udev is a device manager for the linux kernel.
when
you connect some device (pendrive, camera, microphone) on a linux
machine, you have a daemon listen to events that are fired when the
kernel register a new device. All devices are mapped to a file on /dev so you could check some info about the device with this command:
udevadm info -a -n /dev/DEVICE_NAME
i.e.: udevadm info -a -n /dev/ttyUSB2
udevadm info -a -n /dev/ttyUSB2 | grep '{devpath}' | head -n 1
udevadm info -a -n /dev/ttyUSB2 | grep '{idVendor}' | head -n 1
udevadm info -a -n /dev/ttyUSB2 | grep '{idProduct}' | head -n 1
udevadm info -a -n /dev/ttyUSB2 | grep '{serial}' | head -n 1
knowing
the device info we could make a rule based in the information and
execute a new action: make a file, make a symlink or execute a script.
For example I want to create a rule that match with the attributes
devpath, idVendor, idProduct, serial and generate a new symlink.
we need to create a file into:
/etc/udev/rules.d/99-some-rule.rules
- in this file we need to use "" in the values.
- each line is a rule.
- each line has many or one attribute and one action. When the attributes matched the action is executed, that's all.
- if you edit or create a new rule, you need to load and trigger all rules again.
udevadm control --reload-rules && udevadm trigger
- so sometimes is tricky check why some rule is not executed. For debug you have:
- udevadm test /devices/platform/soc/3f980000.usb/usb1/1-1/1-1.3/1-1.3:1.0/ttyUSB0/tty/ttyUSB0
the devices path you could obtain with the comand udevadm info.
- udevadm monitor
show the events that are fired.
- udevadm control --log-priority=debug
set the environment on debug mode.
- udevadm test $(udevadm info --query=path --name=ttyUSB0)
test all rules associated to the device name ttyUSB0
- when you have an syntax error on some rules file, this will appear in the logs:
- journalctl -f
- restart service:
- systemctl restart systemd-udevd
- kernel messages
- dmesg
- example: file called 99-new-rule.rules:
SUBSYSTEM=="tty",
ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523",
ATTRS{devpath}=="1.2", ATTRS{bcdDevice}=="0264", SYMLINK+="FANCY_NAME"
The 99 number indicated the priority for execution. in this case would be the last one to execute.
It's important to put double quotes instead single quote.
