tty — controlling terminal interface
is, in each process, a synonym for the control terminal
associated with the process group of that process, if any.
It is useful for programs or shell sequences that need to be sure
of writing messages on the terminal no matter how output has been redirected.
It can also be used for programs that demand
the name of a file for output, when typed output is desired,
and it is tiresome to find out what terminal
is currently in use.
tty: AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1