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udev Rules Tricks and More

Persitant Name

Very often this is the reason to write a rule. Imagine you have USB-2-serial adapters that you don't want the name to change. You plug it in and in /dev you find

$ ll /dev
crw-rw---- 1 root nut     188,   0 2017-05-18 21:21 ttyUSB0

So first we run this command

$ udevadm info -a -n /dev/ttyUSB0

Mixed in with the output is

  looking at parent device '/devices/pci0000:00/0000:00:14.0/usb1/1-8':
    KERNELS=="1-8"
    SUBSYSTEMS=="usb"
    DRIVERS=="usb"
    ATTRS{authorized}=="1"
    ATTRS{avoid_reset_quirk}=="0"
    ATTRS{bConfigurationValue}=="1"
    ATTRS{bDeviceClass}=="00"
    ATTRS{bDeviceProtocol}=="00"
    ATTRS{bDeviceSubClass}=="00"
    ATTRS{bMaxPacketSize0}=="8"
    ATTRS{bMaxPower}=="90mA"
    ATTRS{bNumConfigurations}=="1"
    ATTRS{bNumInterfaces}==" 1"
    ATTRS{bcdDevice}=="0600"
    ATTRS{bmAttributes}=="a0"
    ATTRS{busnum}=="1"
    ATTRS{configuration}==""
    ATTRS{devnum}=="17"
    ATTRS{devpath}=="8"
    ATTRS{idProduct}=="6001"
    ATTRS{idVendor}=="0403"
    ATTRS{ltm_capable}=="no"
    ATTRS{manufacturer}=="FTDI"
    ATTRS{maxchild}=="0"
    ATTRS{product}=="FT232R USB UART"
    ATTRS{quirks}=="0x0"
    ATTRS{removable}=="removable"
    ATTRS{serial}=="AJV9MKOY"
    ATTRS{speed}=="12"
    ATTRS{urbnum}=="87835"
    ATTRS{version}==" 2.00"

Most of it we don't need, but if we grab:

ATTRS{idVendor}=="0403"
ATTRS{idProduct}=="6001"
ATTRS{serial}=="AJV9MKOY"

We have everything needed to make our own custom udev rule. The last fields - SYMLINK+ creates our persistant name - and while we are here we set the group.

ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6001", ATTRS{serial}=="AJV9MKOY", SYMLINK+="ttyUSB-nut",GROUP = "nut"
#this device is refered to by /etc/nut/ups.conf

We save it in /etc/udev/rules.d/52_nut-serialups.rules ( or something similar in name - the number matters - controls the order they are run ). A gentel nudge to reload may be needed..

$ udevadm control --reload

So now we get:

$ ll /dev |grep nut
crw-rw---- 1 root nut     188,   0 2017-05-18 21:38 ttyUSB0
lrwxrwxrwx 1 root root           7 2017-05-18 21:04 ttyUSB-nut -> ttyUSB0

Correct group and a persistant symlink.

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