Is it possible to `tail -f` the output of `dmesg`?

Question

I want to do something like

dmesg | tail -f

but it doesn't work:

I use Mac OS X v10.6.7 (Snow Leopard). By doing that, tail will exit, instead of monitoring the output.

I wonder if there is a way to do it, or an equivalent command.

P.S., I don't think a while loop will be a good enough idea.

Answer 1

You are probably looking for some combination of messages from various log files. Try:

tail -f /var/log/{messages,kernel,dmesg,syslog}

…to get a pretty good overview of the system. If you want more or less than that, research what log file the messages you want to see are being placed in.

Also look into using multitail to file and color code and filter multiple log files at once.

Edit: This wasn't very relevant when I answered this, but as this page gets a lot of hits I'm thought it worth mentioning that newer systems running systemd have this.

dmesg -w

Answer 2

Just make it @#$%ing work

1. You want to print output of dmesg, constantly, immediately
2. Dmesg is printing the kernel ring buffer (see man dmesg)
3. The kernel ring buffer is a special proc file, /proc/kmsg (see man proc)
4. Read /proc/kmsg directly, ie cat /proc/kmsg.

Now, if you read the friendly proc manual, it'll sternly warn you to let only one user (who must be privileged) read /proc/kmsg at a time. Whatever syslog implementation you have should be doing this, and presumably it works with dmesg. I dunno, I'm out of my league here, just paraphrasing the manual. So while this is the "just make it @#$%ing work" way, consider the next couple methods first.

Man page approved: watch + dmesg

On one linux box I use with systemd init*, dmesg.log isn't written to very often, perhaps not at all? The best way I found to read the kernel log buffer continuously is with watch. Something like this should get you started (adjust for how many lines fit in your terminal):

watch 'dmesg | tail -50'

watch + dmesg + daemon + tail -f

A more convoluted solution might use watch to write dmesg output to file, which you could then tail -f. You'd probably want this running as a daemon. A proper daemon would also gzip and rotate logs. The following bash code is untested, unworking, and only intended to convey an idea. @Brooks Moses's answer has a working version.

watch 'dmesg >> /var/log/dmesg.log | tail -1'

* tangent, cause this is a question about an apple desktop os: when systemd is around, don't bother with dmesg; use journalctl -xf (maybe w/ -n 100 to also show the previous 100 lines)

Answer 3

On Linux, since kernel kernel 3.5.0 you can use:

dmesg -w

Also on systems with systemd you can use:

journalctl -kf

Answer 4

Here's a variant on djeikyb's answer that's actually tested, and fixes a couple of bugs.

watch 'sudo dmesg -c >> /tmp/dmesg.log; tail -n 40 /tmp/dmesg.log'

The important trick is that we're doing dmesg -c, which clears the ring buffer after it prints -- thus, each time through we're only printing what's new since the last time.

You'll need to be root to do that, thus the sudo. There's also a bugfix; instead of trying to both dump the output to a file and pipe it to tail (which doesn't work), we're just reading from the newly-written file.

We could do just dmesg > /tmp/dmesg.log and overwrite the whole file each iteration, but that's a lot of I/O and also risks losing the file if the computer crashes in the middle of an overwrite.

You could also do something similar that's more closely like tail -f with a while loop that executes dmesg -c and sleep 1 forever (see Ben Harris's answer). However, since this is actually clearing the kernel message buffer as it's running, you may also want to pipe things into a logfile in case you want them later.

Answer 5

This may work for you

while true;do sudo dmesg -c;done

Keep in mind that the '-c' flag clears the message buffer into stdout. The 'sudo' is unnecessary if you are root. If you feel this is eating too much of your CPU resource, try adding a 'sleep 1' before the loop is done.

Answer 6

I did this before seeing this post:

#!/usr/bin/env perl

use strict;
use warnings;

# "tail -f" for dmesg
# Keeps last printed line. Anything sorting "gt" will be newer

$|=1;

my $y = '';

while(1) {
    for my $k (`dmesg`) {
        if ($k gt $y) {
            print $k;
            $y = $k;
        }
    }
    sleep 1;
}
exit;

Answer 7

I use this alias in /root/.bashrc;

alias dwatch='watch -n 0.1 "dmesg | tail -n $((LINES-6))"'

which follows dmesg and adjusts the lines for whatever terminal it's called in.

Answer 8

You might be able to do:

tail -f /var/log/messages

Answer 9

Here are some ideas for limited environments

Environments such as embedded or pre-boot, where watch, tail, cat, dd and other commands might not be available, might need different gymnastics.

This is what some lightweight Linux distributions do:

while dmesg -c >> /tmp/dmesg.log; do sleep 0.1; done & tail -f /tmp/dmesg.log

It backgrounds the while loop (with &) while tailing the generated output.

If you can't write to /tmp:

mount -t tmpfs - /tmp 

# or 
mount -t ramfs - /tmp 

# or use /dev/shm instead of /tmp - which is available in newer environments

If you don't have tail, you can

cat /tmp/dmesg.log

# or 
dd if=/tmp/dmesg.log 

# or
dd if=/tmp/dmesg.log 2>/dev/null

Or you might be in a busybox environment that doesn't have dmesg linked, then just:

busybox dmesg -c

You might also need to

busybox sleep

instead of sleep

If you don't have sleep:

while dmesg -c; do echo >/dev/null; done 

If you don't have "dmesg":

while sleep 0.1; do cat -v /proc/kmsg; done

This only works if nothing else is reading from here. You might also have a /dev/kmsg.

Bonus tip:

If you don't know what you have, and you don't have "ls", just:

busybox ls

# or simply:

echo *

Answer 10

I used this code to look for a special kernel event and piped it a "callback" process:

while true ; do dmesg -c ; sleep .1 ; done \
| grep --line-buffered  -o $pattern \
| ...

Answer 11

Under the current Ubuntu (I am using Ubuntu 12.04 (Precise Pangolin)),

tail -f /var/log/syslog
6< <( cat /var/log/syslog |grep -F  'kernel: '; sudo cat /proc/kmsg) cat /dev/fd/6

( the sudo command need sudo privilege )

Please also try another one like: 6< <( dmesg; sudo cat /proc/kmsg) cat /dev/fd/6

Answer 12

This might be useful:

dmesg | tail -f -

pipes the output of dmesg through tail using the - operator as a shortcut to standard output.