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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 con‐
              version 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 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 --help output. See also the COLORS section.

       -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 alert prevents all messages, except emergency (panic)  mes‐
              sages,  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  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  mes‐
              sages.   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  possi‐
              ble  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-
              DDHH:MM:SS,<-+>.

              The iso format has the same issue as ctime: the time may be inaccurate when a  sys‐
              tem 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.

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

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 ⟨ftp://ftp.kernel.org/pub/linux/utils/util-linux/⟩.

util-linux                                  July 2012                                    DMESG(1)

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