1DMESG(1)                         User Commands                        DMESG(1)
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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
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DESCRIPTION

18       dmesg is used to examine or control the kernel ring buffer.
19
20       The default action is to read all messages from kernel ring buffer.
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OPTIONS

23       The  --clear,  --read-clear, --console-on, --console-off and --console-
24       level options are mutually exclusive.
25
26       -C, --clear
27              Clear the ring buffer.
28
29       -c, --read-clear
30              Clear the ring buffer contents after printing.
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32       -D, --console-off
33              Disable printing messages to the console.
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35       -d, --show-delta
36              Display the timestamp and time delta spent between messages.  If
37              used together with --notime then only the time delta without the
38              timestamp is printed.
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40       -e, --reltime
41              Display the local time and delta in human readable format.
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43       -E, --console-on
44              Enable printing messages to the console.
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46       -F, --file file
47              Read log from file.
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49       -f, --facility list
50              Restrict output to defined (comma separated) list of facilities.
51              For example
52
53                     dmesg --facility=daemon
54
55              will print messages from system daemons only.  For all supported
56              facilities see dmesg --help output.
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58       -H, --human
59              Enable human readable output.  See also --color,  --reltime  and
60              --nopager.
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62       -h, --help
63              Print a help text and exit.
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65       -k, --kernel
66              Print kernel messages.
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68       -L, --color
69              Colorize important messages.
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71       -l, --level list
72              Restrict  output  to  defined  (comma separated) list of levels.
73              For example
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75                     dmesg --level=err,warn
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77              will print error and warning messages only.  For  all  supported
78              levels see dmesg --help output.
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80       -n, --console-level level
81              Set  the  level at which logging of messages is done to the con‐
82              sole.  The level is a level number or abbreviation of the  level
83              name.  For all supported levels see dmesg --help output.
84
85              For  example,  -n  1  or  -n alert prevents all messages, except
86              emergency (panic) messages, from appearing on the console.   All
87              levels  of  messages  are  still  written to /proc/kmsg, so sys‐
88              logd(8) can still be used to control exactly where  kernel  mes‐
89              sages  appear.  When the -n option is used, dmesg will not print
90              or clear the kernel ring buffer.
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92       -P, --nopager
93              Do not pipe output into  a  pager,  the  pager  is  enabled  for
94              --human output.
95
96       -r, --raw
97              Print  the  raw message buffer, i.e., do not strip the log level
98              prefixes.
99
100              Note that the real raw format depends  on  method  how  dmesg(1)
101              reads  kernel messages. The /dev/kmsg uses different format than
102              syslog(2).  For backward  compatibility  dmesg(1)  returns  data
103              always  in syslog(2) format. The real raw data from /dev/kmsg is
104              possible  to  read  for  example  by  command  'dd  if=/dev/kmsg
105              iflag=nonblock'.
106
107       -S, --syslog
108              Force to use syslog(2) kernel interface to read kernel messages.
109              The default is to use /dev/kmsg rather than syslog(2) since ker‐
110              nel 3.5.0.
111
112       -s, --buffer-size size
113              Use  a  buffer of size to query the kernel ring buffer.  This is
114              16392 by default.  (The default kernel syslog  buffer  size  was
115              4096  at first, 8192 since 1.3.54, 16384 since 2.1.113.)  If you
116              have set the kernel buffer to be larger than  the  default  then
117              this option can be used to view the entire buffer.
118
119       -T, --ctime
120              Print human readable timestamps.  The timestamp could be inaccu‐
121              rate!
122
123              The time source used for the logs is not  updated  after  system
124              SUSPEND/RESUME.
125
126       -t, --notime
127              Do not print kernel's timestamps.
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129       -u, --userspace
130              Print userspace messages.
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132       -V, --version
133              Output version information and exit.
134
135       -w, --follow
136              Wait for new messages. This feature is supported on systems with
137              readable /dev/kmsg only (since kernel 3.5.0).
138
139       -x, --decode
140              Decode facility and level (priority) number  to  human  readable
141              prefixes.
142

SEE ALSO

144       syslogd(8)
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AUTHORS

147       Karel Zak ⟨kzak@redhat.com⟩
148       Theodore Ts'o ⟨tytso@athena.mit.edu⟩
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AVAILABILITY

151       The  dmesg  command  is part of the util-linux package and is available
152       from Linux Kernel  Archive  ⟨ftp://ftp.kernel.org/pub/linux/utils/util-
153       linux/⟩.
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157util-linux                         July 2012                          DMESG(1)