A USB device is not getting the device node you expect. You want to watch, live, the kernel uevents and the resulting udev events as you unplug and replug it. Which command does that?
LPIC-1 Exam 101-500, objective 101. System architecture hard
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The options
Not correct dmesg -w
Wrong. dmesg -w follows the kernel ring buffer, so you see the kernel's own messages about the device, but udev's rule processing and the resulting device naming never appear there.
Not correct udevadm info --query=all --name=/dev/sdb
Wrong for this purpose. udevadm info queries the udev database for one device's attributes and properties, a static snapshot rather than a live event stream. It is what you use afterwards to find attributes to match on in a rule.
Not correct udevadm trigger
Wrong. udevadm trigger re-emits events for devices already present, which makes new rules apply without a reboot. It generates events, it does not display them.
Correct udevadm monitor
Correct. udevadm monitor prints kernel uevents (KERNEL lines) and the events udev emits after processing its rules (UDEV lines) as they happen, which is the standard way to see what rules actually fire.
Why
The usual udev debugging loop is: udevadm monitor to see the event, udevadm info to read the device's attributes, a rule written under /etc/udev/rules.d (which takes precedence over the rules shipped by packages in /usr/lib/udev/rules.d, still spelled /lib/udev/rules.d on some Debian-derived systems), then udevadm control --reload and udevadm trigger to apply it. dmesg shows the kernel side of the same plug event but nothing about udev.
Where this comes from
- Cited
- LPI exam objective 101.1
- What it says
- Basic understanding of udev, its rule locations and the tools used to inspect device events.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 101 and space the ones you get wrong.