Provided by: util-linux_2.30.1-0ubuntu4_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 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 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  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

       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/⟩.