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  4. dmesg(1)

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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 buffer.

OPTIONS

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 reads kernel messages. The /dev/kmsg device uses a different format than syslog(2). For backward compatibility, dmesg 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.

COLORS

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.

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.

AUTHORS

Karel Zak <kzak@redhat.com>

dmesg was originally written by Theodore Ts’o <tytso@athena.mit.edu>.

SEE ALSO

terminal-colors.d(5), syslogd(8)

REPORTING BUGS

For bug reports, use the issue tracker at <https://github.com/karelzak/util-linux/issues>.

AVAILABILITY

The dmesg command is part of the util-linux package which can be downloaded from Linux Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.

2022-02-14 util-linux 2.37.4