392 lines
9.4 KiB
HTML
392 lines
9.4 KiB
HTML
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<HTML><HEAD><TITLE>Man page of DMESG</TITLE>
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</HEAD><BODY>
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<H1>DMESG</H1>
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Section: User Commands (1)<BR>Updated: July 2012<BR><A HREF="#index">Index</A>
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<A HREF="/cgi-bin/man/man2html">Return to Main Contents</A><HR>
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<A NAME="lbAB"> </A>
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<H2>NAME</H2>
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dmesg - print or control the kernel ring buffer
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<A NAME="lbAC"> </A>
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<H2>SYNOPSIS</H2>
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<B>dmesg</B>
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[options]
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<P>
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<B>dmesg --clear</B>
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<BR>
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<B>dmesg --read-clear </B>[options]
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<BR>
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<B>dmesg --console-level </B><I>level</I>
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<BR>
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<B>dmesg --console-on</B>
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<BR>
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<B>dmesg --console-off</B>
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<A NAME="lbAD"> </A>
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<H2>DESCRIPTION</H2>
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<B>dmesg</B>
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is used to examine or control the kernel ring buffer.
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<P>
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The default action is to display all messages from the kernel ring buffer.
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<A NAME="lbAE"> </A>
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<H2>OPTIONS</H2>
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The
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<B>--clear</B>,
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<B>--read-clear</B>,
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<B>--console-on</B>,
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<B>--console-off</B>,
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and
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<B>--console-level</B>
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options are mutually exclusive.
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<P>
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<DL COMPACT>
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<DT id="1"><B>-C</B>, <B>--clear</B><DD>
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Clear the ring buffer.
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<DT id="2"><B>-c</B>, <B>--read-clear</B><DD>
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Clear the ring buffer after first printing its contents.
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<DT id="3"><B>-D</B>, <B>--console-off</B><DD>
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Disable the printing of messages to the console.
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<DT id="4"><B>-d</B>, <B>--show-delta</B><DD>
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Display the timestamp and the time delta spent between messages. If used
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together with
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<B>--notime</B>
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then only the time delta without the timestamp is printed.
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<DT id="5"><B>-E</B>, <B>--console-on</B><DD>
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Enable printing messages to the console.
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<DT id="6"><B>-e</B>, <B>--reltime</B><DD>
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Display the local time and the delta in human-readable format. Be aware that
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conversion to the local time could be inaccurate (see <B>-T</B> for more
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details).
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<DT id="7"><B>-F</B>, <B>--file </B><I>file</I><DD>
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Read the syslog messages from the given
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<I>file</I>.
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Note that <B>-F</B> does not support messages in kmsg format. The old syslog format is supported only.
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<DT id="8"><B>-f</B>, <B>--facility </B><I>list</I><DD>
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Restrict output to the given (comma-separated)
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<I>list</I>
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of facilities. For example:
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</DL>
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<P>
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<DL COMPACT><DT id="9"><DD>
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<B>dmesg --facility=daemon</B>
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</DL>
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<DL COMPACT>
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<DT id="10"><DD>
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will print messages from system daemons only. For all supported facilities
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see the
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<B>--help</B>
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output.
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<DT id="11"><B>-H</B>, <B>--human</B><DD>
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Enable human-readable output. See also <B>--color</B>, <B>--reltime</B>
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and <B>--nopager</B>.
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<DT id="12"><B>-k</B>, <B>--kernel</B><DD>
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Print kernel messages.
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<DT id="13"><B>-L</B>, <B>--color</B>[=<I>when</I>]<DD>
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Colorize the output. The optional argument <I>when</I>
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can be <B>auto</B>, <B>never</B> or <B>always</B>. If the <I>when</I> argument is omitted,
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it defaults to <B>auto</B>. The colors can be disabled; for the current built-in default
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see the <B>--help</B> output. See also the <B>COLORS</B> section below.
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<DT id="14"><B>-l</B>, <B>--level </B><I>list</I><DD>
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Restrict output to the given (comma-separated)
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<I>list</I>
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of levels. For example:
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</DL>
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<P>
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<DL COMPACT><DT id="15"><DD>
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<B>dmesg --level=err,warn</B>
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</DL>
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<DL COMPACT>
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<DT id="16"><DD>
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will print error and warning messages only. For all supported levels see the
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<B>--help</B>
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output.
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<DT id="17"><B>-n</B>, <B>--console-level </B><I>level</I><DD>
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Set the
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<I>level</I>
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at which printing of messages is done to the console. The
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<I>level</I>
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is a level number or abbreviation of the level name. For all supported
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levels see the
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<B>--help</B>
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output.
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<P>
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For example,
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<B>-n 1</B>
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or
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<B>-n emerg</B>
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prevents all messages, except emergency (panic) messages, from appearing on
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the console. All levels of messages are still written to
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<I>/proc/kmsg</I>,
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so
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<B><A HREF="/cgi-bin/man/man2html?8+syslogd">syslogd</A></B>(8)
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can still be used to control exactly where kernel messages appear. When the
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<B>-n</B>
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option is used,
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<B>dmesg</B>
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will
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<I>not</I>
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print or clear the kernel ring buffer.
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<DT id="18"><B>-P</B>, <B>--nopager</B><DD>
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Do not pipe output into a pager. A pager is enabled by default for <B>--human</B> output.
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<DT id="19"><B>-p</B>, <B>--force-prefix</B><DD>
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Add facility, level or timestamp information to each line of a multi-line message.
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<DT id="20"><B>-r</B>, <B>--raw</B><DD>
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Print the raw message buffer, i.e. do not strip the log-level prefixes.
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<P>
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Note that the real raw format depends on the method how
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<B><A HREF="/cgi-bin/man/man2html?1+dmesg">dmesg</A></B>(1)
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reads kernel messages. The /dev/kmsg device uses a different format than
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<B><A HREF="/cgi-bin/man/man2html?2+syslog">syslog</A></B>(2).
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For backward compatibility,
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<B><A HREF="/cgi-bin/man/man2html?1+dmesg">dmesg</A></B>(1)
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returns data always in the
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<B><A HREF="/cgi-bin/man/man2html?2+syslog">syslog</A></B>(2)
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format. It is possible to read the real raw data from /dev/kmsg by, for example,
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the command 'dd if=/dev/kmsg iflag=nonblock'.
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<DT id="21"><B>-S</B>, <B>--syslog</B><DD>
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Force <B>dmesg</B> to use the
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<B><A HREF="/cgi-bin/man/man2html?2+syslog">syslog</A></B>(2)
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kernel interface to read kernel messages. The default is to use /dev/kmsg rather
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than
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<B><A HREF="/cgi-bin/man/man2html?2+syslog">syslog</A></B>(2)
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since kernel 3.5.0.
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<DT id="22"><B>-s</B>, <B>--buffer-size </B><I>size</I><DD>
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Use a buffer of
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<I>size</I>
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to query the kernel ring buffer. This is 16392 by default. (The default
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kernel syslog buffer size was 4096 at first, 8192 since 1.3.54, 16384 since
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2.1.113.) If you have set the kernel buffer to be larger than the default,
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then this option can be used to view the entire buffer.
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<DT id="23"><B>-T</B>, <B>--ctime</B><DD>
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Print human-readable timestamps.
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<DT id="24"><DD>
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<B>Be aware that the timestamp could be inaccurate!</B>
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The
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<B>time</B>
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source used for the logs is
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<B>not updated after</B>
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system
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<B>SUSPEND</B>/<B>RESUME</B>.
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<DT id="25"><B>-t</B>, <B>--notime</B><DD>
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Do not print kernel's timestamps.
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<DT id="26"><B>--time-format</B> <I>format</I><DD>
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Print timestamps using the given <I>format</I>, which can be
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<B>ctime</B>,
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<B>reltime</B>,
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<B>delta</B>
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or
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<B>iso</B>.
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The first three formats are aliases of the time-format-specific options.
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The
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<B>iso</B>
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format is a
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<B>dmesg</B>
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implementation of the ISO-8601 timestamp format. The purpose of this format is
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to make the comparing of timestamps between two systems, and any other parsing,
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easy. The definition of the <B>iso</B> timestamp is:
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YYYY-MM-DD<T>HH:MM:SS,<microseconds><-+><timezone offset from UTC>.
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<DT id="27"><DD>
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The
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<B>iso</B>
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format has the same issue as
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<B>ctime</B>:
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the time may be inaccurate when a system is suspended and resumed.
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<DT id="28"><B>-u</B>,<B> --userspace</B>
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<DD>
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Print userspace messages.
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<DT id="29"><B>-w</B>,<B> --follow</B>
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<DD>
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Wait for new messages. This feature is supported only on systems with
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a readable /dev/kmsg (since kernel 3.5.0).
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<DT id="30"><B>-x</B>,<B> --decode</B>
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<DD>
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Decode facility and level (priority) numbers to human-readable prefixes.
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<DT id="31"><B>-V</B>,<B> --version</B>
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<DD>
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Display version information and exit.
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<DT id="32"><B>-h</B>,<B> --help</B>
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<DD>
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Display help text and exit.
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</DL>
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<A NAME="lbAF"> </A>
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<H2>COLORS</H2>
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Implicit coloring can be disabled by an empty file <I>/etc/terminal-colors.d/dmesg.disable</I>.
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See
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<B><A HREF="/cgi-bin/man/man2html?5+terminal-colors.d">terminal-colors.d</A></B>(5)
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for more details about colorization configuration.
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<P>
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The logical color names supported by
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<B>dmesg</B>
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are:
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<DL COMPACT>
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<DT id="33"><B>subsys</B>
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<DD>
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The message sub-system prefix (e.g. "ACPI:").
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<DT id="34"><B>time</B>
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<DD>
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The message timestamp.
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<DT id="35"><B>timebreak</B>
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<DD>
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The message timestamp in short ctime format in <B>--reltime</B>
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or <B>--human</B> output.
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<DT id="36"><B>alert</B>
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<DD>
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The text of the message with the alert log priority.
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<DT id="37"><B>crit</B>
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<DD>
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The text of the message with the critical log priority.
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<DT id="38"><B>err</B>
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<DD>
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The text of the message with the error log priority.
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<DT id="39"><B>warn</B>
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<DD>
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The text of the message with the warning log priority.
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<DT id="40"><B>segfault</B>
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<DD>
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The text of the message that inform about segmentation fault.
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</DL>
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<A NAME="lbAG"> </A>
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<H2>EXIT STATUS</H2>
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<B>dmesg</B>
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can fail reporting permission denied error. This is usually caused by
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<B>dmesg_restrict</B>
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kernel setting, please see
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<B><A HREF="/cgi-bin/man/man2html?2+syslog">syslog</A></B>(2)
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for more details.
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<A NAME="lbAH"> </A>
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<H2>SEE ALSO</H2>
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<B><A HREF="/cgi-bin/man/man2html?5+terminal-colors.d">terminal-colors.d</A></B>(5),
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<B><A HREF="/cgi-bin/man/man2html?8+syslogd">syslogd</A></B>(8)
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<A NAME="lbAI"> </A>
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<H2>AUTHORS</H2>
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Karel Zak
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<P>
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<BR>
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<B>dmesg</B>
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was originally written by
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Theodore Ts'o
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<A NAME="lbAJ"> </A>
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<H2>AVAILABILITY</H2>
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The dmesg command is part of the util-linux package and is available from
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Linux Kernel Archive
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<P>
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<HR>
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<A NAME="index"> </A><H2>Index</H2>
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<DL>
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<DT id="41"><A HREF="#lbAB">NAME</A><DD>
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<DT id="42"><A HREF="#lbAC">SYNOPSIS</A><DD>
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<DT id="43"><A HREF="#lbAD">DESCRIPTION</A><DD>
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<DT id="44"><A HREF="#lbAE">OPTIONS</A><DD>
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<DT id="45"><A HREF="#lbAF">COLORS</A><DD>
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<DT id="46"><A HREF="#lbAG">EXIT STATUS</A><DD>
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<DT id="47"><A HREF="#lbAH">SEE ALSO</A><DD>
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<DT id="48"><A HREF="#lbAI">AUTHORS</A><DD>
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<DT id="49"><A HREF="#lbAJ">AVAILABILITY</A><DD>
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</DL>
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<HR>
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This document was created by
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<A HREF="/cgi-bin/man/man2html">man2html</A>,
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using the manual pages.<BR>
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Time: 00:05:11 GMT, March 31, 2021
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</BODY>
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</HTML>
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