Control characters may be described as doing something when the user inputs them, such as code 3 (End-of-Text character, ETX, ^C) to interrupt the running process, or code 4 (End-of-Transmission character, EOT, ^D), used to end text input or to exit a Unix shell. These uses usually have little to do with their use when they are in text being output, and on modern systems usually do not involve the transmission of the code number at all (instead the program gets the fact that the user is holding down the Ctrl key and pushing the key marked with a 'C').