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  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 facili‐
              ties 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 col‐
              ors can be disabled; for the current built-in default see the --help out‐
              put.  See also the COLORS section below.

       -l, --level list
              Restrict output to the given (comma-separated) list of levels.  For exam‐
              ple:

                     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  sup‐
              ported 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 con‐
              trol 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 sys‐
              log(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  mes‐
              sages.   The default is to use /dev/kmsg rather than syslog(2) since ker‐
              nel 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,<microsec‐
              onds><-+><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-col‐
       ors.d/dmesg.disable.  See terminal-colors.d(5) for more details about  coloriza‐
       tion 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 out‐
              put.

       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.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)

Usage:
 dmesg [options]

Display or control the kernel ring buffer.

Options:
 -C, --clear                 clear the kernel ring buffer
 -c, --read-clear            read and clear all messages
 -D, --console-off           disable printing messages to console
 -E, --console-on            enable printing messages to console
 -F, --file <file>           use the file instead of the kernel log buffer
 -f, --facility <list>       restrict output to defined facilities
 -H, --human                 human readable output
 -k, --kernel                display kernel messages
 -L, --color[=<when>]        colorize messages (auto, always or never)
                               colors are enabled by default
 -l, --level <list>          restrict output to defined levels
 -n, --console-level <level> set level of messages printed to console
 -P, --nopager               do not pipe output into a pager
 -r, --raw                   print the raw message buffer
 -S, --syslog                force to use syslog(2) rather than /dev/kmsg
 -s, --buffer-size <size>    buffer size to query the kernel ring buffer
 -u, --userspace             display userspace messages
 -w, --follow                wait for new messages
 -x, --decode                decode facility and level to readable string
 -d, --show-delta            show time delta between printed messages
 -e, --reltime               show local time and time delta in readable format
 -T, --ctime                 show human-readable timestamp (may be inaccurate!)
 -t, --notime                don't show any timestamp with messages
     --time-format <format>  show timestamp using the given format:
                               [delta|reltime|ctime|notime|iso]
Suspending/resume will make ctime and iso timestamps inaccurate.

 -h, --help     display this help and exit
 -V, --version  output version information and exit

Supported log facilities:
    kern - kernel messages
    user - random user-level messages
    mail - mail system
  daemon - system daemons
    auth - security/authorization messages
  syslog - messages generated internally by syslogd
     lpr - line printer subsystem
    news - network news subsystem

Supported log levels (priorities):
   emerg - system is unusable
   alert - action must be taken immediately
    crit - critical conditions
     err - error conditions
    warn - warning conditions
  notice - normal but significant condition
    info - informational
   debug - debug-level messages


For more details see dmesg(1).