DMESG("1") manual page
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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
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.
- -F, --file file
- Read the messages from the given
file.
- -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.
- -h,
--help
- Display help text and exit.
- -k, --kernel
- Print kernel messages.
- -L, --color[=when]
- Colorize
important messages (enabled by default). The optional argument when can
be auto, never or always. If the when argument is omitted, 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 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 alert 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.
- -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 prefixes.
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 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.
- -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 systems with a readable /dev/kmsg (since kernel 3.5.0).
- -x, --decode
- Decode
facility and level (priority) numbers to human-readable prefixes.
- --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.
Implicit coloring can be disabled
by an empty file /etc/terminal-colors.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.
- 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.
syslogd(8)
terminal-colors.d(5)
Karel Zak
dmesg was originally written by Theodore Ts’o
The dmesg command
is part of the util-linux package and is available from Linux Kernel Archive
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