kitty has a few extensions to the xterm protocol, to enable advanced features. These are typically in the form of new or re-purposed escape codes. While these extensions are currently kitty specific, it would be nice to get some of them adopted more broadly, to push the state of terminal emulators forward.
The goal of these extensions is to be as small and unobtrusive as possible, while filling in some gaps in the existing xterm protocol. In particular, one of the goals of this specification is explicitly not to "re-imagine" the tty. The tty should remain what it is -- a device for efficiently processing text received as a simple byte stream. Another objective is to only move the minimum possible amount of extra functionality into the terminal program itself. This is to make it as easy to implement these protocol extensions as possible, thereby hopefully encouraging their widespread adoption.
If you wish to discuss these extensions, propose additions/changes to them please do so by opening issues in the github bug tracker.
kitty supports colored and styled (wavy) underlines. This is of particular use in terminal editors such as vim and emacs to display red, wavy underlines under mis-spelled words and/or syntax errors. This is done by re-purposing some SGR escape codes that are not used in modern terminals (CSI codes)
To set the underline style:
<ESC>[4:0m # this is no underline <ESC>[4:1m # this is a straight underline <ESC>[4:2m # this is a double underline <ESC>[4:3m # this is a curly underline <ESC>[4:4m # this is a dotted underline (not implemented in kitty) <ESC>[4:5m # this is a dashed underline (not implemented in kitty) <ESC>[4m # this is a straight underline (for backwards compat) <ESC>[24m # this is no underline (for backwards compat)
To set the underline color (this is reserved and as far as I can tell not actually used for anything):
This works exactly like the codes
38, 48 that are used to set foreground and
background color respectively.
To reset the underline color (also previously reserved and unused):
The underline color must remain the same under reverse video, if it has a color, if not, it should follow the foreground color.
To detect support for this feature in a terminal emulator, query the terminfo database
Su boolean capability.
See The terminal graphics protocol for a description of this protocol to enable drawing of arbitrary raster images in the terminal.
There are various problems with the current state of keyboard handling. They include:
No way to use modifiers other than
No way to reliably use multiple modifier keys, other than,
No way to handle different types of keyboard events, such as press, release or repeat
No reliable way to distinguish single
Esckeypresses from the start of a escape sequence. Currently, client programs use fragile timing related hacks for this, leading to bugs, for example: neovim #2035.
There are already two distinct keyboard handling modes, normal mode and
application mode. These modes generate different escape sequences for the
various special keys (arrow keys, function keys, home/end etc.) Most terminals
start out in normal mode, however, most shell programs like
bash switch them to
application mode. We propose adding a third mode, named full mode that addresses
the shortcomings listed above.
Switching to the new full mode is accomplished using the standard private mode DECSET escape sequence:
and to leave full mode, use DECRST:
2017 above is not used for any existing modes, as far as I know.
Client programs can query if the terminal emulator is in full mode by using
the standard DECRQM escape sequence.
The new mode works as follows:
All printable key presses without modifier keys are sent just as in the normal mode. This means all printable ASCII characters and in addition,
Backspace. Also any unicode characters generated by platform specific extended input modes, such as using the
AltGrkey. This is done so that client programs that are not aware of this mode can still handle basic text entry, so if a full mode using program crashes and does not reset, the user can still issue a
resetcommand in the shell to restore normal key handling. Note that this includes pressing the
Shiftmodifier and printable keys. Note that this means there are no repeat and release events for these keys and also for the left and right shift keys.
For non printable keys and key combinations including one or more modifiers, an escape sequence encoding the key event is sent. For details on the escape sequence, see below.
The escape sequence encodes the following properties:
Type of event:
Modifiers pressed at the time of the event
The actual key being pressed
<type> is one of
p -- press,
r -- release and
t -- repeat.
Modifiers is a bitmask represented as a single base64 digit. Shift --
0x2, Control --
0x4 and Super --
<key> is a number
(encoded in base85) corresponding to the key pressed. The key name to number
mapping is defined in this table.
Client programs must ignore events for keys they do not know. The mapping in the above table is stable and will never change, however, new codes might be added to it in the future, for new keys.
<ESC>_KpGp<ESC>\ is <Ctrl>+<Alt>+x (press) <ESC>_KrP8<ESC>\ is <Ctrl>+<Alt>+<Shift>+<Super>+PageUp (release)
This encoding means each key event is represented by 8 or 9 printable ascii only bytes, for maximum robustness.
To see the full mode in action, run:
kitty +kitten key_demo
Support for this mode is indicated by the
fullkbd boolean capability
in the terminfo database, in case querying for it via DECQRM is inconvenient.
There already exists an escape code to set some text attributes in arbitrary regions of the screen, DECCARA. However, it is limited to only a few attributes. kitty extends this to work with all SGR attributes. So, for example, this can be used to set the background color in an arbitrary region of the screen.
The motivation for this extension is the various problems with the existing solution for erasing to background color, namely the background color erase (bce) capability. See this discussion and this FAQ for a summary of problems with bce.
For example, to set the background color to blue in a rectangular region of the screen from (3, 4) to (10, 11), you use:
It is often useful for a full screen application with its own color themes to set the default foreground, background, selection and cursor colors. This allows for various performance optimizations when drawing the screen. The problem is that if the user previously used the escape codes to change these colors herself, then running the full screen application will lose her changes even after it exits. To avoid this, kitty introduces a new pair of OSC escape codes to push and pop the current color values from a stack:
<ESC>]30001<ESC>\ # push onto stack <ESC>]30101<ESC>\ # pop from stack
These escape codes save/restore the so called dynamic colors, default background, default foreground, selection background, selection foreground and cursor color.
kitty implements the OSC 52 escape code protocol to get/set the clipboard
contents (controlled via the
clipboard_control setting). There is one
difference in kitty's implementation compared to some other terminal emulators.
kitty allows sending arbitrary amounts of text to the clipboard. It does so
by modifying the protocol slightly. Successive OSC 52 escape codes to set the
clipboard will concatenate, so:
will result in the clipboard having the contents
payload1 + payload2. To
send a new string to the clipboard send an OSC 52 sequence with an invalid payload
first, for example:
! is not valid base64 encoded text, so it clears the clipboard.
Further, since it is invalid, it should be ignored by terminal emulators
that do not support this extension, thereby making it safe to use, simply
always send it before starting a new OSC 52 paste, even if you aren't chunking
up large pastes, that way kitty won't concatenate your paste, and it will have
no ill-effects in other terminal emulators.
In case you're using software that can't be easily adapted to this
protocol extension, it can be disabled by specifying
no-append to the