Difference between /dev and /sys/class?

Question

What is the difference between the device representation in /dev and the one in /sys/class?

Is one preferred over the other? Is there something one offers and the other doesn't?

Answer 1

The files in /dev are actual devices files which UDEV creates at run time. The directory /sys/class is exported by the kernel at run time, exposing the hierarchy of the hardware through sysfs.

From the libudev and Sysfs Tutorial

excerpt

On Unix and Unix-like systems, hardware devices are accessed through special files (also called device files or nodes) located in the /dev directory. These files are read from and written to just like normal files, but instead of writing and reading data on a disk, they communicate directly with a kernel driver which then communicates with the hardware. There are many online resources describing /dev files in more detail. Traditonally, these special files were created at install time by the distribution, using the mknod command. In recent years, Linux systems began using udev to manage these /dev files at runtime. For example, udev will create nodes when devices are detected and delete them when devices are removed (including hotplug devices at runtime). This way, the /dev directory contains (for the most part) only entries for devices which actually exist on the system at the current time, as opposed to devices which could exist.

another excerpt

The directories in Sysfs contain the heirarchy of devices, as they are attached to the computer. For example, on my computer, the hidraw0 device is located under:

/sys/devices/pci0000:00/0000:00:12.2/usb1/1-5/1-5.4/1-5.4:1.0/0003:04D8:003F.0001/hidraw/hidraw0

Based on the path, the device is attached to (roughly, starting from the end) configuration 1 (:1.0) of the device attached to port number 4 of device 1-5, connected to USB controller 1 (usb1), connected to the PCI bus. While interesting, this directory path doesn't do us very much good, since it's dependent on how the hardware is physically connected to the computer.

Fortunately, Sysfs also provides a large number of symlinks, for easy access to devices without having to know which PCI and USB ports they are connected to. In /sys/class there is a directory for each different class of device.

Usage?

In general you use rules in /etc/udev/rules.d to augment your system. Rules can be constructed to run scripts when various hardware is present.

Once a system is up you can write scripts to work against either /dev or /sys, and it really comes down to personal preferences, but I would usually try and work against /sys and make use of tools such as udevadm to query UDEV for locations of various system resources.

$ udevadm info -a -p  $(udevadm info -q path -n /dev/sda) | head -15

Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

  looking at device '/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/block/sda':
    KERNEL=="sda"
    SUBSYSTEM=="block"
    DRIVER==""
    ATTR{ro}=="0"
    ATTR{size}=="976773168"
    ATTR{stat}==" 6951659  2950164 183733008 41904530 16928577 18806302 597365181 580435555        0 138442293 622621324"
    ATTR{range}=="16"
...