3.2 The sysfs Device Path

The traditional Unix /dev directory is a convenient way for user processes to reference and interface with devices supported by the kernel, but it’s also a very simplistic scheme. The name of the device in /dev tells you a little about the device, but usually not enough to be helpful. Another problem is that the kernel assigns devices in the order in which they are found, so a device may have a different name between reboots.

To provide a uniform view for attached devices based on their actual hardware attributes, the Linux kernel offers the sysfs interface through a system of files and directories. The base path for devices is /sys/devices. For example, the SATA hard disk at /dev/sda might have the following path in sysfs:

/sys/devices/pci0000:00/0000:00:17.0/ata3/host0/target0:0:0/0:0:0:0/block/sda

p. 50

As you can see, this path is quite long compared with the /dev/sda filename, which is also a directory. But you can’t really compare the two paths because they have different purposes. The /dev file enables user processes to use the device, whereas the /sys/devices path is used to view information and manage the device. If you list the contents of a device path such as the preceding one, you’ll see something like the following:

alignment_offset  discard_alignment  holders   removable  size       uevent
bdi               events             inflight  ro         slaves
capability        events_async       power     sda1       stat
dev               events_poll_msecs  queue     sda2       subsystem
device            ext_range          range     sda5       trace

The files and subdirectories here are meant to be read primarily by programs rather than humans, but you can get an idea of what they contain and represent by looking at an example such as the /dev file. Running cat dev in this directory displays the numbers 8:0, which happen to be the major and minor device numbers of /dev/sda.

There are a few shortcuts in the /sys directory. For example, /sys/block should contain all of the block devices available on a system. However, those are just symbolic links; you’d run ls -l /sys/block to reveal the true sysfs paths.

It can be difficult to find the sysfs location of a device in /dev. Use the udevadm command as follows to show the path and several other interesting attributes:

$ udevadm info --query=all --name=/dev/sda

You’ll find more details about udevadm and the entire udev system in Section 3.5.