DMESG(1) User Commands DMESG(1)
NAME
dmesg - print or control the kernel ring buffer
SYNOPSIS
dmesg [options]
dmesg --clear
dmesg --read-clear [options]
dmesg --console-level level
dmesg --console-on
dmesg --console-off
DESCRIPTION
dmesg is used to examine or control the kernel ring buffer.
The default action is to display all messages from the kernel ring buf-
fer.
OPTIONS
The --clear, --read-clear, --console-on, --console-off, and --con-
sole-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 facili-
ties. 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 con-
sole. 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 sys-
logd(8) can still be used to control exactly where kernel mes-
sages appear. When the -n option is used, dmesg will not print
or clear the kernel ring buffer.
-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.
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 com-
mand '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 SUS-
PEND/RESUME.
-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 sys-
tems with a readable /dev/kmsg (since kernel 3.5.0).
-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.
COLORS
Implicit coloring can be disabled by an empty file /etc/terminal-col-
ors.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.
EXIT STATUS
dmesg can fail reporting permission denied error. This is usually
caused by dmesg_restrict kernel setting, please see syslog(2) for more
details.
SEE ALSO
terminal-colors.d(5), syslogd(8)
AUTHORS
Karel Zak <kzak AT redhat.com>
dmesg was originally written by Theodore Ts'o <tytso AT athena.edu>
AVAILABILITY
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/>.
util-linux July 2012 DMESG(1)
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