1DMESG(1)                         User Commands                        DMESG(1)
2
3
4

NAME

6       dmesg - print or control the kernel ring buffer
7

SYNOPSIS

9       dmesg [options]
10
11       dmesg --clear
12       dmesg --read-clear [options]
13       dmesg --console-level level
14       dmesg --console-on
15       dmesg --console-off
16

DESCRIPTION

18       dmesg is used to examine or control the kernel ring buffer.
19
20       The default action is to display all messages from the kernel ring buf‐
21       fer.
22

OPTIONS

24       The --clear,  --read-clear,  --console-on,  --console-off,  and  --con‐
25       sole-level options are mutually exclusive.
26
27       -C, --clear
28              Clear the ring buffer.
29
30       -c, --read-clear
31              Clear the ring buffer after first printing its contents.
32
33       -D, --console-off
34              Disable the printing of messages to the console.
35
36       -d, --show-delta
37              Display the timestamp and the time delta spent between messages.
38              If used together with --notime then only the time delta  without
39              the timestamp is printed.
40
41       -E, --console-on
42              Enable printing messages to the console.
43
44       -e, --reltime
45              Display  the  local time and the delta in human-readable format.
46              Be aware that conversion to the local time could  be  inaccurate
47              (see -T for more details).
48
49       -F, --file file
50              Read the syslog messages from the given file.  Note that -F does
51              not support messages in kmsg format. The old  syslog  format  is
52              supported only.
53
54       -f, --facility list
55              Restrict  output  to the given (comma-separated) list of facili‐
56              ties.  For example:
57
58                     dmesg --facility=daemon
59
60              will print messages from system daemons only.  For all supported
61              facilities see the --help output.
62
63       -H, --human
64              Enable  human-readable  output.  See also --color, --reltime and
65              --nopager.
66
67       -k, --kernel
68              Print kernel messages.
69
70       -L, --color[=when]
71              Colorize the output.  The optional argument when  can  be  auto,
72              never  or  always.  If the when argument is omitted, it defaults
73              to auto.  The colors can be disabled; for the  current  built-in
74              default  see  the  --help  output.   See also the COLORS section
75              below.
76
77       -l, --level list
78              Restrict output to the given (comma-separated) list  of  levels.
79              For example:
80
81                     dmesg --level=err,warn
82
83              will  print  error and warning messages only.  For all supported
84              levels see the --help output.
85
86       -n, --console-level level
87              Set the level at which printing of messages is done to the  con‐
88              sole.   The level is a level number or abbreviation of the level
89              name.  For all supported levels see the --help output.
90
91              For example, -n 1 or -n  alert  prevents  all  messages,  except
92              emergency  (panic) messages, from appearing on the console.  All
93              levels of messages are still  written  to  /proc/kmsg,  so  sys‐
94              logd(8)  can  still be used to control exactly where kernel mes‐
95              sages appear.  When the -n option is used, dmesg will not  print
96              or clear the kernel ring buffer.
97
98       -P, --nopager
99              Do  not pipe output into a pager.  A pager is enabled by default
100              for --human output.
101
102       -p, --force-prefix
103              Add facility, level or timestamp information to each line  of  a
104              multi-line message.
105
106       -r, --raw
107              Print  the  raw  message buffer, i.e. do not strip the log-level
108              prefixes.
109
110              Note that the real raw format depends on the method how dmesg(1)
111              reads  kernel  messages.   The /dev/kmsg device uses a different
112              format than syslog(2).   For  backward  compatibility,  dmesg(1)
113              returns  data always in the syslog(2) format.  It is possible to
114              read the real raw data from /dev/kmsg by, for example, the  com‐
115              mand 'dd if=/dev/kmsg iflag=nonblock'.
116
117       -S, --syslog
118              Force dmesg to use the syslog(2) kernel interface to read kernel
119              messages.  The default is to use /dev/kmsg rather than syslog(2)
120              since kernel 3.5.0.
121
122       -s, --buffer-size size
123              Use  a  buffer of size to query the kernel ring buffer.  This is
124              16392 by default.  (The default kernel syslog  buffer  size  was
125              4096  at first, 8192 since 1.3.54, 16384 since 2.1.113.)  If you
126              have set the kernel buffer to be larger than the  default,  then
127              this option can be used to view the entire buffer.
128
129       -T, --ctime
130              Print human-readable timestamps.
131
132              Be  aware  that  the  timestamp  could  be inaccurate!  The time
133              source used for the  logs  is  not  updated  after  system  SUS‐
134              PEND/RESUME.
135
136       -t, --notime
137              Do not print kernel's timestamps.
138
139       --time-format format
140              Print  timestamps  using  the  given format, which can be ctime,
141              reltime, delta or iso.  The first three formats are  aliases  of
142              the  time-format-specific  options.   The  iso format is a dmesg
143              implementation of the ISO-8601 timestamp format.  The purpose of
144              this  format  is to make the comparing of timestamps between two
145              systems, and any other parsing, easy.  The definition of the iso
146              timestamp  is: YYYY-MM-DD<T>HH:MM:SS,<microseconds><-+><timezone
147              offset from UTC>.
148
149              The iso format has the same issue as  ctime:  the  time  may  be
150              inaccurate when a system is suspended and resumed.
151
152       -u, --userspace
153              Print userspace messages.
154
155       -w, --follow
156              Wait  for  new messages.  This feature is supported only on sys‐
157              tems with a readable /dev/kmsg (since kernel 3.5.0).
158
159       -x, --decode
160              Decode facility and level (priority) numbers  to  human-readable
161              prefixes.
162
163       -V, --version
164              Display version information and exit.
165
166       -h, --help
167              Display help text and exit.
168

COLORS

170       Implicit  coloring  can be disabled by an empty file /etc/terminal-col‐
171       ors.d/dmesg.disable.  See terminal-colors.d(5) for more  details  about
172       colorization configuration.
173
174       The logical color names supported by dmesg are:
175
176       subsys The message sub-system prefix (e.g. "ACPI:").
177
178       time   The message timestamp.
179
180       timebreak
181              The  message  timestamp  in  short  ctime format in --reltime or
182              --human output.
183
184       alert  The text of the message with the alert log priority.
185
186       crit   The text of the message with the critical log priority.
187
188       err    The text of the message with the error log priority.
189
190       warn   The text of the message with the warning log priority.
191
192       segfault
193              The text of the message that inform about segmentation fault.
194

EXIT STATUS

196       dmesg can fail reporting permission  denied  error.   This  is  usually
197       caused  by dmesg_restrict kernel setting, please see syslog(2) for more
198       details.
199

SEE ALSO

201       terminal-colors.d(5), syslogd(8)
202

AUTHORS

204       Karel Zak ⟨kzak@redhat.com⟩
205
206       dmesg was originally written by Theodore Ts'o ⟨tytso@athena.mit.edu⟩
207

AVAILABILITY

209       The dmesg command is part of the util-linux package  and  is  available
210       from Linux Kernel Archive ⟨https://www.kernel.org/pub/linux/utils/util-
211       linux/⟩.
212
213
214
215util-linux                         July 2012                          DMESG(1)
Impressum