |
| Print human-readable timestamps. |
| .IP |
| .B Be aware that the timestamp could be inaccurate! |
| The |
| .B time |
| source used for the logs is |
| .B not updated after |
| system |
| .BR SUSPEND / RESUME . |
| .IP "\fB\-t\fR, \fB\-\-notime\fR" |
| Do not print kernel's timestamps. |
| .IP "\fB\-\-time\-format\fR \fIformat\fR" |
| Print timestamps using the given \fIformat\fR, which can be |
| .BR ctime , |
| .BR reltime , |
| .BR delta |
| or |
| .BR iso . |
| The first three formats are aliases of the time-format-specific options. |
| The |
| .B iso |
| format is a |
| .B 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 \fBiso\fR timestamp is: |
| YYYY-MM-DD<T>HH:MM:SS,<microseconds><-+><timezone offset from UTC>. |
| .IP |
| The |
| .B iso |
| format has the same issue as |
| .BR ctime : |
| the time may be inaccurate when a system is suspended and resumed. |
| .TP |
| .BR \-u , " \-\-userspace" |
| Print userspace messages. |
| .TP |
| .BR \-w , " \-\-follow" |
| Wait for new messages. This feature is supported only on systems with |
| a readable /dev/kmsg (since kernel 3.5.0). |
| .TP |
| .BR \-x , " \-\-decode" |
| Decode facility and level (priority) numbers to human-readable prefixes. |
| .TP |
| .BR \-V , " \-\-version" |
| Display version information and exit. |
| .TP |
| .BR \-h , " \-\-help" |
| Display help text and exit. |
| .SH COLORS |
| Implicit coloring can be disabled by an empty file \fI/etc/terminal-colors.d/dmesg.disable\fR. |
| See |
| .BR terminal-colors.d (5) |
| for more details about colorization configuration. |
| .PP |
| The logical color names supported by |
| .B dmesg |
| are: |
| .TP |
| .B subsys |
| The message sub-system prefix (e.g. "ACPI:"). |
| .TP |
| .B time |
| The message timestamp. |
| .TP |
| .B timebreak |
| The message timestamp in short ctime format in \fB\-\-reltime\fR |
| or \fB\-\-human\fR output. |
| .TP |
| .B alert |
| The text of the message with the alert log priority. |
| .TP |
| .B crit |
| The text of the message with the critical log priority. |
| .TP |
| .B err |
| The text of the message with the error log priority. |
| .TP |
| .B warn |
| The text of the message with the warning log priority. |
| .TP |
| .B segfault |
| The text of the message that inform about segmentation fault. |
| |
| .SH SEE ALSO |
| .BR syslogd (8) |
| .BR terminal-colors.d (5) |
| .SH AUTHORS |
| .MT kzak@redhat.com |
| Karel Zak |
| .ME |
| |
| .br |
| .B dmesg |
| was originally written by |
| .MT tytso@athena.mit.edu |
| Theodore Ts'o |
| .ME |
| .SH AVAILABILITY |
| The dmesg command is part of the util-linux package and is available from |
| .UR ftp://\:ftp.kernel.org\:/pub\:/linux\:/utils\:/util-linux/ |
| Linux Kernel Archive |
| .UE . |