dmesg

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(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.

       -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).

       -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.

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

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/⟩.



util-linux                         July 2012                          DMESG(1)