Ssh – What does ‘?’ represent under TTY

On UNIX, a tty (like many other things) appears as a file. Data written to the tty device goes to the terminal and data coming from the terminal is available for reading on the tty.

If the tty is a hardware serial port, then data written to it gets sent on the wire and data coming from the wire appears on the tty.

If the tty is a machine's video console then data written to it gets displayed on the screen and data coming from the keyboard appears on the tty.

If the tty is a pseudo-tty (virtual terminal) connected to an X terminal emulator such as gnome-terminal then data written to it gets delivered to the X terminal emulator software and in turn gets shown in a window, while data typed into that window is available for reading on the virtual terminal. The software is said to be connected to the "master" end of the pseudo-terminal. Real terminals don't have a "master" end because there's a real device behind them (like the serial port), not a virtual device implemented by a piece of software.

So you can see already that it wouldn't make any sense to ask a X terminal emulator to operate on an arbitrary tty device such as /dev/ttyS0 (a real serial port, on Linux). It must be a pseudo-tty.

But can the terminal emulator choose the numeric ID of the pseudo-terminal device that it uses? In principle, it would be possible for a kernel to permit this, but in fact the kernel interface for allocating a new pseudo-terminal does not support it: the kernel makes it own choice. (In the SysV model pesudo-terminals are created by opening a special device called /dev/ptmx and the lowest-numbered available pseudo-terminal device is automatically allocated.)

But: why would you need to choose the number of the pseudo-terminal device that gets allocated? The kernel chooses an unused one that's guaranteed to be available and usable. Do you have a reason why you'd prefer if it chose a different one?

As to you other question:

I saw a lot of tty files in /dev and a lot of pts files in /dev/pts/ I don't know why there are so many of them. Are they all being used?

It depends on the system. On some systems, all possible pseudo-terminal devices are pre-created in /dev/pts or /dev, whether they are in use or not. On others, the device nodes only exist if the pseudo-terminal is in use. You say you are using Ununbu, which uses Linux, which is the latter type. So, yes, all of the devices nodes you see in /dev/pts are presently in use.

But for temporary access, it is not very convenient. I want to use socat to forward a tty or pty to client. Then client can start a terminal with that pty or tty.

If you want socat or any other piece of software to connect to the master end of a pseudo-terminal, then you need for that software to specifically support doing that. But you're in luck because socat does. For example, if I run:

socat PTY,link=/tmp/socat.pty TCP-LISTEN:2222 &
sleep 1 && ( setsid bash ) </tmp/socat.pty >/tmp/socat.pty 2>&1

I can get a shell by connecting to port 2222 from somewhere else. Very dangerous from a security point of view!!!

The who utility by default displays information relating to the login. If you logged in an tty2, then who will say so.

Use the w command to show information about the users currently logged into the machine and on what terminal they are doing what on.

Update: "What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'?" describes some differences between a TTY and a PTY. :0 is the particular host display that you are connected to.