hwdb - Man Page

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​.

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 lexicographic 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 that hwdb file entirely​. hwdb files must have the extension ​.hwdb; other extensions are ignored​.

Each 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 lookup string against​. Multiple match lines are specified in 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​.

Match patterns consist of literal characters, and shell-style wildcards:

• Asterisk "*" matches any number of characters
• Question mark "?" matches a single character
• Character list "[chars]" matches one of the characters chars listed between "[" and "]"​. A range may be specified as with a dash as "[first-last]"​. The match may be inverted with a caret "[^​...]"​.

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​.

In case multiple records match a given lookup string, the key-value pairs from all records are combined​. If a key is specified multiple times, the value from the record with the highest priority is used (each key can have only a single value)​. The priority is higher when the record is in a file that sorts later lexicographically, and in case of records in the same file, later records have higher priority​.

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​.

Example ​1. ​General syntax of hwdb files

# /usr/lib/udev/hwdb​.d/example​.hwdb
# Comments can be placed before any records​. This is a good spot
# to describe what that file is used for, what kind of properties
# it defines, and the ordering convention​.

# A record with three matches and one property
mouse:*:name:*Trackball*:*
mouse:*:name:*trackball*:*
mouse:*:name:*TrackBall*:*
 ID_INPUT_TRACKBALL=1

# The rule above could be also be written in a form that
# matches Tb, tb, TB, tB:
mouse:*:name:*[tT]rack[bB]all*:*
 ID_INPUT_TRACKBALL=1

# A record with a single match and five properties
mouse:usb:v046dp4041:name:Logitech MX Master:*
 MOUSE_DPI=1000@166
 MOUSE_WHEEL_CLICK_ANGLE=15
 MOUSE_WHEEL_CLICK_ANGLE_HORIZONTAL=26
 MOUSE_WHEEL_CLICK_COUNT=24
 MOUSE_WHEEL_CLICK_COUNT_HORIZONTAL=14

Example ​2. ​Overriding of properties

# /usr/lib/udev/hwdb​.d/60-keyboard​.hwdb
evdev:atkbd:dmi:bvn*:bvr*:bd*:svnAcer*:pn*:*
 KEYBOARD_KEY_a1=help
 KEYBOARD_KEY_a2=setup
 KEYBOARD_KEY_a3=battery

# Match vendor name "Acer" and any product name starting with "X123"
evdev:atkbd:dmi:bvn*:bvr*:bd*:svnAcer:pnX123*:*
 KEYBOARD_KEY_a2=wlan

# /etc/udev/hwdb​.d/70-keyboard​.hwdb
# disable wlan key on all at keyboards
evdev:atkbd:*
 KEYBOARD_KEY_a2=reserved
 PROPERTY_WITH_SPACES=some string

If the hwdb consists of those two files, a keyboard with the lookup string "evdev:atkbd:dmi:bvnAcer:bvr:bdXXXXX:bd08/05/2010:svnAcer:pnX123:" will match all three records, and end up with the following properties:

KEYBOARD_KEY_a1=help
KEYBOARD_KEY_a2=reserved
KEYBOARD_KEY_a3=battery
PROPERTY_WITH_SPACES=some string

systemd-hwdb(8)

gmobile.udev(5), machine-info(5), sd-hwdb(3), sd_hwdb_get(3), systemd.directives(7), systemd-hwdb(8), systemd-imdsd@.service(8), systemd-imds-generator(8), systemd.index(7).