Provided by: util-linux_2.38-4ubuntu1_amd64
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 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 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 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, --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. -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. 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. 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 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 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.
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.
dmesg can fail reporting permission denied error. This is usually caused by dmesg_restrict kernel setting, please see syslog(2) for more details.
Karel Zak <firstname.lastname@example.org> dmesg was originally written by Theodore Ts’o <email@example.com>.
For bug reports, use the issue tracker at https://github.com/util-linux/util-linux/issues.
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/>.