tty —
return user's terminal name
The tty utility writes the name of the terminal attached
  to standard input to standard output. The name that is written 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:
  - -s
- Don't write the terminal name; only the exit status is affected when this
      option is specified. The -soption is deprecated
      in favor of the “test -t 0”
    command.
tty exits 0 if the standard input is a
    terminal, 1 if the standard input is not a terminal, and >1 if an error
    occurs.
The tty utility conforms to IEEE Std
  1003.2-1992 (“POSIX.2”).
A tty utility appeared in
  Version 1 AT&T UNIX.