isatty

The isatty function is a standard C library function, also available in many other programming languages, used to determine if a file descriptor is connected to a terminal (TTY) device. A terminal is an interactive input/output interface, typically a command-line shell. When a program's standard input, standard output, or standard error is connected to a terminal, isatty will return true (a non-zero value). If the file descriptor is connected to a regular file, a pipe, or a socket, isatty will return false (zero).

This function is particularly useful for programs that behave differently depending on whether they are being run interactively by a user or as part of a script or pipeline. For example, a program might provide more detailed, colorful output when connected to a terminal but produce simpler, machine-readable output when its output is redirected to a file. Similarly, a program might prompt for user input only when its standard input is a terminal.

The typical use case involves checking the return value of isatty before performing certain actions. If isatty returns true, the program might enable interactive features. If it returns false, the program will proceed with non-interactive behavior. The function takes a single integer argument, which is the file descriptor to be checked. Common file descriptors are 0 for standard input, 1 for standard output, and 2 for standard error.