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How to Write udev Rules for USB Devices

Udev-tux If you use a custom USB device for which there isn't a suitable udev rule installed on your Linux system, you might notice that only the root user has read and write access for it. In order to change that you need to write a new udev rule. Here is an easy way how to do this.

First we need to collect some information about that USB device. We get some basic information from lsusb. Its output should look something like this:

Bus 001 Device 003: ID 1111:2222 FooDev

Now we know the path of this device in the Linux filesystem:

/dev/bus/usb/001/003

We can use this path now to ask udev about some detail information:

udevadm info -a -p $(udevadm info -q path -n /dev/bus/usb/001/003)

Maybe you have to use udevinfo instead of udevadm info. The output should look similar to this:

looking at device '/devices/pci0000:00/0000:00:06.0/usb1/1-2':
    KERNEL=="1-2"
    SUBSYSTEM=="usb"
    DRIVER=="usb"
    ATTR{bDeviceSubClass}=="00"
    ATTR{bDeviceProtocol}=="00"
    ATTR{devpath}=="2"
    ATTR{idVendor}=="1111"
    ATTR{speed}=="12"
    ATTR{bNumInterfaces}==" 1"
    ATTR{bConfigurationValue}=="1"
    ATTR{bMaxPacketSize0}=="64"
    ATTR{busnum}=="1"
    ATTR{devnum}=="3"
    ATTR{configuration}==""
    ATTR{bMaxPower}=="500mA"
    ATTR{authorized}=="1"
    ATTR{bmAttributes}=="80"
    ATTR{bNumConfigurations}=="1"
    ATTR{maxchild}=="0"
    ATTR{bcdDevice}=="0106"
    ATTR{avoid_reset_quirk}=="0"
    ATTR{quirks}=="0x0"
    ATTR{serial}==" 10T7371 9028039"
    ATTR{version}==" 1.10"
    ATTR{urbnum}=="10"
    ATTR{ltm_capable}=="no"
    ATTR{manufacturer}=="Foo Inc"
    ATTR{removable}=="unknown"
    ATTR{idProduct}=="2222"
    ATTR{bDeviceClass}=="00"
    ATTR{product}=="Some FooDev"

With this information we can finally start to write the new udev rule. To do this create a new file:

sudo vim /etc/udev/rules.d/10-local.rules

If for instance we would like to give everyone in the group "users" read and write access to the device, we would put following line in that file:

SUBSYSTEMS=="usb", ATTRS{product}=="Some FooDev", GROUP="users"

Please note that it's ATTRS in the rules file, not ATTR.

Now we can test the new rule. For this we need the /devices/... path from the udevadm info output from before.

udevadm test /devices/pci0000:00/0000:00:06.0/usb1/1-2

Like before, you might have to use udevtest instead of udevadm test. The output should also contain information about the permissions for this device:

[...] handling device node '/dev/bus/usb/001/003', devnum=c189:2, mode=0664, uid=0, gid=100 set permissions /dev/bus/usb/001/003, 020664, uid=0, gid=100 [...]

gid=100 is the group id of the "users" group. So this is the proof that it works.

If your rule was not applied, it might be because it was overridden by another rule. In that case use the ":=" operator instead of "=". This tells udev to not override this property.

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