dmesg(1) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | OPTIONS | COLORS | EXIT STATUS | AUTHORS | SEE ALSO | AVAILABILITY | COLOPHON

DMESG(1)                      User Commands                     DMESG(1)

NAME         top

       dmesg - print or control the kernel ring buffer

SYNOPSIS         top

       dmesg [options]

       dmesg --clear
       dmesg --read-clear [options]
       dmesg --console-level level
       dmesg --console-on
       dmesg --console-off

DESCRIPTION         top

       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         top

       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.

COLORS         top

       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         top

       dmesg can fail reporting permission denied error.  This is
       usually caused by dmesg_restrict kernel setting, please see
       syslog(2) for more details.

AUTHORS         top

       Karel Zak ⟨kzak@redhat.com⟩

       dmesg was originally written by Theodore Ts'o
       ⟨tytso@athena.mit.edu⟩

SEE ALSO         top

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

AVAILABILITY         top

       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/⟩.

COLOPHON         top

       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)

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