Linux identifies USB hardware from numeric vendor and product IDs exposed by the kernel, then systemd-udevd can turn those values and other USB descriptors into properties used by rules and applications. Use a udev rule when you need to match a device event and change udev behavior; use systemd hwdb when a hardware-aware consumer expects match-keyed properties. First inspect the device and its parent chain: a USB device, one of its interfaces, and a child such as a tty node are different match targets.
What Linux means by a USB hardware ID
When the kernel enumerates a USB device, it exposes descriptor values through its device model and sysfs. Common raw attributes include idVendor, idProduct, bcdDevice, manufacturer, product, and serial. The vendor and product IDs are numeric identifiers; manufacturer and product strings are separate descriptive fields and may be absent or inconsistent. The Linux USB API documentation also describes vendor and product IDs as values used for USB driver matching.
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A numeric ID is usually a steadier match key than a text label, but it identifies a product identity rather than necessarily one physical unit. If a device provides a serial number and you include it in a match, you may narrow the match to an individual unit. Device strings can help make output readable; they should not be assumed to be present.
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systemd-udevd processes device events and can derive properties from USB attributes using its usb_id builtin. Properties such as ID_VENDOR_ID, ID_MODEL_ID, ID_VENDOR, ID_MODEL, ID_SERIAL, and ID_BUS are udev metadata, not the original USB descriptor fields. The builtin normalizes readable strings and can fall back to numeric IDs when a manufacturer or product string is unavailable. Its implementation also handles serial and interface-related information; see the systemd usb_id source.
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Keep three layers distinct when diagnosing a device:
- Raw USB attributes: descriptor-derived values exposed in sysfs, such as
idVendorandidProduct. - udev properties: values derived by udev builtins or rules and available to userspace consumers, such as
ID_VENDOR_IDandID_SERIAL. - Kernel driver matching: driver-level matching against USB IDs or class/interface information; it is related to identification but is not the same mechanism as a userspace udev rule.
Distinguish the USB device from its interfaces and child nodes
A physical USB product can expose multiple interfaces. For example, a composite device may present separate functions, each with its own interface class, subclass, and protocol. A rule targeting the USB device is not automatically equivalent to a rule targeting a child such as ttyUSB0, hidraw0, or an input event node.
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Inspect the actual sysfs path and ancestry before choosing a match. Use udevadm info --attribute-walk /dev/DEVICE for a device node, substituting the real path, or inspect the relevant /sys entry. Attributes belonging to the event device itself are commonly matched with ATTR; attributes found on a parent in its ancestry may require parent-aware matching such as ATTRS. Check the installed udev manual for the rule semantics on your system. The systemd rules examples show matching patterns that combine USB attributes and interface information.
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When to use a udev rule or systemd hwdb
| Mechanism | What it is for | What to check |
|---|---|---|
| udev rule | Match a device event and apply supported udev behavior, such as setting a property. | Target subsystem, event device versus parent, rule ordering, and the installed udev rule syntax. |
| systemd hwdb | Store match-keyed hardware properties for consumers that read them. | Whether the specific consumer uses hwdb for the property or quirk you intend to change. |
These mechanisms are related but not interchangeable. A rule expresses event-time matching and behavior; hwdb supplies hardware-specific data to consumers that consult the database. Do not assume that every hardware quirk belongs in hwdb: projects can use their own configuration systems. For example, the libinput device-configuration documentation describes hwdb workflows and notes that libinput has moved its device quirks to a separate system. Follow the documentation for the subsystem whose behavior you need to change.
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Write and test a local udev rule
The following is an illustrative match that sets a property; replace the example IDs and assignment with values and behavior appropriate to the device. It does not change permissions or power behavior:
ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="vvvv", ATTR{idProduct}=="pppp", ENV{EXAMPLE_MATCHED}="1"For a child node, the USB attributes may be on a parent rather than the event device, so inspect the tree and adjust the matching keys accordingly. Add administrator rules in the local configuration directory, commonly /etc/udev/rules.d, using a filename whose sort order is appropriate. Do not edit packaged rules under /usr/lib/udev/rules.d or /lib/udev/rules.d; package updates can replace them. The udev manual documents rule-file syntax, matching operators, substitutions, and assignments; details can vary with the installed systemd version.
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Update and verify a hwdb change
If you edit an hwdb file, rebuild the database and refresh the relevant device so the consumer can see the resulting properties. The libinput documentation gives this example workflow; replace the example input path with the actual affected device path:
sudo systemd-hwdb update
sudo udevadm trigger /sys/class/input/eventX
udevadm info /sys/class/input/eventX
sudo udevadm test /sys/class/input/eventXCommand availability and behavior depend on the systemd version and distribution. udevadm info lets you inspect the properties associated with the device. udevadm test is useful for diagnosing rule matching and parsing, but it does not execute RUN directives, so it cannot simulate every side effect. Consult the installed systemd-hwdb and udevadm manual pages alongside the consumer’s documentation.
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A practical identification workflow
- Locate the target. Determine whether your goal concerns the USB device itself, an interface, or a child node such as a tty, hidraw, or input event device.
- Inspect raw attributes and ancestry. Record the vendor and product IDs, relevant interface attributes, and—only if needed—the serial number. Use
udevadm info --attribute-walkon the relevant device node or inspect its sysfs path. - Inspect the current udev properties. Use
udevadm infoto see derived properties. If a property is missing, determine whether the underlying raw attribute is missing or whether it was not imported or set by a rule. - Choose the mechanism by consumer and outcome. Use a local rule for event matching and udev behavior; use hwdb only when the consumer documents that it reads the relevant hardware property there.
- Apply and refresh. For a rule, follow your system’s rule reload and device-trigger procedure. For hwdb, run
systemd-hwdb updateand trigger the relevant device as appropriate. - Verify the result. Re-check properties with
udevadm info, and useudevadm testto diagnose matching issues while accounting for its limits aroundRUNdirectives.
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