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 syslog messages from the given
  53 .IR file .
  54 Note that \fB\-F\fR does not support messages in kmsg format. The old syslog format is supported only.
  55 .IP "\fB\-f\fR, \fB\-\-facility \fIlist\fR"
  56 Restrict output to the given (comma-separated)
  57 .I list
  58 of facilities.  For example:
  59 .PP
  60 .RS 14
  61 .B dmesg \-\-facility=daemon
  62 .RE
  63 .IP
  64 will print messages from system daemons only.  For all supported facilities
  65 see the
  66 .B \-\-help
  67 output.
  68 .IP "\fB\-H\fR, \fB\-\-human\fR"
  69 Enable human-readable output.  See also \fB\-\-color\fR, \fB\-\-reltime\fR
  70 and \fB\-\-nopager\fR.
  71 .IP "\fB\-k\fR, \fB\-\-kernel\fR"
  72 Print kernel messages.
  73 .IP "\fB\-L\fR, \fB\-\-color\fR[=\fIwhen\fR]"
  74 Colorize the output.  The optional argument \fIwhen\fP
  75 can be \fBauto\fR, \fBnever\fR or \fBalways\fR.  If the \fIwhen\fR argument is omitted,
  76 it defaults to \fBauto\fR.  The colors can be disabled; for the current built-in default
  77 see the \fB\-\-help\fR output.  See also the \fBCOLORS\fR section below.
  78 .IP  "\fB\-l\fR, \fB\-\-level \fIlist\fR"
  79 Restrict output to the given (comma-separated)
  80 .I list
  81 of levels.  For example:
  82 .PP
  83 .RS 14
  84 .B dmesg \-\-level=err,warn
  85 .RE
  86 .IP
  87 will print error and warning messages only.  For all supported levels see the
  88 .B \-\-help
  89 output.
  90 .IP "\fB\-n\fR, \fB\-\-console\-level \fIlevel\fR
  91 Set the
  92 .I level
  93 at which printing of messages is done to the console.  The
  94 .I level
  95 is a level number or abbreviation of the level name.  For all supported
  96 levels see the
  97 .B \-\-help
  98 output.
  99 .sp
 100 For example,
 101 .B \-n 1
 102 or
 103 .B \-n alert
 104 prevents all messages, except emergency (panic) messages, from appearing on
 105 the console.  All levels of messages are still written to
 106 .IR /proc/kmsg ,
 107 so
 108 .BR syslogd (8)
 109 can still be used to control exactly where kernel messages appear.  When the
 110 .B \-n
 111 option is used,
 112 .B dmesg
 113 will
 114 .I not
 115 print or clear the kernel ring buffer.
 116 .IP "\fB\-P\fR, \fB\-\-nopager\fR"
 117 Do not pipe output into a pager.  A pager is enabled by default for \fB\-\-human\fR output.
 118 .IP "\fB\-p\fR, \fB\-\-force\-prefix\fR"
 119 Add facility, level or timestamp information to each line of a multi-line message.
 120 .IP "\fB\-r\fR, \fB\-\-raw\fR"
 121 Print the raw message buffer, i.e. do not strip the log-level prefixes.
 122
 123 Note that the real raw format depends on the method how
 124 .BR dmesg (1)
 125 reads kernel messages.  The /dev/kmsg device uses a different format than
 126 .BR syslog (2).
 127 For backward compatibility,
 128 .BR dmesg (1)
 129 returns data always in the
 130 .BR syslog (2)
 131 format.  It is possible to read the real raw data from /dev/kmsg by, for example,
 132 the command 'dd if=/dev/kmsg iflag=nonblock'.
 133 .IP "\fB\-S\fR, \fB\-\-syslog\fR"
 134 Force \fBdmesg\fR to use the
 135 .BR syslog (2)
 136 kernel interface to read kernel messages.  The default is to use /dev/kmsg rather
 137 than
 138 .BR syslog (2)
 139 since kernel 3.5.0.
 140 .IP "\fB\-s\fR, \fB\-\-buffer\-size \fIsize\fR
 141 Use a buffer of
 142 .I size
 143 to query the kernel ring buffer.  This is 16392 by default.  (The default
 144 kernel syslog buffer size was 4096 at first, 8192 since 1.3.54, 16384 since
 145 2.1.113.)  If you have set the kernel buffer to be larger than the default,
 146 then this option can be used to view the entire buffer.
 147 .IP "\fB\-T\fR, \fB\-\-ctime\fR"
 148 Print human-readable timestamps.
 149 .IP
 150 .B Be aware that the timestamp could be inaccurate!
 151 The
 152 .B time
 153 source used for the logs is
 154 .B not updated after
 155 system
 156 .BR SUSPEND / RESUME .
 157 .IP "\fB\-t\fR, \fB\-\-notime\fR"
 158 Do not print kernel's timestamps.
 159 .IP "\fB\-\-time\-format\fR \fIformat\fR"
 160 Print timestamps using the given \fIformat\fR, which can be
 161 .BR ctime ,
 162 .BR reltime ,
 163 .B delta
 164 or
 165 .BR iso .
 166 The first three formats are aliases of the time-format-specific options.
 167 The
 168 .B iso
 169 format is a
 170 .B dmesg
 171 implementation of the ISO-8601 timestamp format.  The purpose of this format is
 172 to make the comparing of timestamps between two systems, and any other parsing,
 173 easy.  The definition of the \fBiso\fR timestamp is:
 174 YYYY-MM-DD<T>HH:MM:SS,<microseconds><-+><timezone offset from UTC>.
 175 .IP
 176 The
 177 .B iso
 178 format has the same issue as
 179 .BR ctime :
 180 the time may be inaccurate when a system is suspended and resumed.
 181 .TP
 182 .BR \-u , " \-\-userspace"
 183 Print userspace messages.
 184 .TP
 185 .BR \-w , " \-\-follow"
 186 Wait for new messages.  This feature is supported only on systems with
 187 a readable /dev/kmsg (since kernel 3.5.0).
 188 .TP
 189 .BR \-x , " \-\-decode"
 190 Decode facility and level (priority) numbers to human-readable prefixes.
 191 .TP
 192 .BR \-V , " \-\-version"
 193 Display version information and exit.
 194 .TP
 195 .BR \-h , " \-\-help"
 196 Display help text and exit.
 197 .SH COLORS
 198 Implicit coloring can be disabled by an empty file \fI/etc/terminal-colors.d/dmesg.disable\fR.
 199 See
 200 .BR terminal-colors.d (5)
 201 for more details about colorization configuration.
 202 .PP
 203 The logical color names supported by
 204 .B dmesg
 205 are:
 206 .TP
 207 .B subsys
 208 The message sub-system prefix (e.g. "ACPI:").
 209 .TP
 210 .B time
 211 The message timestamp.
 212 .TP
 213 .B timebreak
 214 The message timestamp in short ctime format in \fB\-\-reltime\fR
 215 or \fB\-\-human\fR output.
 216 .TP
 217 .B alert
 218 The text of the message with the alert log priority.
 219 .TP
 220 .B crit
 221 The text of the message with the critical log priority.
 222 .TP
 223 .B err
 224 The text of the message with the error log priority.
 225 .TP
 226 .B warn
 227 The text of the message with the warning log priority.
 228 .TP
 229 .B segfault
 230 The text of the message that inform about segmentation fault.
 231 .SH EXIT STATUS
 232 .B dmesg
 233 can fail reporting permission denied error.  This is usually caused by
 234 .B dmesg_restrict
 235 kernel setting, please see
 236 .BR syslog (2)
 237 for more details.
 238 .SH SEE ALSO
 239 .BR terminal-colors.d (5),
 240 .BR syslogd (8)
 241 .SH AUTHORS
 242 .MT kzak@redhat.com
 243 Karel Zak
 244 .ME
 245
 246 .br
 247 .B dmesg
 248 was originally written by
 249 .MT tytso@athena.mit.edu
 250 Theodore Ts'o
 251 .ME
 252 .SH AVAILABILITY
 253 The dmesg command is part of the util-linux package and is available from
 254 .UR https://\:www.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
 255 Linux Kernel Archive
 256 .UE .