utility writes the name of the terminal attached to standard input
to standard output.
The name that is written is the string returned by
If the standard input is not a terminal, the message
not a tty
The options are as follows:
Don't write the terminal name; only the exit status is affected
when this option is specified.
option is deprecated in favor of the
"test -t 0"
utility exits 0 if the standard input is a terminal, 1 if the standard input
is not a terminal, or >1 if an error occurred.