#include <unistd.h> int isatty(int fildes);
The isatty() function tests whether fildes, an open file descriptor, is associated with a terminal device.
The isatty() function returns 1 if fildes is associated with a terminal; otherwise it returns 0 and may set errno to indicate the error.
The isatty() function may fail if:
The fildes argument is not a valid open file descriptor.
The fildes argument is not associated with a terminal.
The isatty() function does not necessarily indicate that a human being is available for interaction via fildes. It is quite possible that non-terminal devices are connected to the communications line.
See attributes(5) for descriptions of the following attributes: