How to tail dmesg on CentOS 7

Question

In a CentOS 7 server, I typed dmesg and noted the time stamp of the last entry.

Next, I typed tail -f /var/log/dmesg in a DIFFERENT TERMINAL ON THE SAME MACHINE, so that I could watch the log tail.

Third, I ran some scripts on the same machine using the same terminal. Nothing changed in the terminal in which I had typed tail -f /var/log/dmesg.

Finally, I typed dmesg again, extracted the log into Notepad++, and noticed that over 1,500 lines of new logs had been created in the dmesg output while I was running the other scripts.

What specific commands must be typed in order to get the tail of the dmesg logs to actually display on the screen?

Solution

dmesg is a ring buffer. Which is to say, when it reaches a certain size, old data are thrown out. Because of this, it doesn't really work like a normal file. It's kind of like the data in /proc - when you cat or otherwise display it, you see only the state at that instant.

To get around this, newer implementations of dmesg include the -w or --follow switches, which work effectively like tail -f. If you want to capture this into a conventional file rather than a ring buffer, you can use a construct such as:

dmesg --follow >> /path/to/file

Or, if you wish to also see what is being captured:

dmesg --follow | tee -a /path/to/file

As with tail -f, this can be terminated with a simple INT signal or Ctrl-C.

Comments

[Pankaj Goyal]

I was mistaken; I had forgotten that dmesg is a ring buffer. CentOS 7 might have dmesg -w or dmesg --follow.