LPIC-1 System Architecture Practice Question
A Linux server has a USB serial adapter that is sometimes detected as /dev/ttyUSB0 and sometimes as /dev/ttyUSB1 after reboot. An administrator wants to create a udev rule that always assigns the name /dev/ttyUSB-radio to this specific adapter based on its serial number. Which udev rule syntax should be used?
⚠ Common exam trap
The trap here is matching on a non-unique attribute like vendor ID or using a USB subsystem rule, which either affects multiple devices or fails to name the tty node correctly.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
SUBSYSTEM=="tty", ATTRS{serial}=="A50285BI", SYMLINK+="ttyUSB-radio"
A reliable udev rule for a USB serial adapter must match the tty subsystem and a unique attribute such as the adapter's serial number, then create a persistent symlink. ATTRS{serial} searches the device and its parent devices, which is where USB serial numbers are exposed. SYMLINK+ adds the link without disturbing other symlinks, ensuring the adapter is always reachable at /dev/ttyUSB-radio.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
KERNEL=="ttyUSB*", ATTR{idVendor}=="0403", SYMLINK="ttyUSB-radio"
Why it's wrong here
Matching by idVendor alone is not unique because many adapters share the same vendor ID, so the rule could apply to the wrong device. Additionally, SYMLINK without the plus sign replaces any existing symlink list, potentially removing other useful links. The scenario requires uniqueness by serial number.
- ✓
SUBSYSTEM=="tty", ATTRS{serial}=="A50285BI", SYMLINK+="ttyUSB-radio"
Why this is correct
This rule matches the tty subsystem and the device's serial attribute, then creates a persistent symbolic link named ttyUSB-radio. Using SYMLINK+ adds a link without removing existing ones, and ATTRS matches attributes of the device or its parents, which is appropriate for USB serial adapters whose serial number appears on the USB device.
- ✗
SUBSYSTEM=="usb", ATTR{serial}=="A50285BI", NAME="ttyUSB-radio"
Why it's wrong here
The usb subsystem rule would match the USB device node, not the tty device, and NAME= on a non-tty device will not create the desired serial port name. Assigning NAME to a USB device node can also disrupt the default device node creation, making the adapter inaccessible under its expected tty name.
- ✗
ACTION=="add", SUBSYSTEM=="tty", RUN+="/bin/ln -s /dev/%k /dev/ttyUSB-radio"
Why it's wrong here
Using RUN to create a symlink manually bypasses udev's own symlink management and may run before the device node is fully ready or with a different working directory. It also does not provide a persistent match key such as serial, so it could link the wrong device if multiple adapters appear. The native SYMLINK mechanism is preferred.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official LPI exam blueprint
This LPIC-1 practice question is part of Courseiva's free LPI certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LPIC-1 exam.