dmesg(1)



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 messages from the given file.

       -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
              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  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  alert  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.

       -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)
              returns 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 SUS-
              PEND/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  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.

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)

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