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 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 be-
              low.

       -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  emerg  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) re-
              turns  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 SUSPEND/RE-
              SUME.

       -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 im-
              plementation 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  in-
              accurate 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@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)