The dmesg command in Linux is used to examine or control the kernel ring buffer. The kernel ring buffer is a data structure that stores log messages generated by the kernel, including messages related to hardware devices, drivers, and other kernel-related information. This command is particularly useful for debugging hardware and driver issues.
Understanding dmesg
The dmesg command prints the messages stored in the kernel ring buffer. These messages can include information about the system boot process, hardware detection, driver initialization, and errors.
Basic Usage
To display the messages in the kernel ring buffer, simply run:
dmesg
Example Output
Here’s an example of what the dmesg command might return:
-c / --read-clear: Read and clear the ring buffer.
-D / --console-off: Disable printing messages to the console.
-E / --console-on: Enable printing messages to the console.
-H / --human: Display output in a human-readable format, with timestamps in a readable format.
-T / --ctime: Show human-readable timestamps.
-l / --level [level]: Show messages of a certain log level.
-k / --kernel: Show only kernel messages.
Practical Examples
Display Kernel Messages with Human-Readable Timestamps:
Display Kernel Messages with Human-Readable Formatting:
Filter for Specific Message Levels: To show only error messages:
Clear the Kernel Ring Buffer:
Read and Clear the Kernel Ring Buffer:
Monitor New Kernel Messages in Real-Time:
This command will keep running and display new messages as they are added to the buffer.
Related Commands
journalctl: For systems using systemd, journalctl can be used to view system logs, including kernel messages.
/var/log/kern.log: Kernel messages are also logged in /var/log/kern.log on some systems. Viewing this file can be done with cat, less, or tail.
/var/log/syslog: General system messages and logs, including kernel messages, can be found in /var/log/syslog on some systems.
Conclusion
The dmesg command is an essential tool for system administrators and users who need to troubleshoot hardware and driver issues. By examining the kernel ring buffer, you can gain valuable insights into the system's hardware interactions and kernel events.
Options:
-C, --clear clear the kernel ring buffer
-c, --read-clear read and clear all messages
-D, --console-off disable printing messages to console
-E, --console-on enable printing messages to console
-F, --file <file> use the file instead of the kernel log buffer
-f, --facility <list> restrict output to defined facilities
-H, --human human readable output
-J, --json use JSON output format
-k, --kernel display kernel messages
-L, --color[=<when>] colorize messages (auto, always or never)
colors are enabled by default
-l, --level <list> restrict output to defined levels
-n, --console-level <level> set level of messages printed to console
-P, --nopager do not pipe output into a pager
-p, --force-prefix force timestamp output on each line of multi-line messages
-r, --raw print the raw message buffer
--noescape don't escape unprintable character
-S, --syslog force to use syslog(2) rather than /dev/kmsg
-s, --buffer-size <size> buffer size to query the kernel ring buffer
-u, --userspace display userspace messages
-w, --follow wait for new messages
-W, --follow-new wait and print only new messages
-x, --decode decode facility and level to readable string
-d, --show-delta show time delta between printed messages
-e, --reltime show local time and time delta in readable format
-T, --ctime show human-readable timestamp (may be inaccurate!)
-t, --notime don't show any timestamp with messages
--time-format <format> show timestamp using the given format:
[delta|reltime|ctime|notime|iso]
Suspending/resume will make ctime and iso timestamps inaccurate.
--since <time> display the lines since the specified time
--until <time> display the lines until the specified time
-h, --help display this help
-V, --version display version
Supported log facilities:
kern - kernel messages
user - random user-level messages
mail - mail system
daemon - system daemons
auth - security/authorization messages
syslog - messages generated internally by syslogd
lpr - line printer subsystem
news - network news subsystem
Supported log levels (priorities):
emerg - system is unusable
alert - action must be taken immediately
crit - critical conditions
err - error conditions
warn - warning conditions
notice - normal but significant condition
info - informational
debug - debug-level messages
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. 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.