Return to Answer

The terminal may transmit the same escape sequence for some modifier combinations (e.g. many terminals transmit a space character for both Space and Shift+Space). A few keys are not transmitted at all, for example modifier keys or keys that trigger a binding of the terminal emulator (e.g. a copy or paste command).

The terminal may transmit the same escape sequence for some modifier combinations (e.g. many terminals transmit a space character for both Space and Shift+Space; xterm has a mode to distinguish modifier combinations but terminals based on the popular vte library don't). A few keys are not transmitted at all, for example modifier keys or keys that trigger a binding of the terminal emulator (e.g. a copy or paste command).

The X server reads input events through a device file, for example /dev/input/eventNNN on Linux (where NNN is a number). Whenever there is an event, the kernel signals that there is data to read from that device. The device file transmits key up/down events with a scan code, which may or may not be identical to the value transmitted by the hardware (the kernel may translate the scan code from a keyboard-dependent value to a common value, and Linux doesn't retransmit the scan codes that it doesn't know).

Relationship of keyboard layout and xmodmap goes into more detail on keyboard input. How do mouse events work in linux? gives an overview of mouse input at the lower levels.

See What are the purposes of the different types of XWindows fonts? for a discussion of client-side and server-side text rendering under X11.

If your text editor is a text mode application running in a terminal, then it is the terminal which is the application for the purpose of the section above. In this section, I explain the interface between the text mode application and the terminal. First I describe the case of a terminal emulator running under X11. What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'? may be useful background here. After reading this, you may want to read the far more detailed What are the responsibilities of each Pseudo-Terminal (PTY) component (software, master side, slave side)?

In a terminal, if you type Ctrl+V followed by a key combination, this inserts the first byte of the escape sequence from the key combination literally. Since escape sequences normally consist only of printable characters after the first one, this inserts the whole escape sequence literally. See key bindings table? for a discussion of zsh in this context.

Escape sequences supported by the terminal emulator are described in the termcap or terminfo database. Most terminal emulator nowadays are fairly closely aligned with xterm. See Documentation on LESS_TERMCAP_* variables? for a longer discussion of terminal capability information databases, and How to stop cursor from blinking and Can I set my local machine's terminal colors to use those of the machine I ssh into? for some usage examples.

The X server reads input events through a device file, for example /dev/input/eventNNN on Linux (where NNN is a number). Whenever there is an event, the kernel signals that there is data to read from that device. The device file transmits key up/down events with a scan code, which may or may not be identical to the value transmitted by the hardware (the kernel may translate the scan code from a keyboard-dependent value to a common value, and Linux doesn't retransmit the scan codes that it doesn't know).

Relationship of keyboard layout and xmodmap goes into more detail on keyboard input. How do mouse events work in linux? gives an overview of mouse input at the lower levels.

See What are the purposes of the different types of XWindows fonts? for a discussion of client-side and server-side text rendering under X11.

If your text editor is a text mode application running in a terminal, then it is the terminal which is the application for the purpose of the section above. In this section, I explain the interface between the text mode application and the terminal. First I describe the case of a terminal emulator running under X11. What is the exact difference between a 'terminal', a 'shell', a 'tty' and a 'console'? may be useful background here. After reading this, you may want to read the far more detailed What are the responsibilities of each Pseudo-Terminal (PTY) component (software, master side, slave side)?

In a terminal, if you type Ctrl+V followed by a key combination, this inserts the first byte of the escape sequence from the key combination literally. Since escape sequences normally consist only of printable characters after the first one, this inserts the whole escape sequence literally. See key bindings table? for a discussion of zsh in this context.

Escape sequences supported by the terminal emulator are described in the termcap or terminfo database. Most terminal emulator nowadays are fairly closely aligned with xterm. See Documentation on LESS_TERMCAP_* variables? for a longer discussion of terminal capability information databases, and How to stop cursor from blinking and Can I set my local machine's terminal colors to use those of the machine I ssh into? for some usage examples.

Applications connect to the X server and receive a notification when a key is pressed while a window of that application has the focus. The notification incidates that a certain keysym was pressed or released as well as what modifiers are currently pressed. You can see keysyms by running the program xev from a terminal. What the application does with the information is up to it; some applications have configurable key bindings.

Output is rather simpler than input. If the application outputs a character to the pty device file, the terminal emulator displays it at the current cursor posititon. (The terminal emulator maintains a cursor position, and scrolls if the cursor would fall under the bottom of the screen.) The application can also output escape sequences (mostly beginning with ^[ or ^]) to tell the terminal to perform actions such as moving the cursor, changing the text attributes (color, bold, …), or erasing part of the screen.

This is mostly transparent, except that sometimes the remote terminal database may not know all the capabilites of the local terminal.

Applications connect to the X server and receive a notification when a key is pressed while a window of that application has the focus. The notification indicates that a certain keysym was pressed or released as well as what modifiers are currently pressed. You can see keysyms by running the program xev from a terminal. What the application does with the information is up to it; some applications have configurable key bindings.

Output is rather simpler than input. If the application outputs a character to the pty device file, the terminal emulator displays it at the current cursor position. (The terminal emulator maintains a cursor position, and scrolls if the cursor would fall under the bottom of the screen.) The application can also output escape sequences (mostly beginning with ^[ or ^]) to tell the terminal to perform actions such as moving the cursor, changing the text attributes (color, bold, …), or erasing part of the screen.

This is mostly transparent, except that sometimes the remote terminal database may not know all the capabilities of the local terminal.