tty writes the name of the terminal that is connected to standard input onto standard output. If the standard input is not a
terminal, "not a tty" will be written to standard output.
Write the synchronous line number of the terminal on a separate line following the terminal
name line. If the standard input is not a synchronous terminal then "not on an active synchronous line" is written.
-s, --silent|quiet
Disable the terminal name line. Use [[ -t 0 ]] instead.