143

I'm writing a device driver that prints error message into ring buffer dmesg output. I want to see the output of dmesg as it changes.

How can I do this?

182

Relatively recent dmesg versions provide a follow option (-w, --follow) which works analogously to tail -f.

Thus, just use following command:

$ dmesg -wH

(-H, --human enables user-friendly features like colors, relative time)

Those options are available for example in Fedora 19.

  • 2
    Nice find! Gentoo doesn't grok -H but otherwise spot-on – unperson325680 Oct 13 '13 at 16:19
  • The user space tools need to be version 2.22+. Ubuntu users have to wait for version 14.10 "utopic" – Daniel Alder Aug 25 '14 at 7:41
  • 1
    Ubuntu doesn't grok -w - need to use watch (below) – Brent Faust Feb 24 '16 at 0:19
  • 2
    Wow a sysadmin answer that uses --descriptively-named-flags instead of cryptic single-character flags. BRAVO, SIR. BRAVO. – allyourcode Apr 2 '16 at 4:59
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    -w should work in every version of Ubuntu from Utopic (14.10) onwards. (launchpad.net/ubuntu/+source/util-linux/+publishinghistory suggests early Utopic packages were for 2.20, but reached 2.25 by the time it was released.) – mwfearnley Aug 20 '17 at 13:58
54

You can use the watch command which is intended exactly for things like this

watch -n 0.1 "dmesg | tail -n $((LINES-6))"

the $((LINES-6)) part should make it fit nicely into your terminal.

  • 2
    If you use single quotes instead of double quotes (or escape the dollar sign), you'll to get a fresh value of $LINES for every invocation, so it will adjust if you change your terminal size. – P Daddy Mar 31 '16 at 15:22
  • Single quotes will inhibit variable expansion. Moreover, the variable is expanded only once in this example - upon invocation of watch. Thus it won't really change between invocations of dmesg. One would need to use a wrapper that would query the terminal state. – peterph Apr 2 '16 at 14:29
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    That's the whole point. The single quotes prohibit the variable expansion when the shell passes the arguments to watch, therefore watch will execute the command with the variable expression in there, which will be expanded by the shell it invokes. Each time. Try it, it works. – P Daddy Apr 2 '16 at 14:37
  • Hmm, you are right - I assume watch uses popen(), which means another shell is spawned and the environment variable is then supplied by it (and thus updated on every execute run). Nice find. – peterph Apr 2 '16 at 15:14
12

you can't really monitor the output of dmesg directly.

however, chances are high that your module is not printing directly into the ring-buffer of dmesg, but instead uses the kernel logging facilities (which will then be displayed by dmesg). if your syslog has some sane (e.g. default) settings, these messages will most likely also show up in the kern.log logfile.

so you can do something like:

 tail -f /var/log/kern.log
  • /var/log/kern.log is fairly Linux-specific. For OpenBSD (and possibly others) the dmesg stuff gets logged in /var/log/messages. Granted, some other stuff goes there too though. – kurtm Oct 13 '13 at 14:27
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    tail -f /var/log/{messages,kernel,dmesg,syslog} via superuser: is-it-possible-to-tail-f-the-output-of-dmesg – here May 2 '14 at 7:55
9

You use dmesg to get log messages of the kernel.

The kernel itself logs into a ring buffer, i.e. just in memory. Now all dmesg does is output the content of that ring buffer. If you do dmesg -c it will also delete the ring buffer afterwards.

Therefore you could do something like while true; do dmesg -c; sleep 1; done to have something like the equivalent of a not working dmesg|tail. But this deletes the ring buffer and therefore needs root powers.

The other way is the file /proc/kmsg which allows a view on the ring buffer. You could do tail -f /proc/kmsg, but this is only allow to one process, and this is usually your logging daemon. - It's job is to read the messages and write it to real files (usually in /var/log) where they can be read. It can be configured to output all messages to a single file or different parts into different files. (But configuration depends on the logging daemon of your system.)

Therefore have a look at /var/log if there is any file which suits your needs and configure your logging daemon otherwise.

  • // , On the CEntOS 6 system I'm using, tailing and viewing /proc/kmsg produces no output. [~]$ sudo tail -f /proc/kmsg ♥% [~]$ sudo cat /proc/kmsg ♥% [~]$ – Nathan Basanese Sep 4 '15 at 19:36
8

If you are using an embedded system the busybox that is common on systems like OpenWRT has very limited functionality and only 2-3 flags are supported.

If you want a quick and dirty way of printing dmesg output on screen continually as events change, a simple bash loop works fine. It's not ideal but as I mentioned the BusyBox dmesg is missing a lot of functions. I find the following has the same effect when entered in to the command line:

$ while true; do dmesg -c ; sleep 1 ; done

you can quit the loop with Ctrl-C the sleep 1 is to stop it battering the CPU needlessly, and the -c flag clears the buffer on each call so you don't see repeated output every second,

  • 1
    Really useful for debugging android. – val Aug 30 '17 at 11:22
5

On systems that use systemd you can also:

# journalctl -kf
  • 1
    Ubuntu 14.10: ➜ ~ journalctl -kf No journal files were found. – Nathan Basanese May 6 '15 at 11:46
0

Use these 2 commands from seperate terminals :

  1. while true; do dmesg -c >> test.txt;sleep 1; done
  2. tail -f test.txt

It will achieve similar result.

  • // , This basically instructs dmesg to append output to test.txt, right? And the second command just watches that test.txt file? – Nathan Basanese Sep 4 '15 at 19:31
  • Thanks .. yes.. the 2nd command watches the changes to dmesg. cat /proc/kmsg can achieve the similar output but it doesn't save the logs to a file. – K_K Sep 7 '15 at 6:56

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