/dev is the directory that exposes hardware devices and kernel abstractions as files. Reading and writing a file in /dev communicates directly with a device driver. This is the “everything is a file” principle made concrete: a disk, a terminal, a random number generator, and a null sink all appear as regular file paths that ordinary open/read/write/close calls work on.

Device files come in two kinds. Block devices (b in ls -l) transfer data in fixed-size chunks and support seeking; disks and partitions are block devices (/dev/sda, /dev/nvme0n1). Character devices (c) transfer data as a stream without seeking; terminals, serial ports, and input devices are character devices (/dev/tty, /dev/ttyS0, /dev/input/event0). Each device file has a major number (identifies the driver) and a minor number (identifies the specific device instance).

/dev/sda          block device: first SATA/SCSI disk
/dev/sda1         block device: first partition of sda
/dev/nvme0n1      block device: first NVMe disk
/dev/tty          char device:  controlling terminal of the current process
/dev/ttyS0        char device:  first serial port (COM1)
/dev/null         char device:  discards all writes; reads return EOF
/dev/zero         char device:  reads return an infinite stream of zero bytes
/dev/urandom      char device:  reads return cryptographically secure random bytes
/dev/mem          char device:  direct access to physical memory (requires root)

/dev is populated dynamically by udev (or devtmpfs in the kernel). When the kernel detects new hardware, it emits a uevent; udev receives it and creates the corresponding file in /dev with the correct permissions and symlinks. Plug in a USB drive and /dev/sdb appears; unplug it and the file disappears.