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 -J, --json
72 Use JSON output format. The time output format is in "sec.usec"
73 format only, log priority level is not decoded by default (use
74 --decode to split into facility and priority), the other options to
75 control the output format or time format are silently ignored.
76 -k, --kernel
78 Print kernel messages.
79 -L, --color[=when]
81 Colorize the output. The optional argument when can be auto, never
82 or always. If the when argument is omitted, it defaults to auto.
83 The colors can be disabled; for the current built-in default see
84 the --help output. See also the COLORS section below.
85 -l, --level list
87 Restrict output to the given (comma-separated) list of levels. For
88 example:
89 dmesg --level=err,warn
91 will print error and warning messages only. For all supported
93 levels see the --help output.
94 Appending a plus + to a level name also includes all higher levels.
96 For example:
97 dmesg --level=err+
99 will print levels err, crit, alert and emerg.
101 Prepending it will include all lower levels.
103 -n, --console-level level
105 Set the level at which printing of messages is done to the console.
106 The level is a level number or abbreviation of the level name. For
107 all supported levels see the --help output.
108 For example, -n 1 or -n emerg prevents all messages, except
110 emergency (panic) messages, from appearing on the console. All
111 levels of messages are still written to /proc/kmsg, so syslogd(8)
112 can still be used to control exactly where kernel messages appear.
113 When the -n option is used, dmesg will not print or clear the
114 kernel ring buffer.
115 --noescape
117 The unprintable and potentially unsafe characters (e.g., broken
118 multi-byte sequences, terminal controlling chars, etc.) are escaped
119 in format \x<hex> for security reason by default. This option
120 disables this feature at all. It’s usable for example for debugging
121 purpose together with --raw. Be careful and don’t use it by
122 default.
123 -P, --nopager
125 Do not pipe output into a pager. A pager is enabled by default for
126 --human output.
127 -p, --force-prefix
129 Add facility, level or timestamp information to each line of a
130 multi-line message.
131 -r, --raw
133 Print the raw message buffer, i.e., do not strip the log-level
134 prefixes, but all unprintable characters are still escaped (see
135 also --noescape).
136 Note that the real raw format depends on the method how dmesg reads
138 kernel messages. The /dev/kmsg device uses a different format than
139 syslog(2). For backward compatibility, dmesg returns data always in
140 the syslog(2) format. It is possible to read the real raw data from
141 /dev/kmsg by, for example, the command 'dd if=/dev/kmsg
142 iflag=nonblock'.
143 -S, --syslog
145 Force dmesg to use the syslog(2) kernel interface to read kernel
146 messages. The default is to use /dev/kmsg rather than syslog(2)
147 since kernel 3.5.0.
148 -s, --buffer-size size
150 Use a buffer of size to query the kernel ring buffer. This is 16392
151 by default. (The default kernel syslog buffer size was 4096 at
152 first, 8192 since 1.3.54, 16384 since 2.1.113.) If you have set the
153 kernel buffer to be larger than the default, then this option can
154 be used to view the entire buffer.
155 -T, --ctime
157 Print human-readable timestamps.
158 Be aware that the timestamp could be inaccurate! The time source
160 used for the logs is not updated after system SUSPEND/RESUME.
161 Timestamps are adjusted according to current delta between boottime
162 and monotonic clocks, this works only for messages printed after
163 last resume.
164 --since time
166 Display record since the specified time. Supported is the subsecond
167 granularity. The time is possible to specify in absolute way as
168 well as by relative notation (e.g. '1 hour ago'). Be aware that the
169 timestamp could be inaccurate and see --ctime for more details.
170 --until time
172 Display record until the specified time. Supported is the subsecond
173 granularity. The time is possible to specify in absolute way as
174 well as by relative notation (e.g. '1 hour ago'). Be aware that the
175 timestamp could be inaccurate and see --ctime for more details.
176 -t, --notime
178 Do not print kernel’s timestamps.
179 --time-format format
181 Print timestamps using the given format, which can be ctime,
182 reltime, delta or iso. The first three formats are aliases of the
183 time-format-specific options. The iso format is a dmesg
184 implementation of the ISO-8601 timestamp format. The purpose of
185 this format is to make the comparing of timestamps between two
186 systems, and any other parsing, easy. The definition of the iso
187 timestamp is: YYYY-MM-DD<T>HH:MM:SS,<microseconds>←+><timezone
188 offset from UTC>.
189 The iso format has the same issue as ctime: the time may be
191 inaccurate when a system is suspended and resumed.
192 -u, --userspace
194 Print userspace messages.
195 -w, --follow
197 Wait for new messages. This feature is supported only on systems
198 with a readable /dev/kmsg (since kernel 3.5.0).
199 -W, --follow-new
201 Wait and print only new messages.
202 -x, --decode
204 Decode facility and level (priority) numbers to human-readable
205 prefixes.
206 -h, --help
208 Display help text and exit.
209 -V, --version
211 Print version and exit.
212
214 The output colorization is implemented by terminal-colors.d(5)
215 functionality. Implicit coloring can be disabled by an empty file
216 /etc/terminal-colors.d/dmesg.disable
218 for the dmesg command or for all tools by
220 /etc/terminal-colors.d/disable
222 The user-specific $XDG_CONFIG_HOME/terminal-colors.d or
224 $HOME/.config/terminal-colors.d overrides the global setting.
225 Note that the output colorization may be enabled by default, and in
227 this case terminal-colors.d directories do not have to exist yet.
228 The logical color names supported by dmesg are:
230 subsys
232 The message sub-system prefix (e.g., "ACPI:").
233 time
235 The message timestamp.
236 timebreak
238 The message timestamp in short ctime format in --reltime or --human
239 output.
240 alert
242 The text of the message with the alert log priority.
243 crit
245 The text of the message with the critical log priority.
246 err
248 The text of the message with the error log priority.
249 warn
251 The text of the message with the warning log priority.
252 segfault
254 The text of the message that inform about segmentation fault.
255
257 dmesg can fail reporting permission denied error. This is usually
258 caused by dmesg_restrict kernel setting, please see syslog(2) for more
259 details.
260
262 Karel Zak <kzak@redhat.com>
263 dmesg was originally written by Theodore Ts’o <tytso@athena.mit.edu>.
265
267 terminal-colors.d(5), syslogd(8)
268
270 For bug reports, use the issue tracker at
271 https://github.com/util-linux/util-linux/issues.
272
274 The dmesg command is part of the util-linux package which can be
275 downloaded from Linux Kernel Archive
276 <https://www.kernel.org/pub/linux/utils/util-linux/>.
277util-linux 2.39.2 2023-06-14 DMESG(1)