1DMESG(1) User Commands DMESG(1)
2
6 dmesg - print or control the kernel ring buffer
7
9 dmesg [options]
10 dmesg --clear
12 dmesg --read-clear [options]
14 dmesg --console-level level
16 dmesg --console-on
18 dmesg --console-off
20
22 dmesg is used to examine or control the kernel ring buffer.
23 The default action is to display all messages from the kernel ring
25 buffer.
26
28 The --clear, --read-clear, --console-on, --console-off, and
29 --console-level options are mutually exclusive.
30 -C, --clear
32 Clear the ring buffer.
33 -c, --read-clear
35 Clear the ring buffer after first printing its contents.
36 -D, --console-off
38 Disable the printing of messages to the console.
39 -d, --show-delta
41 Display the timestamp and the time delta spent between messages. If
42 used together with --notime then only the time delta without the
43 timestamp is printed.
44 -E, --console-on
46 Enable printing messages to the console.
47 -e, --reltime
49 Display the local time and the delta in human-readable format. Be
50 aware that conversion to the local time could be inaccurate (see -T
51 for more details).
52 -F, --file file
54 Read the syslog messages from the given file. Note that -F does not
55 support messages in kmsg format. The old syslog format is supported
56 only.
57 -f, --facility list
59 Restrict output to the given (comma-separated) list of facilities.
60 For example:
61 dmesg --facility=daemon
63 will print messages from system daemons only. For all supported
65 facilities see the --help output.
66 -H, --human
68 Enable human-readable output. See also --color, --reltime and
69 --nopager.
70 -k, --kernel
72 Print kernel messages.
73 -L, --color[=when]
75 Colorize the output. The optional argument when can be auto, never
76 or always. If the when argument is omitted, it defaults to auto.
77 The colors can be disabled; for the current built-in default see
78 the --help output. See also the COLORS section below.
79 -l, --level list
81 Restrict output to the given (comma-separated) list of levels. For
82 example:
83 dmesg --level=err,warn
85 will print error and warning messages only. For all supported
87 levels see the --help output.
88 -n, --console-level level
90 Set the level at which printing of messages is done to the console.
91 The level is a level number or abbreviation of the level name. For
92 all supported levels see the --help output.
93 For example, -n 1 or -n emerg prevents all messages, except
95 emergency (panic) messages, from appearing on the console. All
96 levels of messages are still written to /proc/kmsg, so syslogd(8)
97 can still be used to control exactly where kernel messages appear.
98 When the -n option is used, dmesg will not print or clear the
99 kernel ring buffer.
100 --noescape
102 The unprintable and potentially unsafe characters (e.g., broken
103 multi-byte sequences, terminal controlling chars, etc.) are escaped
104 in format \x<hex> for security reason by default. This option
105 disables this feature at all. It’s usable for example for debugging
106 purpose together with --raw. Be careful and don’t use it by
107 default.
108 -P, --nopager
110 Do not pipe output into a pager. A pager is enabled by default for
111 --human output.
112 -p, --force-prefix
114 Add facility, level or timestamp information to each line of a
115 multi-line message.
116 -r, --raw
118 Print the raw message buffer, i.e., do not strip the log-level
119 prefixes, but all unprintable characters are still escaped (see
120 also --noescape).
121 Note that the real raw format depends on the method how dmesg reads
123 kernel messages. The /dev/kmsg device uses a different format than
124 syslog(2). For backward compatibility, dmesg returns data always in
125 the syslog(2) format. It is possible to read the real raw data from
126 /dev/kmsg by, for example, the command 'dd if=/dev/kmsg
127 iflag=nonblock'.
128 -S, --syslog
130 Force dmesg to use the syslog(2) kernel interface to read kernel
131 messages. The default is to use /dev/kmsg rather than syslog(2)
132 since kernel 3.5.0.
133 -s, --buffer-size size
135 Use a buffer of size to query the kernel ring buffer. This is 16392
136 by default. (The default kernel syslog buffer size was 4096 at
137 first, 8192 since 1.3.54, 16384 since 2.1.113.) If you have set the
138 kernel buffer to be larger than the default, then this option can
139 be used to view the entire buffer.
140 -T, --ctime
142 Print human-readable timestamps.
143 Be aware that the timestamp could be inaccurate! The time source
145 used for the logs is not updated after system SUSPEND/RESUME.
146 Timestamps are adjusted according to current delta between boottime
147 and monotonic clocks, this works only for messages printed after
148 last resume.
149 --since time
151 Display record since the specified time. The time is possible to
152 specify in absolute way as well as by relative notation (e.g. '1
153 hour ago'). Be aware that the timestamp could be inaccurate and see
154 --ctime for more details.
155 --until time
157 Display record until the specified time. The time is possible to
158 specify in absolute way as well as by relative notation (e.g. '1
159 hour ago'). Be aware that the timestamp could be inaccurate and see
160 --ctime for more details.
161 -t, --notime
163 Do not print kernel’s timestamps.
164 --time-format format
166 Print timestamps using the given format, which can be ctime,
167 reltime, delta or iso. The first three formats are aliases of the
168 time-format-specific options. The iso format is a dmesg
169 implementation of the ISO-8601 timestamp format. The purpose of
170 this format is to make the comparing of timestamps between two
171 systems, and any other parsing, easy. The definition of the iso
172 timestamp is: YYYY-MM-DD<T>HH:MM:SS,<microseconds>←+><timezone
173 offset from UTC>.
174 The iso format has the same issue as ctime: the time may be
176 inaccurate when a system is suspended and resumed.
177 -u, --userspace
179 Print userspace messages.
180 -w, --follow
182 Wait for new messages. This feature is supported only on systems
183 with a readable /dev/kmsg (since kernel 3.5.0).
184 -W, --follow-new
186 Wait and print only new messages.
187 -x, --decode
189 Decode facility and level (priority) numbers to human-readable
190 prefixes.
191 -V, --version
193 Display version information and exit.
194 -h, --help
196 Display help text and exit.
197
199 Implicit coloring can be disabled by an empty file
200 /etc/terminal-colors.d/dmesg.disable. See terminal-colors.d(5) for more
201 details about colorization configuration.
202 The logical color names supported by dmesg are:
204 subsys
206 The message sub-system prefix (e.g., "ACPI:").
207 time
209 The message timestamp.
210 timebreak
212 The message timestamp in short ctime format in --reltime or --human
213 output.
214 alert
216 The text of the message with the alert log priority.
217 crit
219 The text of the message with the critical log priority.
220 err
222 The text of the message with the error log priority.
223 warn
225 The text of the message with the warning log priority.
226 segfault
228 The text of the message that inform about segmentation fault.
229
231 dmesg can fail reporting permission denied error. This is usually
232 caused by dmesg_restrict kernel setting, please see syslog(2) for more
233 details.
234
236 Karel Zak <kzak@redhat.com>
237 dmesg was originally written by Theodore Ts’o <tytso@athena.mit.edu>.
239
241 terminal-colors.d(5), syslogd(8)
242
244 For bug reports, use the issue tracker at
245 https://github.com/karelzak/util-linux/issues.
246
248 The dmesg command is part of the util-linux package which can be
249 downloaded from Linux Kernel Archive
250 <https://www.kernel.org/pub/linux/utils/util-linux/>.
251util-linux 2.37.2 2021-07-20 DMESG(1)