dmesg(1) - print or control the kernel ring buffer

  • 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 display all messages from the kernel ring buf‐
           The --clear,  --read-clear,  --console-on,  --console-off,  and  --con‐
           sole-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 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
           -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
           -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.
           -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
                  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.
           --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 sys‐
                  tems with a readable /dev/kmsg (since kernel 3.5.0).
           -x, --decode
                  Decode facility and level (priority) numbers  to  human-readable
           -V, --version
                  Display version information and exit.
           -h, --help
                  Display help text and exit.
           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.
                  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.
                  The text of the message that inform about segmentation fault.
           dmesg can fail reporting permission  denied  error.   This  is  usually
           caused  by dmesg_restrict kernel setting, please see syslog(2) for more
           terminal-colors.d(5), syslogd(8)
           Karel Zak ⟨[email protected]⟩
           dmesg was originally written by Theodore Ts'o ⟨[email protected]⟩
           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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