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Linux identifies USB hardware using numeric vendor and product IDs exposed by the kernel, then udev can turn those and other device details into properties or use them to match rules. The key to a reliable configuration is matching the right part of the device tree: a physical USB device, one of its interfaces, or a child such as a serial or hidraw node.
What a USB hardware ID tells Linux
When the kernel enumerates a USB device, it exposes its descriptors and attributes through sysfs. The numeric vendor ID and product ID are available as attributes such as idVendor and idProduct. USB driver matching can also use these identifiers, though kernel driver matching is separate from userspace udev rules. See the Linux USB API documentation.
Manufacturer and product strings are descriptive values supplied by the device, not substitutes for numeric IDs. They may be absent or inconsistent. systemd’s USB-ID builtin can fall back to numeric identifiers when readable strings are unavailable. The systemd usb_id implementation also derives identity data such as serial and revision information.
- Raw USB attributes come from the device in sysfs, including
idVendor,idProduct,bcdDevice,manufacturer,product, andserial. - udev properties such as
ID_VENDOR_ID,ID_MODEL_ID,ID_VENDOR,ID_MODEL,ID_SERIAL, andID_BUSare derived by udev builtins and rules; they are metadata, not the original descriptor fields. - A serial number can distinguish one physical unit from other units of the same model. Use it only when that narrower match is wanted.
Check whether the target is a device, interface, or child node
One physical USB product can expose multiple interfaces, each with its own class, subclass, and protocol. A composite device may therefore have one device-level vendor/product identity but several interface-level functions. A rule for a USB device is not automatically the right rule for a child node such as ttyUSB* or hidraw*.
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Start by inspecting the actual device path and its parent chain. The udev manual documents rule matching and attributes; systemd’s packaged examples also show matching USB identifiers and interface class before setting a device attribute. The systemd default rules illustrate that device and interface attributes can both matter.
udevadm info --attribute-walk /dev/ttyUSB0
udevadm info /dev/ttyUSB0
Replace /dev/ttyUSB0 with the actual device node. The attribute walk helps reveal which values belong to the node itself and which come from its ancestors. In a rule, ATTR{...} matches an attribute on the event device; ATTRS{...} can match an ancestor. Choose based on the inspected tree rather than assuming a USB ID is directly attached to every child node.
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How to write a udev rule for a USB match
udev processes device events against rule files. Rules can match event properties, subsystem, kernel name, and device or parent attributes, then assign properties or perform supported actions. The manual for the installed systemd version is the authority for available operators and behavior.
ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="vvvv", ATTR{idProduct}=="pppp", ENV{EXAMPLE_MATCHED}="1"This is an illustrative match only: replace the ID values and assignment with values appropriate to the target and intended result. It matches an add event on the USB subsystem and sets an example environment property. It does not prescribe changing permissions or power behavior.
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For a child device, include an appropriate condition for that child and use parent-aware matching where the USB attributes are on an ancestor. Do not edit packaged rules in /usr/lib/udev/rules.d or /lib/udev/rules.d. Put administrator rules in the distribution’s local configuration directory, commonly /etc/udev/rules.d, and choose a filename whose lexical position suits the intended rule ordering. Check the installed udev(7) manual for the exact rule syntax and rule-directory behavior on your system.
When to use systemd hwdb instead
Rules and hwdb are related but serve different purposes. A udev rule matches an event and controls udev behavior. The hardware database associates match patterns with hardware properties that a subsystem or other consumer can use. A hardware-specific change belongs in hwdb only when the relevant consumer reads that database and documents hwdb as its configuration mechanism.
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That distinction matters because consumers can change their configuration approach. For example, libinput’s device-configuration documentation describes rebuilding hwdb and checking properties, while noting that libinput has moved device quirks to a separate system. Follow the documentation for the component whose behavior you intend to alter; do not assume every quirk should be expressed in hwdb.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Apply and verify a hwdb change
For the libinput workflow documented at the cited URL, rebuild the database, trigger the affected input device, inspect its properties, and run a test for diagnostics:
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- Rebuild the database:
sudo systemd-hwdb update - Trigger the relevant device:
sudo udevadm trigger /sys/class/input/eventX - Inspect its properties:
udevadm info /sys/class/input/eventX - Diagnose rule processing:
sudo udevadm test /sys/class/input/eventX
Substitute the actual sysfs path for /sys/class/input/eventX. These commands and their precise behavior can vary with the installed systemd version and distribution; consult local manual pages if a command is missing or behaves differently. udevadm test helps diagnose matching and parsing, but it does not execute RUN directives, so it is not a simulation of every side effect.
Quick Recap
A practical identification workflow
- Locate the relevant node. Decide whether the target is the USB device itself, one of its interfaces, or a child such as a tty or hidraw node.
- Inspect the attributes and ancestry. Record vendor and product IDs, interface details where relevant, and a serial only if the rule should target an individual unit.
- Check existing udev properties. Use
udevadm infoto distinguish a missing raw device attribute from a property that has not been derived or assigned. - Choose the configuration mechanism by consumer. Use a udev rule for event matching and udev behavior; use hwdb only when the intended subsystem documents it as a source of hardware properties.
- Apply and refresh appropriately. Reload or trigger as appropriate for a rule change; rebuild hwdb before refreshing a hwdb-based change.
- Verify the result. Inspect resulting properties and use
udevadm testto investigate match or syntax problems, accounting for its inability to runRUNcommands.
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