DMESG man page on Archlinux

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DMESG(1)			 User Commands			      DMESG(1)

       dmesg - print or control the kernel ring buffer

       dmesg [options]

       dmesg --clear
       dmesg --read-clear [options]
       dmesg --console-level level
       dmesg --console-on
       dmesg --console-off

       dmesg is used to examine or control the kernel ring buffer.

       The default action is to read all messages from the kernel ring buffer.

       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 printing 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, --reltime
	      Display the local time and the delta in human-readable format.

       -E, --console-on
	      Enable printing messages to the console.

       -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 dmesg --help output.

       -H, --human
	      Enable human-readable output.  See also --color,	--reltime  and

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

       -k, --kernel
	      Print kernel messages.

       -L, --color[=when]
	      Colorize	important messages.  The optional argument when can be
	      auto, never or always.  If the when argument is omitted, then 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 dmesg --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 dmesg --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

	      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‐

       -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

       --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.


       Karel Zak ⟨⟩
       Theodore Ts'o ⟨⟩

       The  dmesg  command  is part of the util-linux package and is available
       from Linux Kernel  Archive  ⟨

util-linux			   July 2012			      DMESG(1)

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