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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | COLORS | EXIT STATUS | AUTHORS | SEE ALSO | AVAILABILITY | COLOPHON |
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DMESG(1) User Commands DMESG(1)
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, --clear
Clear the ring buffer.
-c, --read-clear
Clear the ring buffer after first printing its contents.
-D, --console-off
Disable the printing of messages to the console.
-d, --show-delta
Display 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-on
Enable printing messages to the console.
-e, --reltime
Display 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 file
Read the syslog messages from the given file. Note that
-F does not support messages in kmsg format. The old
syslog format is supported only.
-f, --facility list
Restrict 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, --human
Enable human-readable output. See also --color, --reltime
and --nopager.
-k, --kernel
Print 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 list
Restrict 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 level
Set 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 emerg 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.
--noescape
The unprintable and potentially unsafe characters (e.g.,
broken multi-byte sequences, terminal controlling chars,
etc.) are escaped in format \x<hex> for security reason by
default. This option disables this feature at all. It's
usable for example for debugging purpose together with
--raw. Be careful and don't use it by default.
-P, --nopager
Do not pipe output into a pager. A pager is enabled by
default for --human output.
-p, --force-prefix
Add facility, level or timestamp information to each line
of a multi-line message.
-r, --raw
Print the raw message buffer, i.e., do not strip the log-
level prefixes, but all unprintable characters are still
escaped (see also --noescape).
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, --syslog
Force 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 size
Use 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, --ctime
Print 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. Timestamps are adjusted according to
current delta between boottime and monotonic clocks, this
works only for messages printed after last resume.
--since time
Display record since the specified time. The time is
possible to specify in absolute way as well as by relative
notation (e.g. '1 hour ago'). Be aware that the timestamp
could be inaccurate and see --ctime for more details.
--until time
Display record until the specified time. The time is
possible to specify in absolute way as well as by relative
notation (e.g. '1 hour ago'). Be aware that the timestamp
could be inaccurate and see --ctime for more details.
-t, --notime
Do not print kernel's timestamps.
--time-format format
Print 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, --userspace
Print userspace messages.
-w, --follow
Wait for new messages. This feature is supported only on
systems with a readable /dev/kmsg (since kernel 3.5.0).
-W, --follow-new
Wait and print only new messages.
-x, --decode
Decode facility and level (priority) numbers to human-
readable prefixes.
-V, --version
Display version information and exit.
-h, --help
Display 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:
subsys The message sub-system prefix (e.g., "ACPI:").
time The message timestamp.
timebreak
The message timestamp in short ctime format in --reltime
or --human output.
alert The text of the message with the alert log priority.
crit The text of the message with the critical log priority.
err The text of the message with the error log priority.
warn The text of the message with the warning log priority.
segfault
The text of the message that inform about segmentation
fault.
dmesg can fail reporting permission denied error. This is
usually caused by dmesg_restrict kernel setting, please see
syslog(2) for more details.
Karel Zak ⟨kzak@redhat.com⟩
dmesg was originally written by Theodore Ts'o
⟨tytso@athena.mit.edu⟩
terminal-colors.d(5), syslogd(8)
The dmesg command is part of the util-linux package and is
available from Linux Kernel Archive
⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩.
This page is part of the util-linux (a random collection of Linux
utilities) project. Information about the project can be found
at ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you
have a bug report for this manual page, send it to
util-linux@vger.kernel.org. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2020-12-18. (At that time, the date of the most recent commit
that was found in the repository was 2020-12-17.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
util-linux July 2012 DMESG(1)
Pages that refer to this page: babeltrace2-log(1), dmesg(1), syslog(2), proc(5), systemd.exec(5), terminal-colors.d(5), babeltrace2-plugin-text(7), babeltrace2-source.text.dmesg(7), iptables-extensions(8), tc-bpf(8), wg(8)
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