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. See -K instead.

       -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.

       -J, --json
           Use JSON output format. The time output format is in "sec.usec" format only, log
           priority level is not decoded by default (use --decode to split into facility and
           priority), the other options to control the output format or time format are silently
           ignored.

       -K, --kmsg-file file
           Read the /dev/kmsg messages from the given file. Different record as expected to be
           separated by a NULL byte.

       -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.

           Appending a plus + to a level name also includes all higher levels. For example:

           dmesg --level=err+

           will print levels err, crit, alert and emerg.

           Prepending it will include all lower levels.

       -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.

       --noescape
           The unprintable and potentially unsafe characters (e.g., broken multi-byte sequences,
           terminal controlling chars, etc.) are escaped in format \x<hex> for security reason by
           default. This option disables this feature at all. It’s usable for example for
           debugging purpose together with --raw. Be careful and don’t use it by default.

       -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, but all
           unprintable characters are still escaped (see also --noescape).

           Note that the real raw format depends on the method how dmesg reads kernel messages.
           The /dev/kmsg device uses a different format than syslog(2). For backward
           compatibility, dmesg 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. Timestamps are adjusted according to current
           delta between boottime and monotonic clocks, this works only for messages printed
           after last resume.

       --since time
           Display record since the specified time. Supported is the subsecond granularity. The
           time is possible to specify in absolute way as well as by relative notation (e.g. '1
           hour ago'). Be aware that the timestamp could be inaccurate and see --ctime for more
           details.

       --until time
           Display record until the specified time. Supported is the subsecond granularity. The
           time is possible to specify in absolute way as well as by relative notation (e.g. '1
           hour ago'). Be aware that the timestamp could be inaccurate and see --ctime for more
           details.

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

       --time-format format
           Print timestamps using the given format, which can be ctime, reltime, delta, iso or
           raw. The first three formats are aliases of the time-format-specific options. The raw
           format uses the default timestamp format showing seconds since boot. 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.

           --time-format may be used multiple times with different values for format to output
           each specified format.

           The delta always follows ctime or raw if specified together.

       -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).

       -W, --follow-new
           Wait and print only new messages.

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

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

       -V, --version
           Print version and exit.

COLORS

       The output colorization is implemented by terminal-colors.d(5) functionality. Implicit
       coloring can be disabled by an empty file

          /etc/terminal-colors.d/dmesg.disable

       for the dmesg command or for all tools by

          /etc/terminal-colors.d/disable

       The user-specific $XDG_CONFIG_HOME/terminal-colors.d or $HOME/.config/terminal-colors.d
       overrides the global setting.

       Note that the output colorization may be enabled by default, and in this case
       terminal-colors.d directories do not have to exist yet.

       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.

AUTHORS

       Karel Zak <kzak@redhat.com>

       dmesg was originally written by Theodore Ts’o <tytso@athena.mit.edu>.

SEE ALSO

       terminal-colors.d(5), syslogd(8)

REPORTING BUGS

       For bug reports, use the issue tracker at https://github.com/util-linux/util-linux/issues.

AVAILABILITY

       The dmesg command is part of the util-linux package which can be downloaded from Linux
       Kernel Archive <https://www.kernel.org/pub/linux/utils/util-linux/>.