Udev rule for USB storage device and partitions

Question

I am trying to set up a udev rule which symlinks /dev/sda to /dev/cm, /dev/sda1 to /dev/cm1 and /dev/sda2 to /dev/cm2. Of course, depending on what else is connected to the computer, instead of sda, the device could turn up as sdb or anything else. So I have this rule:

SUBSYSTEMS=="usb", ATTRS{idVendor}=="0a5c", ATTRS{idProduct}=="0001", GROUP="disk", SYMLINK+="cm"

This rule matches my device and yields the /dev/cm symlink to the correct device node, but not /dev/cm1 or /dev/cm2, even though I can see that /dev/sda1 and /dev/sda2 have turned up. What am I missing?

Answer 1

Here is one solution:

SUBSYSTEMS=="usb", ATTRS{idVendor}=="0a5c", ATTRS{idProduct}=="0001", GROUP="disk", SYMLINK+="cm%n"

See? the symlink name is not cm, but cm%n. Apparently, the %n is the name of the partition. For me, this rule yields the following symlinks:

• /dev/cm
• /dev/cm0 (this one points to /dev/sg0, which is apparently the generic SCSI layer)
• /dev/cm1 (points to the first partition)
• /dev/cm2 (points to the second partition)

https://wiki.debian.org/udev is where to go to learn some of this.