Here is the video version, if you prefer it:
The dmesg command is used to display kernel messages. (Ward, 2014) You use this command when you want to see if a kernel is giving you an error for a particular hardware device, for example.
An example:
mislav@mislavovo-racunalo:~/Linux_folder$ sudo dmesg
[2344706.157876] x86: Booting SMP configuration:
[2344706.157877] smpboot: Booting Node 0 Processor 1 APIC 0x2
[2344706.158274] cache: parent cpu1 should not be sleeping
[2344706.158419] CPU1 is up
[2344706.158436] smpboot: Booting Node 0 Processor 2 APIC 0x4
[2344706.158833] cache: parent cpu2 should not be sleeping
[2344706.158987] CPU2 is up
…
dmesg gives a lot of output, so be prepared. You can pipe it and use less to read through it, for example, or redirect its output in a file.
Another way to look at kernel messages (besides dmesg) is by looking at the contents of a file which is located in /var/log/kern.log. (Ward, 2014) Keep in mind that catting this file will produce a lot of output.
Thank you for reading!
References
Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 94-95