Linux – Find Out if a Specific Device is a USB Mass Storage

block-devicedeviceslinuxremovable-storageusb-drive

Context

I'm automating SD card imaging from an existing dd factory image. The SD card is always connected through an external USB card reader and thus appears in the system as a SCSI block device /dev/sd*.

Currently the syntax of my command is: write-image DEVICE where DEVICE is the SD card block device, eg. /dev/sdd.

Problem

I'm already doing a basic check on DEVICE to verify it is of the form /dev/sd* but this is not enough: I fear the users (production people not used to Linux) make a mistake and specify another seemingly valid device, eg. /dev/sda. You can see the looming catastrophe, especially since my imaging script needs root privileges (not to write the image itself, mind you, but to modify the SD card afterwards, including adjusting a partition's size depending on the SD card's real size)…

Question

I would like to verify that the specified device actually is some USB mass storage (or at the very least a removable device) so that I can protect the system disks from being trashed accidentally. How can I do that?

I found nothing relevant in /proc or on the web, I'm quite at loss now.

Best Answer

Have a look under the /sys/ directory. In particular, /sys/block/ contains symlinks to block devices in /sys/devices/.

/sys/block/sdX/removable looks like it will read as 1 for a removable device, and 0 otherwise. This gives you a basic check for removability.

I'm not sure if there's a better way to check if it's a USB device, but readlink /sys/block/sde will spit out something like ../devices/pci0000:00/0000:00:1d.0/usb6/6-1/6-1.2/6-1.2:1.0/host7/target7:0:0/7:0:0:0/block/sde. Checking if that contains a usb* folder might work as a simple check.

You can get other device details like vendor and model from /sys/block/sdX/device/, which might also come in handy.

Related Solutions

How to make sure that certain devices always use the same device file

I noticed that the devpath attribute is constant for my USB ports. You could add a new udev rule and create specific symlinks based on the ports the devices are plugged in then. Just add a new file into your /etc/udev/rules.d directory, that looks like this:

#new symlink for my front USB port:
KERNELS=="2-1.8", SUBSYSTEMS=="usb", ATTRS{devpath}=="1.8", SYMLINK+="usb_port1"

and make sure it's name ends in .rules, e.g. port1.rules. From now on you can make the fstab entry using /dev/usb_port1 as any device plugged into that specific port will be listed under said symlink in /dev/ . Same for the SD card and you should be settled.

What is happening is that we check for unique properties of the specific port ( the == parts), and once these match add (+=) a new symlink to /dev/.

The information on attributes you want to match canbe retrieved via udevadm info -a /dev/sdb1 (for a device sdb1). Make sure you take attributes from only one parent device block, and that should be the one that have unique devpath and KERNELS entries - try with a few USB ports if available to see which ones change.

PS: udev will also allow to run a script when plugging in a device - might come in handy for your backup needs. However I do not know if booting your system will count as plugging in, so you'll have to check the behaviour.