Frequently Asked Question

What are the files in /dev and what's the difference between block and character devices?

/dev holds device files: special files that the kernel uses as handles for hardware and pseudo-hardware. Reading /dev/sda gives you the raw bytes of the first SATA disk; writing to /dev/null discards them; reading /dev/urandom gives you a stream of cryptographic-quality random bytes. The ls -l listing shows the type as the first character: b for block devices and c for character devices. After the permissions come two numbers, like 8, 0, which are the major and minor device numbers, the kernel uses them to look up which driver handles the device.

Block devices (disks, SSDs, USB sticks, loop devices, RAID arrays) move fixed-size blocks at a time, can seek arbitrarily, and can be cached by the kernel's page cache. Character devices (terminals, serial ports, audio, /dev/null, /dev/random) are byte streams. On modern systems /dev is itself a devtmpfs automatically populated by the kernel as devices are discovered, with udev applying naming rules and permissions on top.