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    Oct 2005

    Create your own udev rules to control removable devices

    I'm sure many people who use removable devices have noticed that sometimes they don't appear where they were before.

    You plug your USB drive in, and use fdisk to find the device node and then mount it. Sometimes the USB appears as /dev/sda1, sometimes /dev/sdb1. It can depend on what order you plug in your USB devices, and where you plug them in. This is a real pain if you mount devices manually or if you are trying to customise your /etc/fstab.

    udev allows the assignment of a persistant device node, /dev/..., based on a rule match defined by your specific hardware. In other words, if a device is attached that matches certain criteria it is given it's own device node, rather than being assigned a dynamic one.

    It's actually really easy to setup.


    To start with you need to know the dynamic device node that is given to a device when attached for the first time. The way that I would do this is to use the tail command
    tail -f /var/log/messages
    When a device is plugged in, the screen will update with the newest messages, usually imparting some knowledge on the location of the new device. My USB flash disc for example, produced this output when plugged in.
    Apr 30 16:37:01 localhost kernel: [4294885.683000] usb 1-3: new full speed USB device using ohci_hcd and address 6
    Apr 30 16:37:01 localhost kernel: [4294885.853000] scsi5 : SCSI emulation for USB Mass Storage devices
    Apr 30 16:37:02 localhost usb.agent[10421]: usb-storage: already loaded
    Apr 30 16:37:06 localhost kernel: [4294890.859000] Vendor: Model: TS128MJFLASHA Rev: 1.00
    Apr 30 16:37:06 localhost kernel: [4294890.859000] Type: Direct-Access ANSI SCSI revision: 02
    Apr 30 16:37:06 localhost kernel: [4294890.883000] SCSI device sdd: 253400 512-byte hdwr sectors (130 MB)
    Apr 30 16:37:06 localhost kernel: [4294890.896000] sdd: Write Protect is off
    Apr 30 16:37:06 localhost kernel: [4294890.924000] SCSI device sdd: 253400 512-byte hdwr sectors (130 MB)
    Apr 30 16:37:06 localhost kernel: [4294890.937000] sdd: Write Protect is off
    Apr 30 16:37:07 localhost kernel: [4294890.937000] /dev/scsi/host5/bus0/target0/lun0: p1
    Apr 30 16:37:07 localhost kernel: [4294891.046000] Attached scsi removable disk sdd at scsi5, channel 0, id 0, lun 0
    Apr 30 16:37:07 localhost scsi.agent[10469]: sd_mod: loaded sucessfully (for disk)
    The output shows that the USB device attached is assigned the device node /dev/sdd. You may also want to determine the partition number using
    sudo fdisk -l
    For my USB flash disc, the partition is /dev/sdd1.

    So next thing is to find out some unique information from the device, information that will be used in defining the udev rule, remembering a match is required to assign the persistant node. The next command I have used is from the Writing udev rules link at the bottom of this HOWTO
    udevinfo -a -p $(udevinfo -q path -n /dev/sdd)
    This command outputs a lot information about the hardware associated with /dev/sdd. I've left out a lot of the information, leaving only the section I think is most relevant. This section contains the more specific information about my USB flash disc.

    Note the bolded text in the output. It is important that information used in a udev rule is contained in the one section.
    udevinfo starts with the device the node belongs to and then walks up the
    device chain, to print for every device found, all possibly useful attributes
    in the udev key format.
    Only attributes within one device section may be used together in one rule,
    to match the device for which the node will be created.
      looking at the device chain at '/sys/devices/pci0000:00/0000:00:02.0/usb1/1-3':
        SYSFS{bNumInterfaces}==" 1"
        SYSFS{version}==" 1.10"
    The items that contain specific information regarding my USB disc have been coloured red. These are the two items of interest to me, the device is connected to the USB bus, and the product is identified by TS128MJFLASHA


    The next step is to create the udev rule concerning this device. I'll start by creating my own .rules file
    sudo nano /etc/udev/rules.d/10-local.rules
    By naming my set of rules 10-local.rules they should be looked at before the other rules set in this folder.

    The rule I will use for the flash disc looks like this.
    BUS=="usb", SYSFS{product}=="TS128MJFLASHA", KERNEL=="sd?1", NAME="transcend128mb", SYMLINK="usbdevices/transcend128mb"
    A quick explanation.

    - The BUS==”usb” and SYSFS{product}==”TS128MJFLASHA” options are the same as those I picked out from the udevinfo output.

    - The option KERNEL="sd?1" will only match locations like /dev/sda1, /dev/sdb1 and more importantly, it won't match nodes like /dev/sda, /dev/sdb, which can be fdisk'ed. The 'Writing udev rules' guide also mentions this.

    - The options NAME="128FLASH" and SYMLINK="usbdisc/128FLASH" will create the persistant node at /dev/transcend128mb and a symlink /dev/usbdisc/transcend128mb that points to the persistant node /dev/transcend128mb. The SYMLINK option is not required. The reason I have included it is so that all my USB devices will have a symlink starting with /dev/usbdevices/...
    I just think its neater.

    There are other options that could be used to create udev rules, such as GROUP=”some_group”, if you want to assigned the group ownership of the device node to a specific group, and MODE=”0660”, which would give the owner/group read and write permissions, like chmod.

    The Writing udev rules guide contains some more detailed information on these other options.


    To start using these new rules, you need to run the command udevstart
    sudo udevstart
    You can also restart udev using
    sudo /etc/init.d/udev restart
    This should work for later versions of udev, that don't appear to use the udevstart command

    - thanks to ash211 for pointing this out

    Now to quickly check that the new node for my example has been created.
    user@ubuntu:~$ ls -l /dev/trans*
    brw-r-----  1 root plugdev 8, 49 2006-04-30 16:37 /dev/transcend128mb
    user@ubuntu:~$ ls -l /dev/usbdevices/
    lrwxrwxrwx  1 root root 17 2006-04-30 16:37 transcend128mb -> ../transcend128mb

    Finally the fstab can be edited to include the new persistant device node, but first we'll back it up
    sudo cp /etc/fstab /etc/fstab_backup
    sudo nano /etc/fstab
    Then we can add an entry for the example USB device using either the device node or the symlink (if used), so in my example I could either use the new device node
    /dev/transcend128mb  /media/usb128mb  vfat  iocharset=utf8,umask=000   0   0
    or the symlink to the node, which I prefer as all my USB devices have symlinks in /dev/usbdevices. I think makes the fstab look neater.
    /dev/usbdevices/transcend128mb  /media/usb128mb vfat iocharset=utf8,umask=000  0  0
    When the entry has been correctly entered, you can save the file (Ctrl+O) and exit (Ctrl+X), and then mount the device, in this example my USB disc using
    sudo mount /media/usb128mb
    sudo mount -a
    Once this is all completed, the example USB drive will always appear at /dev/transcend128mb so the entry in the fstab can remain unchanged and will always find the device.

    And you're all done!

    Hope that helps some people, like it did me.

    Please let me know if this works for you, and of course if there are any typos, errors or things that need clarifying.

    The useful links I needed to get this working - udev

    Writing udev rules
    Last edited by Sutekh; May 27th, 2006 at 02:02 PM.
    I bring Sutekh's gift of [Ubuntu] to all human life


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