ls -la /dev/tty shows the output:
crw-rw-rw- 1 root tty 5, 0 Dec 14 22:21 /dev/tty
The ‘c‘ means it‘s a character device. tty is a special file representing the ‘controlling terminal‘ for the current process.
Character Devices
Unix supports ‘device files‘, which aren‘t really files at all, but file-like access points to hardware devices. A ‘character‘ device is one which is interfaced byte-by-byte (as opposed to buffered IO).
TTY
/dev/tty is a special file, representing the terminal for the current process. So, when you echo 1 > /dev/tty, your message (‘1‘) will appear on your screen. Likewise, when you cat /dev/tty, your subsequent input gets duplicated (until you press Ctrl-C).
/dev/tty doesn‘t ‘contain‘ anything as such, but you can read from it and write to it (for what it‘s worth). I can‘t think of a good use for it, but there are similar files which are very useful for simple IO operations (e.g. /dev/ttyS0 is normally your serial port)
original from: http://stackoverflow.com/questions/8514735/what-is-special-about-dev-tty