Terminals and pseudoterminals

These files in /dev represent different types of terminals:

• /dev/tty* - physical consoles;
• /dev/ttyS* - serial connections;
• /dev/pts/* - pseudoterminals.

Pseudoterminals

Pseudoterminals allow to build terminal emulators and use them instead of a real terminal, where an application expects a terminal device. Pseudoterminal consists of 2 parts:

• A ptmx part which is a leader for the pseudoterminal. This end is used to emulate the user input and read back the program output.
• A pts is a secondary end. This part is given to an application that needs a terminal.

Notable programs using pseudoterminals:

• ssh/sshd - pairs two pseudoterminals and transfers data between them over the net.
• screen/tmux - use a pseudoterminal for every client.
• expect - allows user to script input into an interactive program.
• script - records terminal sessions

Terminal signals

• SIGTTIN - a background process tried to read from a terminal.
• SIGTTOU - a background process tries to write to a terminal when the tostop flag is set or a background process asks to send it to the foreground.
• SIGTSTP - a default response to a C-Z pressed combination.

The noncanonical programs such as vi, emacs and less, need to handle all the above signals in order to reset terminal settings back and forth, redraw a terminal content and place the cursor in the right place.