dmesg — print or control the kernel ring buffer
dmesg [options]
dmesg \-\-clear
dmesg −−read−clear [options]
dmesg −−console−level level
dmesg \-\-console\-on
dmesg \-\-console\-off
dmesg is used to
examine or control the kernel ring buffer.
The default action is to display all messages from the kernel ring buffer.
The −−clear,
−−read−clear,
−−console−on,
−−console−off,
and −−console−level options are
mutually exclusive.
−C, −−clearClear the ring buffer.
−c, −−read−clearClear the ring buffer after first printing its contents.
−D, −−console−offDisable the printing of messages to the console.
−d, −−show−deltaDisplay the timestamp and the time delta spent
between messages. If used together with −−notime then only the time
delta without the timestamp is printed.
−E, −−console−onEnable printing messages to the console.
−e, −−reltimeDisplay the local time and the delta in
human-readable format. Be aware that conversion to the
local time could be inaccurate (see −T for more details).
−F, −−file fileRead the messages from the given file.
−f, −−facility listRestrict output to the given (comma-separated) list of facilities. For example:
dmesg −−facility=daemon
will print messages from system daemons only. For
all supported facilities see the −−help output.
−H, −−humanEnable human-readable output. See also
−−color,
−−reltime
and −−nopager.
−k, −−kernelPrint kernel messages.
−L, −−color[=when]Colorize the output. The optional argument
when can be
auto,
never or
always. If the
when argument
is omitted, it defaults to auto. The
colors can be disabled; for the current built-in
default see the −−help output. See
also the COLORS
section below.
−l, −−level listRestrict output to the given (comma-separated) list of levels. For example:
dmesg −−level=err,warn
will print error and warning messages only. For all
supported levels see the −−help output.
−n, −−console−level
levelSet the level at which
printing of messages is done to the console. The
level is
a level number or abbreviation of the level name.
For all supported levels see the −−help output.
For example, −n
1 or −n
alert prevents all messages, except
emergency (panic) messages, from appearing on the
console. All levels of messages are still written
to /proc/kmsg, so
syslogd(8) can
still be used to control exactly where kernel
messages appear. When the −n option is used,
dmesg
will not print or clear the kernel ring
buffer.
−P, −−nopagerDo not pipe output into a pager. A pager is
enabled by default for −−human output.
−r, −−rawPrint the raw message buffer, i.e. do not strip the log-level prefixes.
Note that the real raw format depends on the method how dmesg(1) reads kernel messages. The /dev/kmsg device uses a different format than syslog(2). For backward compatibility, dmesg(1) returns data always in the syslog(2) format. It is possible to read the real raw data from /dev/kmsg by, for example, the command 'dd if=/dev/kmsg iflag=nonblock'.
−S, −−syslogForce dmesg to use the
syslog(2)
kernel interface to read kernel messages. The
default is to use /dev/kmsg rather than syslog(2) since
kernel 3.5.0.
−s, −−buffer−size
sizeUse a buffer of 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.
−T, −−ctimePrint human-readable timestamps.
Be aware that the
timestamp could be inaccurate! The
time source
used for the logs is not updated after
system SUSPEND/RESUME.
−t, −−notimeDo not print kernel's timestamps.
−−time−format
formatPrint timestamps using the given format, which
can be ctime,
reltime,
delta or
iso. The first
three formats are aliases of the
time-format-specific options. The iso format is a
dmesg
implementation of the ISO-8601 timestamp format.
The purpose of this format is to make the
comparing of timestamps between two systems, and
any other parsing, easy. The definition of the
iso
timestamp is:
YYYY-MM-DD<T>HH:MM:SS,<microseconds><-+><timezone
offset from UTC>.
The iso format has the same issue as ctime: the time may be inaccurate when a system is suspended and resumed.
−u,
−−userspacePrint userspace messages.
−w,
−−followWait for new messages. This feature is supported only on systems with a readable /dev/kmsg (since kernel 3.5.0).
−x,
−−decodeDecode facility and level (priority) numbers to human-readable prefixes.
−V,
−−versionDisplay version information and exit.
−h,
−−helpDisplay help text and exit.
Implicit coloring can be disabled by an empty file
/etc/terminal-colors.d/dmesg.disable. See
terminal-colors.d(5) for
more details about colorization configuration.
The logical color names supported by dmesg are:
The message sub-system prefix (e.g. "ACPI:").
The message timestamp.
The message timestamp in short ctime format in
−−reltime or
−−human
output.
The text of the message with the alert log priority.
The text of the message with the critical log priority.
The text of the message with the error log priority.
The text of the message with the warning log priority.
The text of the message that inform about segmentation fault.
The dmesg command is part of the util-linux package and is available from Linux Kernel Archive
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Copyright 1993 Rickard E. Faith (faithcs.unc.edu) May be distributed under the GNU General Public License |