Read the kernel's own boot diary to find hardware and driver issues fast. 16.11.2025 | reading time: 3 min Boot time can hide the cause of a failing service or dead network; dmesg is the tool that tells what the kernel saw while starting. Run `dmesg -T` to translate timestamps and scan for warnings and errors; the kernel ring buffer often contains the first and most honest report of hardware, driver, and early-boot userspace issues. Show me an example Reproduce the steps and read the output as if diagnosing a server that fails network or disk access: ```bash $ sudo dmesg -T | grep -iE "error|fail|ata|usb|eth" [Mon Nov 16 10:01:23 2025] sd 0:0:0:0: [sda] tag#0 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE [Mon Nov 16 10:01:23 2025] ata1.00: exception Emask 0x0 SAct 0x0 [Mon Nov 16 10:01:24 2025] usb 1-1: device descriptor read/64, error -71 [Mon Nov 16 10:01:25 2025] e1000e 0000:02:00.0: eth0: NIC Link is Down ``` From that output the next steps are clear: check physical cabling and NIC firmware, inspect `/var/log` and journal, and consider reseating or replacing the failing disk. Practical filters and live watching Do not stare at everything; narrow the noise with `dmesg --level=err,warn -T` to show problems only, use `dmesg -w` to follow messages in real time during a reboot or while inserting hardware, and `sudo dmesg -c` to clear the buffer when you want a fresh capture for a test; remember that timestamps without `-T` are seconds since boot and are best for correlating events with systemd or other logs. What the kernel ring buffer means Messages in dmesg come from a circular kernel ring buffer with limited size, so very early or very verbose boots can push older lines out; the entries are authoritative for driver and kernel-level failures but ephemeral on systems without persistent logging, which is why capturing output or combining with the system journal is recommended. Complementary logs and controls For persistent storage of kernel messages and richer queries use `journalctl -k -b` to see kernel messages for the current boot, check `/var/log/kern.log` or `/var/log/messages` on non-systemd systems, and adjust console verbosity with `dmesg -n` or kernel command line loglevel to control what appears on the console during boot. Next steps for the investigator When dmesg points at a module or firmware, test by reloading the module, checking firmware versions, and searching upstream kernels or bug trackers for the exact message string; capture reproducible output, narrow to minimal hardware steps, and use live tracing tools if needed to dig deeper. Clean finish and challenge dmesg gives a direct line into the kernel's view of boot problems and often saves hours of guesswork when used with filters and the system journal; practice by reproducing a simple boot fault and capturing the dmesg output, then pursue deeper certification such as CompTIA Linux+ or LPIC-1 and consider exam-focused training at bitsandbytes.academy for intensive preparation. Join Bits & Bytes Academy First class LINUX exam preparation. boot-process troubleshooting utilities infrastructure processes
