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 emerg
 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\-\-noescape\fR"
 117 The unprintable and potentially unsafe characters (e.g. broken multi-byte
 118 sequences, terminal controlling chars, etc.) are escaped in format \\x<hex> for
 119 security reason by default.  This option disables this feature at all. It's
 120 usable for example for debugging purpose together with \fB\-\-raw\fR.  Be
 121 careful and don't use it by default.
 122 .IP "\fB\-P\fR, \fB\-\-nopager\fR"
 123 Do not pipe output into a pager.  A pager is enabled by default for \fB\-\-human\fR output.
 124 .IP "\fB\-p\fR, \fB\-\-force\-prefix\fR"
 125 Add facility, level or timestamp information to each line of a multi-line message.
 126 .IP "\fB\-r\fR, \fB\-\-raw\fR"
 127 Print the raw message buffer, i.e. do not strip the log-level prefixes, but
 128 all unprintable characters are still escaped (see also \fB\-\-noescape\fR).
 129
 130 Note that the real raw format depends on the method how
 131 .BR dmesg (1)
 132 reads kernel messages.  The /dev/kmsg device uses a different format than
 133 .BR syslog (2).
 134 For backward compatibility,
 135 .BR dmesg (1)
 136 returns data always in the
 137 .BR syslog (2)
 138 format.  It is possible to read the real raw data from /dev/kmsg by, for example,
 139 the command 'dd if=/dev/kmsg iflag=nonblock'.
 140 .IP "\fB\-S\fR, \fB\-\-syslog\fR"
 141 Force \fBdmesg\fR to use the
 142 .BR syslog (2)
 143 kernel interface to read kernel messages.  The default is to use /dev/kmsg rather
 144 than
 145 .BR syslog (2)
 146 since kernel 3.5.0.
 147 .IP "\fB\-s\fR, \fB\-\-buffer\-size \fIsize\fR
 148 Use a buffer of
 149 .I size
 150 to query the kernel ring buffer.  This is 16392 by default.  (The default
 151 kernel syslog buffer size was 4096 at first, 8192 since 1.3.54, 16384 since
 152 2.1.113.)  If you have set the kernel buffer to be larger than the default,
 153 then this option can be used to view the entire buffer.
 154 .IP "\fB\-T\fR, \fB\-\-ctime\fR"
 155 Print human-readable timestamps.
 156 .IP
 157 .B Be aware that the timestamp could be inaccurate!
 158 The
 159 .B time
 160 source used for the logs is
 161 .B not updated after
 162 system
 163 .BR SUSPEND / RESUME .
 164 .IP "\fB\-t\fR, \fB\-\-notime\fR"
 165 Do not print kernel's timestamps.
 166 .IP "\fB\-\-time\-format\fR \fIformat\fR"
 167 Print timestamps using the given \fIformat\fR, which can be
 168 .BR ctime ,
 169 .BR reltime ,
 170 .B delta
 171 or
 172 .BR iso .
 173 The first three formats are aliases of the time-format-specific options.
 174 The
 175 .B iso
 176 format is a
 177 .B dmesg
 178 implementation of the ISO-8601 timestamp format.  The purpose of this format is
 179 to make the comparing of timestamps between two systems, and any other parsing,
 180 easy.  The definition of the \fBiso\fR timestamp is:
 181 YYYY-MM-DD<T>HH:MM:SS,<microseconds><-+><timezone offset from UTC>.
 182 .IP
 183 The
 184 .B iso
 185 format has the same issue as
 186 .BR ctime :
 187 the time may be inaccurate when a system is suspended and resumed.
 188 .TP
 189 .BR \-u , " \-\-userspace"
 190 Print userspace messages.
 191 .TP
 192 .BR \-w , " \-\-follow"
 193 Wait for new messages.  This feature is supported only on systems with
 194 a readable /dev/kmsg (since kernel 3.5.0).
 195 .TP
 196 .BR \-x , " \-\-decode"
 197 Decode facility and level (priority) numbers to human-readable prefixes.
 198 .TP
 199 .BR \-V , " \-\-version"
 200 Display version information and exit.
 201 .TP
 202 .BR \-h , " \-\-help"
 203 Display help text and exit.
 204 .SH COLORS
 205 Implicit coloring can be disabled by an empty file \fI/etc/terminal-colors.d/dmesg.disable\fR.
 206 See
 207 .BR terminal-colors.d (5)
 208 for more details about colorization configuration.
 209 .PP
 210 The logical color names supported by
 211 .B dmesg
 212 are:
 213 .TP
 214 .B subsys
 215 The message sub-system prefix (e.g. "ACPI:").
 216 .TP
 217 .B time
 218 The message timestamp.
 219 .TP
 220 .B timebreak
 221 The message timestamp in short ctime format in \fB\-\-reltime\fR
 222 or \fB\-\-human\fR output.
 223 .TP
 224 .B alert
 225 The text of the message with the alert log priority.
 226 .TP
 227 .B crit
 228 The text of the message with the critical log priority.
 229 .TP
 230 .B err
 231 The text of the message with the error log priority.
 232 .TP
 233 .B warn
 234 The text of the message with the warning log priority.
 235 .TP
 236 .B segfault
 237 The text of the message that inform about segmentation fault.
 238 .SH EXIT STATUS
 239 .B dmesg
 240 can fail reporting permission denied error.  This is usually caused by
 241 .B dmesg_restrict
 242 kernel setting, please see
 243 .BR syslog (2)
 244 for more details.
 245 .SH SEE ALSO
 246 .BR terminal-colors.d (5),
 247 .BR syslogd (8)
 248 .SH AUTHORS
 249 .MT kzak@redhat.com
 250 Karel Zak
 251 .ME
 252
 253 .br
 254 .B dmesg
 255 was originally written by
 256 .MT tytso@athena.mit.edu
 257 Theodore Ts'o
 258 .ME
 259 .SH AVAILABILITY
 260 The dmesg command is part of the util-linux package and is available from
 261 .UR https://\:www.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
 262 Linux Kernel Archive
 263 .UE .