Indicates if a specified file descriptor is associated with a
    terminal.

    Format

      #include  <unistd.h>

      int isatty  (int file_desc);

1 - Argument

 file_desc

    A file descriptor.

2 - Return Values

    1                  If the file descriptor is associated with a
                       terminal.
    0                  If the file descriptor is not associated with
                       a terminal.
    -1                 Indicates an error (for example, if the file
                       descriptor is not associated with an open
                       file).