tty - return user's terminal name
The tty(1) utility writes to standard output the name of the terminal attached to standard input. The name is the string returned by ttyname(3). If the standard input is not a terminal, the message not a tty is written. The options are as follows:
The -s option is deprecated in favor of:
test -t 0
The tty(1) utility exits 0 if the standard input is a terminal, 1 if the standard input is not a terminal, and >1 if an error occurs.