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 -n, --console-level level
96 Set the level at which printing of messages is done to the console.
97 The level is a level number or abbreviation of the level name. For
98 all supported levels see the --help output.
99 For example, -n 1 or -n emerg prevents all messages, except
101 emergency (panic) messages, from appearing on the console. All
102 levels of messages are still written to /proc/kmsg, so syslogd(8)
103 can still be used to control exactly where kernel messages appear.
104 When the -n option is used, dmesg will not print or clear the
105 kernel ring buffer.
106 --noescape
108 The unprintable and potentially unsafe characters (e.g., broken
109 multi-byte sequences, terminal controlling chars, etc.) are escaped
110 in format \x<hex> for security reason by default. This option
111 disables this feature at all. It’s usable for example for debugging
112 purpose together with --raw. Be careful and don’t use it by
113 default.
114 -P, --nopager
116 Do not pipe output into a pager. A pager is enabled by default for
117 --human output.
118 -p, --force-prefix
120 Add facility, level or timestamp information to each line of a
121 multi-line message.
122 -r, --raw
124 Print the raw message buffer, i.e., do not strip the log-level
125 prefixes, but all unprintable characters are still escaped (see
126 also --noescape).
127 Note that the real raw format depends on the method how dmesg reads
129 kernel messages. The /dev/kmsg device uses a different format than
130 syslog(2). For backward compatibility, dmesg returns data always in
131 the syslog(2) format. It is possible to read the real raw data from
132 /dev/kmsg by, for example, the command 'dd if=/dev/kmsg
133 iflag=nonblock'.
134 -S, --syslog
136 Force dmesg to use the syslog(2) kernel interface to read kernel
137 messages. The default is to use /dev/kmsg rather than syslog(2)
138 since kernel 3.5.0.
139 -s, --buffer-size size
141 Use a buffer of size to query the kernel ring buffer. This is 16392
142 by default. (The default kernel syslog buffer size was 4096 at
143 first, 8192 since 1.3.54, 16384 since 2.1.113.) If you have set the
144 kernel buffer to be larger than the default, then this option can
145 be used to view the entire buffer.
146 -T, --ctime
148 Print human-readable timestamps.
149 Be aware that the timestamp could be inaccurate! The time source
151 used for the logs is not updated after system SUSPEND/RESUME.
152 Timestamps are adjusted according to current delta between boottime
153 and monotonic clocks, this works only for messages printed after
154 last resume.
155 --since time
157 Display record since 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 --until time
163 Display record until the specified time. The time is possible to
164 specify in absolute way as well as by relative notation (e.g. '1
165 hour ago'). Be aware that the timestamp could be inaccurate and see
166 --ctime for more details.
167 -t, --notime
169 Do not print kernel’s timestamps.
170 --time-format format
172 Print timestamps using the given format, which can be ctime,
173 reltime, delta or iso. The first three formats are aliases of the
174 time-format-specific options. The iso format is a dmesg
175 implementation of the ISO-8601 timestamp format. The purpose of
176 this format is to make the comparing of timestamps between two
177 systems, and any other parsing, easy. The definition of the iso
178 timestamp is: YYYY-MM-DD<T>HH:MM:SS,<microseconds>←+><timezone
179 offset from UTC>.
180 The iso format has the same issue as ctime: the time may be
182 inaccurate when a system is suspended and resumed.
183 -u, --userspace
185 Print userspace messages.
186 -w, --follow
188 Wait for new messages. This feature is supported only on systems
189 with a readable /dev/kmsg (since kernel 3.5.0).
190 -W, --follow-new
192 Wait and print only new messages.
193 -x, --decode
195 Decode facility and level (priority) numbers to human-readable
196 prefixes.
197 -h, --help
199 Display help text and exit.
200 -V, --version
202 Print version and exit.
203
205 The output colorization is implemented by terminal-colors.d(5)
206 functionality. Implicit coloring can be disabled by an empty file
207 /etc/terminal-colors.d/dmesg.disable
209 for the dmesg command or for all tools by
211 /etc/terminal-colors.d/disable
213 The user-specific $XDG_CONFIG_HOME/terminal-colors.d or
215 $HOME/.config/terminal-colors.d overrides the global setting.
216 Note that the output colorization may be enabled by default, and in
218 this case terminal-colors.d directories do not have to exist yet.
219 The logical color names supported by dmesg are:
221 subsys
223 The message sub-system prefix (e.g., "ACPI:").
224 time
226 The message timestamp.
227 timebreak
229 The message timestamp in short ctime format in --reltime or --human
230 output.
231 alert
233 The text of the message with the alert log priority.
234 crit
236 The text of the message with the critical log priority.
237 err
239 The text of the message with the error log priority.
240 warn
242 The text of the message with the warning log priority.
243 segfault
245 The text of the message that inform about segmentation fault.
246
248 dmesg can fail reporting permission denied error. This is usually
249 caused by dmesg_restrict kernel setting, please see syslog(2) for more
250 details.
251
253 Karel Zak <kzak@redhat.com>
254 dmesg was originally written by Theodore Ts’o <tytso@athena.mit.edu>.
256
258 terminal-colors.d(5), syslogd(8)
259
261 For bug reports, use the issue tracker at
262 https://github.com/util-linux/util-linux/issues.
263
265 The dmesg command is part of the util-linux package which can be
266 downloaded from Linux Kernel Archive
267 <https://www.kernel.org/pub/linux/utils/util-linux/>.
268util-linux 2.38 2022-02-17 DMESG(1)