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 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) 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.

       -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 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.

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/⟩.