sys-utils/dmesg.1

   1 .\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
   2 .\" May be distributed under the GNU General Public License
   3 .TH DMESG "1" "July 2012" "util-linux" "User Commands"
   4 .SH NAME
   5 dmesg \- print or control the kernel ring buffer
   6 .SH SYNOPSIS
   7 .B dmesg
   8 [options]
   9 .sp
  10 .B dmesg \-\-clear
  11 .br
  12 .BR "dmesg \-\-read\-clear " [options]
  13 .br
  14 .BI "dmesg \-\-console\-level " level
  15 .br
  16 .B dmesg \-\-console\-on
  17 .br
  18 .B dmesg \-\-console\-off
  19 .SH DESCRIPTION
  20 .B dmesg
  21 is used to examine or control the kernel ring buffer.
  22 .PP
  23 The default action is to display all messages from the kernel ring buffer.
  24 .SH OPTIONS
  25 The
  26 .BR \-\-clear ,
  27 .BR \-\-read\-clear ,
  28 .BR \-\-console\-on ,
  29 .BR \-\-console\-off ,
  30 and
  31 .B \-\-console\-level
  32 options are mutually exclusive.
  33 .PP
  34 .IP "\fB\-C\fR, \fB\-\-clear\fR"
  35 Clear the ring buffer.
  36 .IP "\fB\-c\fR, \fB\-\-read\-clear\fR"
  37 Clear the ring buffer after first printing its contents.
  38 .IP "\fB\-D\fR, \fB\-\-console\-off\fR"
  39 Disable the printing of messages to the console.
  40 .IP "\fB\-d\fR, \fB\-\-show\-delta\fR"
  41 Display the timestamp and the time delta spent between messages.  If used
  42 together with
  43 .B \-\-notime
  44 then only the time delta without the timestamp is printed.
  45 .IP "\fB\-E\fR, \fB\-\-console\-on\fR"
  46 Enable printing messages to the console.
  47 .IP "\fB\-e\fR, \fB\-\-reltime\fR"
  48 Display the local time and the delta in human-readable format.  Be aware that
  49 conversion to the local time could be inaccurate (see \fB\-T\fR for more
  50 details).
  51 .IP "\fB\-F\fR, \fB\-\-file \fIfile\fR"
  52 Read the messages from the given
  53 .IR file .
  54 .IP "\fB\-f\fR, \fB\-\-facility \fIlist\fR"
  55 Restrict output to the given (comma-separated)
  56 .I list
  57 of facilities.  For example:
  58 .PP
  59 .RS 14
  60 .B dmesg \-\-facility=daemon
  61 .RE
  62 .IP
  63 will print messages from system daemons only.  For all supported facilities
  64 see the
  65 .B \-\-help
  66 output.
  67 .IP "\fB\-H\fR, \fB\-\-human\fR"
  68 Enable human-readable output.  See also \fB\-\-color\fR, \fB\-\-reltime\fR
  69 and \fB\-\-nopager\fR.
  70 .IP "\fB\-k\fR, \fB\-\-kernel\fR"
  71 Print kernel messages.
  72 .IP "\fB\-L\fR, \fB\-\-color\fR[=\fIwhen\fR]"
  73 Colorize the output.  The optional argument \fIwhen\fP
  74 can be \fBauto\fR, \fBnever\fR or \fBalways\fR.  If the \fIwhen\fR argument is omitted,
  75 it defaults to \fBauto\fR.  The colors can be disabled; for the current built-in default
  76 see the \fB\-\-help\fR output.  See also the \fBCOLORS\fR section below.
  77 .IP  "\fB\-l\fR, \fB\-\-level \fIlist\fR"
  78 Restrict output to the given (comma-separated)
  79 .I list
  80 of levels.  For example:
  81 .PP
  82 .RS 14
  83 .B dmesg \-\-level=err,warn
  84 .RE
  85 .IP
  86 will print error and warning messages only.  For all supported levels see the
  87 .B \-\-help
  88 output.
  89 .IP "\fB\-n\fR, \fB\-\-console\-level \fIlevel\fR
  90 Set the
  91 .I level
  92 at which printing of messages is done to the console.  The
  93 .I level
  94 is a level number or abbreviation of the level name.  For all supported
  95 levels see the
  96 .B \-\-help
  97 output.
  98 .sp
  99 For example,
 100 .B \-n 1
 101 or
 102 .B \-n alert
 103 prevents all messages, except emergency (panic) messages, from appearing on
 104 the console.  All levels of messages are still written to
 105 .IR /proc/kmsg ,
 106 so
 107 .BR syslogd (8)
 108 can still be used to control exactly where kernel messages appear.  When the
 109 .B \-n
 110 option is used,
 111 .B dmesg
 112 will
 113 .I not
 114 print or clear the kernel ring buffer.
 115 .IP "\fB\-P\fR, \fB\-\-nopager\fR"
 116 Do not pipe output into a pager.  A pager is enabled by default for \fB\-\-human\fR output.
 117 .IP "\fB\-r\fR, \fB\-\-raw\fR"
 118 Print the raw message buffer, i.e. do not strip the log-level prefixes.
 119
 120 Note that the real raw format depends on the method how
 121 .BR dmesg (1)
 122 reads kernel messages.  The /dev/kmsg device uses a different format than
 123 .BR syslog (2).
 124 For backward compatibility,
 125 .BR dmesg (1)
 126 returns data always in the
 127 .BR syslog (2)
 128 format.  It is possible to read the real raw data from /dev/kmsg by, for example,
 129 the command 'dd if=/dev/kmsg iflag=nonblock'.
 130 .IP "\fB\-S\fR, \fB\-\-syslog\fR"
 131 Force \fBdmesg\fR to use the
 132 .BR syslog (2)
 133 kernel interface to read kernel messages.  The default is to use /dev/kmsg rather
 134 than
 135 .BR syslog (2)
 136 since kernel 3.5.0.
 137 .IP "\fB\-s\fR, \fB\-\-buffer\-size \fIsize\fR
 138 Use a buffer of
 139 .I size
 140 to query the kernel ring buffer.  This is 16392 by default.  (The default
 141 kernel syslog buffer size was 4096 at first, 8192 since 1.3.54, 16384 since
 142 2.1.113.)  If you have set the kernel buffer to be larger than the default,
 143 then this option can be used to view the entire buffer.
 144 .IP "\fB\-T\fR, \fB\-\-ctime\fR"
 145 Print human-readable timestamps.
 146 .IP
 147 .B Be aware that the timestamp could be inaccurate!
 148 The
 149 .B time
 150 source used for the logs is
 151 .B not updated after
 152 system
 153 .BR SUSPEND / RESUME .
 154 .IP "\fB\-t\fR, \fB\-\-notime\fR"
 155 Do not print kernel's timestamps.
 156 .IP "\fB\-\-time\-format\fR \fIformat\fR"
 157 Print timestamps using the given \fIformat\fR, which can be
 158 .BR ctime ,
 159 .BR reltime ,
 160 .BR delta
 161 or
 162 .BR iso .
 163 The first three formats are aliases of the time-format-specific options.
 164 The
 165 .B iso
 166 format is a
 167 .B dmesg
 168 implementation of the ISO-8601 timestamp format.  The purpose of this format is
 169 to make the comparing of timestamps between two systems, and any other parsing,
 170 easy.  The definition of the \fBiso\fR timestamp is:
 171 YYYY-MM-DD<T>HH:MM:SS,<microseconds><-+><timezone offset from UTC>.
 172 .IP
 173 The
 174 .B iso
 175 format has the same issue as
 176 .BR ctime :
 177 the time may be inaccurate when a system is suspended and resumed.
 178 .TP
 179 .BR \-u , " \-\-userspace"
 180 Print userspace messages.
 181 .TP
 182 .BR \-w , " \-\-follow"
 183 Wait for new messages.  This feature is supported only on systems with
 184 a readable /dev/kmsg (since kernel 3.5.0).
 185 .TP
 186 .BR \-x , " \-\-decode"
 187 Decode facility and level (priority) numbers to human-readable prefixes.
 188 .TP
 189 .BR \-V , " \-\-version"
 190 Display version information and exit.
 191 .TP
 192 .BR \-h , " \-\-help"
 193 Display help text and exit.
 194 .SH COLORS
 195 Implicit coloring can be disabled by an empty file \fI/etc/terminal-colors.d/dmesg.disable\fR.
 196 See
 197 .BR terminal-colors.d (5)
 198 for more details about colorization configuration.
 199 .PP
 200 The logical color names supported by
 201 .B dmesg
 202 are:
 203 .TP
 204 .B subsys
 205 The message sub-system prefix (e.g. "ACPI:").
 206 .TP
 207 .B time
 208 The message timestamp.
 209 .TP
 210 .B timebreak
 211 The message timestamp in short ctime format in \fB\-\-reltime\fR
 212 or \fB\-\-human\fR output.
 213 .TP
 214 .B alert
 215 The text of the message with the alert log priority.
 216 .TP
 217 .B crit
 218 The text of the message with the critical log priority.
 219 .TP
 220 .B err
 221 The text of the message with the error log priority.
 222 .TP
 223 .B warn
 224 The text of the message with the warning log priority.
 225 .TP
 226 .B segfault
 227 The text of the message that inform about segmentation fault.
 228
 229 .SH SEE ALSO
 230 .BR terminal-colors.d (5),
 231 .BR syslogd (8)
 232 .SH AUTHORS
 233 .MT kzak@redhat.com
 234 Karel Zak
 235 .ME
 236
 237 .br
 238 .B dmesg
 239 was originally written by
 240 .MT tytso@athena.mit.edu
 241 Theodore Ts'o
 242 .ME
 243 .SH AVAILABILITY
 244 The dmesg command is part of the util-linux package and is available from
 245 .UR https://\:www.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
 246 Linux Kernel Archive
 247 .UE .