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 read 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.

       -F, --file file
              Read the messages from the given file.

       -f, --facility list
              Restrict output to the given (comma-separated) list  of  facili-
              ties.  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.

       -h, --help
              Display help text and exit.

       -k, --kernel
              Print kernel messages.

       -L, --color[=when]
              Colorize  important messages (enabled by default).  The optional
              argument when can be auto, never or always.  If the  when  argu-
              ment is omitted, it defaults to auto.

       -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 con-
              sole.  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 sys-
              logd(8) can still be used to control exactly where  kernel  mes-
              sages  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  com-
              mand '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  SUS-
              PEND/RESUME.

       -t, --notime
              Do not print kernel's timestamps.

       -u, --userspace
              Print userspace messages.

       -V, --version
              Display version information and exit.

       -w, --follow
              Wait  for  new messages.  This feature is supported only on sys-
              tems with a readable /dev/kmsg (since kernel 3.5.0).

       -x, --decode
              Decode facility and level (priority) numbers  to  human-readable
              prefixes.

       --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.

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
       colorization configuration.

       The logical color names supported by dmesg are:

       subsys The message sub-system prefix (e.g. "ACPI:").

       time   The message timestamp.

       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/>.



util-linux                         July 2012                          DMESG(1)
