hwdb man page
hwdb — Hardware Database
The hardware database is a key-value store for associating modalias-like keys to udev-property-like values. It is used primarily by udev to add the relevant properties to matching devices, but it can also be queried directly.
Hardware Database Files
The hwdb files are read from the files located in the system hwdb directory /usr/lib/udev/hwdb.d and the local administration directory /etc/udev/hwdb.d. All hwdb files are collectively sorted and processed in lexical order, regardless of the directories in which they live. However, files with identical filenames replace each other. Files in /etc have the highest priority and take precedence over files with the same name in /usr/lib. This can be used to override a system-supplied hwdb file with a local file if needed; a symlink in /etc with the same name as a hwdb file in /usr/lib, pointing to /dev/null, disables the hwdb file entirely. hwdb files must have the extension .hwdb; other extensions are ignored.
The hwdb file contains data records consisting of matches and associated key-value pairs. Every record in the hwdb starts with one or more match strings, specifying a shell glob to compare the database lookup string against. Multiple match lines are specified in additional consecutive lines. Every match line is compared individually, and they are combined by OR. Every match line must start at the first character of the line.
The match lines are followed by one or more key-value pair lines, which are recognized by a leading space character. The key name and value are separated by "=". An empty line signifies the end of a record. Lines beginning with "#" are ignored.
The content of all hwdb files is read by systemd-hwdb(8) and compiled to a binary database located at /etc/udev/hwdb.bin, or alternatively /usr/lib/udev/hwdb.bin if you want ship the compiled database in an immutable image. During runtime, only the binary database is used.
systemd.directives(7), systemd-hwdb(8), systemd.index(7).