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

dmesglinux

I found the output of dmesg and content of /var/log/dmesg are different. Then what differences are between their purposes?

Best Answer

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.

Related Solutions

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

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.

Linux – How to dmesg content be logged into a file

You need to look at either /etc/rsyslog.conf or /etc/syslog.conf. If you have a line early on such as:

*.*                -/var/log/syslog

Everything, including the stuff from dmesg, should go to that file. To target it better:

kernel.*           -/var/log/dmesg

If that fails for some reason, you could periodically (e.g. via cron):

dmesg > /var/log/dmesg

Depending on how big the dmesg buffer is (this is compiled into the kernel, or set via the log_buf_len parameter) and how long your system has been up, that will keep a record of the kernel log since it started.

If you want to write the dmesg output continuously to a file use the -w (--follow) flag.

dmesg --follow > mydmesg.log

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Related Solutions

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

Linux – How to dmesg content be logged into a file

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