Provided by: util-linux_2.33.1-0.1ubuntu2_amd64 bug

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.

       -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) 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.comdmesg 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/⟩.