Ubuntu Manpage: dmesg - print or control the kernel ring buffer

Provided by: util-linux_2.36.1-8ubuntu1_amd64

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(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.

-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⟩

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/⟩.