focal (1) dmesg.1.gz

Provided by: util-linux_2.34-0.1ubuntu9.6_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/⟩.