ttyS0

ttyS0 is the device file designation for the first built-in serial port on many Unix-like systems. It represents a hardware RS-232 serial interface exposed by the system’s UART controller (such as legacy 8250/16550 or similar cores) and is accessed as a character device under /dev. The ttyS prefix is used for standard serial terminal devices.

In Linux and other Unix-like environments, /dev/ttyS0 is used for serial communication through the first serial port. Applications interact with it through standard serial interfaces, typically configured with termios calls or utilities such as stty. Common tasks include setting baud rate, data bits, parity, and stop bits, or using programs like minicom, screen, or picocom to communicate with attached devices. The kernel may also use ttyS0 as a system console, specified at boot time with kernel parameters such as console=ttyS0,115200.

Typical uses for ttyS0 include debugging and maintenance of headless systems, interfacing with embedded boards, serial modems, or other equipment that communicates over RS-232. On many modern computers, physical serial ports are less common, and USB-to-serial adapters expose devices named /dev/ttyUSB0, /dev/ttyUSB1, etc., while some platforms may still expose a native /dev/ttyS0. On certain single-board computers, naming and role can vary—e.g., a mini UART versus a PL011 UART—so device behavior can differ by hardware.

Permissions and access are controlled by the system’s access controls; users may need to be in a serial or dialout group, or use sudo to interact with /dev/ttyS0. In Linux, ttyS0 typically has major number 4 and minor number 64.