ASCII 4 — EOT

End of Transmission (EOT) signals the completion of all data in a communication session. Generated by Ctrl+D, it serves as the end-of-file indicator in Unix and Linux terminals — typing Ctrl+D on an empty line closes standard input and typically exits the current shell. This EOF behavior is essential for piping data between programs and for interactive scripts that read from stdin. In the Hayes modem command set, EOT also played a role in session termination and management.

Control characters were defined in the original 1963 ASCII standard to manage telecommunications equipment and terminal devices. Unlike printable characters representing visible symbols, control codes perform actions: initiating transmissions, acknowledging received data, triggering device alerts, and structuring information hierarchically. Of ASCII's 128 code points, 33 are designated as control characters (codes 0–31 plus 127), reflecting the standard's deep roots in telegraphy and serial communication systems. While most control codes have fallen out of daily use, several remain essential to modern computing workflows.

In the ASCII encoding table, End of Transmission is assigned code point 4 in decimal (0x04 hexadecimal, 004 octal, 00000100 binary). The 7-bit ASCII standard, first published in 1963 by the American Standards Association, defines exactly 128 characters that remain the foundation of text encoding systems worldwide. UTF-8, the dominant encoding on the modern web, is fully backward compatible with ASCII — every ASCII character is encoded as the identical single byte in UTF-8, guaranteeing that End of Transmission works reliably across all operating systems, programming languages, and internet protocols.