Frequently Asked Question

What is dmesg and when should I read it?

dmesg prints the kernel's ring buffer, a circular log of everything the kernel has said since boot: which devices it found, which drivers it loaded, which filesystems were mounted, what hardware errors it saw, what processes the OOM killer terminated. Run it any time the machine isn't doing what you expect at the hardware or driver level: USB device not appearing, disk throwing I/O errors, a process being killed for memory pressure, network interface flapping.

On a systemd system, journalctl -k shows the same buffer plus persists it across reboots; journalctl -k -b -1 shows the previous boot.