Udev
Jump to navigation
Jump to search
Reference
- udev man page (
man udev)
udev rules
- Rules are contained in files located in /etc/udev/rules. Files must have .rules extensions, and are processed in lexical order. Files usually starts with 2 figures (like 10-local.rules)
Examples
This basic rule simply changes group of all usb hub devices to plugdev.
# Basic rule - set group 'plugdev' for all USB HUB
#SUBSYSTEMS=="usb", ATTR{bDeviceClass}=="09", MODE="660", GROUP="plugdev"
This rule is more advanced. Whenever a specific usb device (here, an usb serial device) is inserted, it changes the group of the hub device the first device is attached to.
# More advanced rule, only set 'plugdev' grp for hub that have an FTDI board connected to them
# SUBSYSTEM=="tty", DRIVERS=="ftdi_sio", RUN+="/etc/udev/rules.d/plugdev_hub.sh"
#
# Even better, this rules also detects board removal, which allow to reset grp to 'root'
SUBSYSTEM=="tty", KERNEL=="ttyUSB*", RUN+="/etc/udev/rules.d/plugdev_hub.sh"
File plugdev_hub.sh:
#! /bin/bash
# Set group to 'plugdev' for given hub device
function setgrp()
{
DEVNAME=/dev/$(udevadm info -q name -p $1)
chgrp plugdev $DEVNAME
}
# Reset group to 'root' if no ttyUSB device found for given hub
function resetgrp()
{
TTYDEVS=$(find $1 -lname "*/ttyUSB*")
if [ -z "$TTYDEVS" ]; then
DEVNAME=/dev/$(udevadm info -q name -p $1)
chgrp root $DEVNAME
fi
}
if [ "$ACTION" = "add" -o "$ACTION" = "remove" ]; then
F=/sys$DEVPATH
while [ "$F" != "/sys" ] ; do
if [ -a "$F/bDeviceClass" ] ; then
if [ "$(cat $F/bDeviceClass)" == "09" ] ; then
if [ "$ACTION" = "add" ]; then
setgrp $F # add
else
resetgrp $F # remove
fi
break
fi
fi
F=$(dirname $F)
done
fi
Detecting device removal
Note that many attributes are no longer available during udev rules processing when a device is removed. To know which variables are still available in the environment, try
udevadm monitor --property
Troubleshooting
udevadm monitor
udevadm monitor --environment --udev # To also get the env var. before each udev actions
chmod 644 99-myrule.rules
chown root:root 99-myrule.rules
udevadm test -a add $(udevadm info -q path -n /dev/ttyACM0)
- Remove not executing
- See [1]. Maybe are using an attribute that no longer exists.
- More troubleshooting
- https://stackoverflow.com/questions/67123997/how-to-find-reasons-why-an-udev-rule-is-not-applied