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