dmesg — print or control the kernel ring buffer
dmesg [options]
dmesg is used to examine or control the kernel ring buffer.
The default action is to read 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 printing 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, −−reltimeDisplay the local time and the delta in human-readable format.
−E, −−console−onEnable printing messages to the console.
−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 dmesg −−help output.
−H, −−humanEnable human-readable output. See also −−color, −−reltime and −−nopager.
−h, −−helpDisplay help text and exit.
−k, −−kernelPrint kernel messages.
−L, −−color[=when]Colorize important messages. The optional argument when can be auto, never or always. If the when argument is omitted, then it defaults to auto.
−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 dmesg −−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 dmesg −−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.
−u, −−userspacePrint userspace messages.
−V, −−versionDisplay version information and exit.
−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.
−−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.
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 |