.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
.\" May be distributed under the GNU General Public License
.TH DMESG "1" "July 2012" "util-linux" "User Commands"
.SH NAME
dmesg \- print or control the kernel ring buffer
.SH SYNOPSIS
.B dmesg
.RB [ options ]
.sp
dmesg \-\-clear
.br
dmesg \-\-read-clear [options]
.br
dmesg \-\-console-level level
.br
dmesg \-\-console-on
.br
dmesg \-\-console-off
.SH DESCRIPTION
.B dmesg
is used to examine or control the kernel ring buffer.
.PP
The default action is to read all messages from kernel ring buffer.
.SH OPTIONS
The \-\-clear, \-\-read-clear, \-\-console-on, \-\-console-off and
\-\-console-level options are mutually exclusive.
.PP
.IP "\fB\-C\fR, \fB\-\-clear\fR"
Clear the ring buffer.
.IP "\fB\-c\fR, \fB\-\-read-clear\fR"
Clear the ring buffer contents after printing.
.IP "\fB\-D\fR, \fB\-\-console-off\fR"
Disable printing messages to the console.
.IP "\fB\-d\fR, \fB\-\-show-delta\fR"
Display the timestamp and time delta spent between messages.  If used
together with
.B \-\-notime
then only the time delta without the timestamp is printed.
.IP "\fB\-e\fR, \fB\-\-reltime\fR"
Display the local time and delta in human readable format.
.IP "\fB\-E\fR, \fB\-\-console-on\fR"
Enable printing messages to the console.
.IP "\fB\-F\fR, \fB\-\-file \fIfile\fR"
Read log from
.IR file .
.IP "\fB\-f\fR, \fB\-\-facility \fIlist\fR"
Restrict output to defined (comma separated)
.I list
of facilities.  For example
.PP
.RS 14
dmesg \-\-facility=daemon
.RE
.IP
will print messages from system daemons only.  For all supported facilities
see
.B dmesg \-\-help
output.
.IP "\fB\-H\fR, \fB\-\-human\fR"
Enable human readable output.  See also \fB\-\-color\fR, \fB\-\-reltime\fR
and \fB\-\-nopager\fR.
.IP "\fB\-h\fR, \fB\-\-help\fR"
Print a help text and exit.
.IP "\fB\-k\fR, \fB\-\-kernel\fR"
Print kernel messages.
.IP "\fB\-L\fR, \fB\-\-color\fR [=\fIwhen\fP]"
Colorize important messages, the optional argumet \fIwhen\fP is 'auto', 'never'
or 'always'. If the \fIwhen\fP argument is omitted then the default is 'auto'.
.IP  "\fB\-l\fR, \fB\-\-level \fIlist\fR"
Restrict output to defined (comma separated)
.I list
of levels.  For example
.PP
.RS 14
dmesg \-\-level=err,warn
.RE
.IP
will print error and warning messages only.  For all supported levels see
.B dmesg \-\-help
output.
.IP "\fB\-n\fR, \fB\-\-console-level \fIlevel\fR
Set the
.I level
at which logging of messages is done to the console.  The
.I level
is a level number or abbreviation of the level name.  For all supported
levels see
.B dmesg \-\-help
output.
.sp
For example,
.B \-n 1
or
.B \-n alert
prevents all messages, except emergency (panic) messages, from appearing on
the console.  All levels of messages are still written to
.IR /proc/kmsg ,
so
.BR syslogd (8)
can still be used to control exactly where kernel messages appear.  When the
.B \-n
option is used,
.B dmesg
will
.I not
print or clear the kernel ring buffer.
.IP "\fB\-P\fR, \fB\-\-nopager\fR"
Do not pipe output into a pager, the pager is enabled for \fB\-\-human\fR output.
.IP "\fB\-r\fR, \fB\-\-raw\fR"
Print the raw message buffer, i.e., do not strip the log level prefixes.

Note that the real raw format depends on method how
.BR dmesg (1)
reads kernel messages. The /dev/kmsg uses different format than
.BR syslog (2) .
For backward compatibility
.BR dmesg (1)
returns data always in
.BR syslog (2)
format. The real raw data from /dev/kmsg is possible to read for example by
command 'dd if=/dev/kmsg iflag=nonblock'.
.IP "\fB\-S\fR, \fB\-\-syslog\fR"
Force to use
.BR syslog (2)
kernel interface to read kernel messages. The default is to use /dev/kmsg rather
than
.BR syslog (2)
since kernel 3.5.0.
.IP "\fB\-s\fR, \fB\-\-buffer-size \fIsize\fR
Use a buffer of
.I size
to query the kernel ring buffer.  This is 16392 by default.  (The default
kernel syslog buffer size was 4096 at first, 8192 since 1.3.54, 16384 since
2.1.113.)  If you have set the kernel buffer to be larger than the default
then this option can be used to view the entire buffer.
.IP "\fB\-T\fR, \fB\-\-ctime\fR"
Print human readable timestamps.  The timestamp could be inaccurate!
.IP
The
.B time
source used for the logs is
.B not updated after
system
.BR SUSPEND / RESUME .
.IP "\fB\-t\fR, \fB\-\-notime\fR"
Do not print kernel's timestamps.
.IP "\fB\-u\fR, \fB\-\-userspace\fR"
Print userspace messages.
.IP "\fB\-V\fR, \fB\-\-version\fR"
Output version information and exit.
.IP "\fB\-w\fR, \fB\-\-follow\fR"
Wait for new messages. This feature is supported on systems with readable
/dev/kmsg only (since kernel 3.5.0).
.IP "\fB\-x\fR, \fB\-\-decode\fR"
Decode facility and level (priority) number to human readable prefixes.
.IP "\fB\-\-time\-format\fR \fIformat\fR"
Print time stam using format
.IR ctime ,
.IR delta ,
.I reltime
or
.IR iso .
The three first time formats are aliases of time format specific options.
The
.I iso
is a
.B dmesg
implementation of ISO-8601 timestamp format.  Purpose of this format is
to make comparing of timestamps in between two system, and other parsing,
easy.  The defition of timestamp is YYYY-MM-DD<T stands for date and time
separator>HH:MM:SS,<microseconds><-+><GMT-0 timezone offset>.
.IP
The
.I iso
time format has the same issue as
.IR ctime ,
the time may be inaccurate if a system is suspended and resumed.
.SH SEE ALSO
.BR syslogd (8)
.SH AUTHORS
.MT kzak@redhat.com
Karel Zak
.ME
.br
.MT tytso@athena.mit.edu
Theodore Ts'o
.ME
.SH AVAILABILITY
The dmesg command is part of the util-linux package and is available from
.UR ftp://\:ftp.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
Linux Kernel Archive
.UE .