What’s the difference of dmesg output and /var/log/messages

dmesgkernellogs

AFAIK dmesg shows information about kernel and kernel modules, and /var/log/messages also shows information produced by kernel and modules.

So what's the difference? Does /var/log/messages ⊂ output of dmesg?

More Info that may be helpful:
– There is a kernel ring buffer, which I think is the very and only place to store kernel log data.
– Article "Kernel logging: APIs and implementation" on IBM DeveloperWorks described APIs and the bird-view picture.

Best Answer

dmesg prints the contents of the ring buffer. This information is also sent in real time to syslogd or klogd, when they are running, and ends up in /var/log/messages; when dmesg is most useful is in capturing boot-time messages from before syslogd and/or klogd started, so that they will be properly logged.

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Linux – Correlating /var/log/* timestamps

Interesting problem, Not sure I've ever tried to do this. But I have notice the timestamp you are talking about and I have always belived it to be seconds since bootup.

In my syslog I have on my server, I have:

Jan 10 19:58:55 wdgitial kernel: [    0.000000] Initializing cgroup subsys cpuset
Jan 10 19:58:55 wdgitial kernel: [    0.000000] Initializing cgroup subsys cpu
Jan 10 19:58:55 wdgitial kernel: [    0.000000] Linux version 2.6.32-21-server (buildd@yellow) (gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5) ) #32-Ubuntu SMP Fri Apr 16     09:17:34 UTC 2010 (Ubuntu 2.6.32-21.32-server 2.6.32.11+drm33.2)
Jan 10 19:58:55 wdgitial kernel: [    0.000000] Command line:  root=/dev/xvda1 ro quiet splash

I would imagine this is fairly consistent among most Linux distro's as this is the kernel spitting out it's stuff.

And here I have the date along with the timestamp.

Linux dmesg – Difference Between Output of dmesg and Content of /var/log/dmesg

The dmesg command shows the current content of the kernel syslog ring buffer messages while the /var/log/dmesg file contains what was in that ring buffer when the boot process last completed.

Best Answer

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Linux – Correlating /var/log/* timestamps

Linux dmesg – Difference Between Output of dmesg and Content of /var/log/dmesg

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