The ttys are not just input/output devices. They also do a
special job of acting as the controlling terminal for a session,
like sending signals (Ctrl+C). /dev/ttyNN are virtual consoles,
which are full screen displays on the monitor.
The terminals start from /dev/tty1. You could switch to these
consoles, usually, by pressing Ctrl+Alt+Fn keys.
e.g, Ctrl+Alt+F1 takes you to the first virtual terminal.
Nowadays, most of the Linux distributions run the X server from
the tty1. So, pressing Ctrl+Alt+F1 may not have an effect.
$ ps ax | grep Xorg | grep -v grep
1504 tty1 Ss+ 44:57 /usr/bin/Xorg :0 -background none -verbose -auth /var/run/gdm/auth-for-gdm-rfsWyA/database -nolisten tcp vt1
Ctrl+Alt+F2 will take you to the second terminal. Usually the
distributions run a login program(agetty) on the virtual
$ ps ax | grep tty2 | grep -v grep
31865 tty2 Ss+ 0:00 /sbin/agetty tty2 38400
The login programs provide you a login prompt and lets you login
with username/password. The init scripts decides, where all the
login program will be run. So depending on that you may or may
not see a login prompt on, say tty9. To go back to your GUI
interface, press Ctrl+Alt+F1(as in example output above).
/dev/tty0 is a special device, which points to the current
terminal. So, irrespective of where you run it from(any virtual
console), anything read from/written to tty0 goes to your current
The second column in 'ps ax' also gives the controlling terminal
of the program. For some programs, like daemons, you may see that
the column is '?', which means they are not bound to a terminal.
/dev/pts/0 etc are psuedo-terminal devices, which are not
attached to the system display. for e.g, terminal you get when
you open a gnome-terminal or any other GUI terminal. These are
client-server based approach where client side will be exported
to programs, like bash. The data send by the program to the
pseudo terminal is sent to the 'server' side (which is usually
monitored by another program, like gnome-terminal). The
controlling process (server side) determines what needs to be
sent to the terminal, which is eventually seen by the client.
These devices help you to open multiple 'GUI terminals' without
any limit on your system, still providing the same old terminal
like controls(ioctl(), colour setting, Sending signals [Ctrl+C]