Extensions to the xterm protocol — kitty 0.20.1 documentation

# Extensions to the xterm protocol

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.

Contents

- Extensions to the xterm protocol

  - Colored and styled underlines

  - Graphics rendering

  - Keyboard handling

  - Setting text styles/colors in arbitrary regions of the screen

  - Saving and restoring colors

  - Pasting to clipboard

  - Desktop notifications

## Colored and styled underlines

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):

    <ESC>[58...m

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):

    <ESC>[59m

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 for the `Su` boolean capability.

## Graphics rendering

See The terminal graphics protocol for a description of this protocol to enable drawing of arbitrary raster images in the terminal.

## Keyboard handling

kitty has a keyboard protocol for reporting key presses to terminal applications that solves all key handling issues in terminal applications.

## Setting text styles/colors in arbitrary regions of the screen

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:

    <ESC>[2*x<ESC>[4;3;11;10;44$r<ESC>[*x

## Saving and restoring colors

It is often useful for a full screen application with its own color themes to set the default foreground, background, selection and cursor colors and the ANSI color table. 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 colors, default background, default foreground, selection background, selection foreground and cursor color and the 256 colors of the ANSI color table.

Note

In July 2020, after several years, XTerm copied this protocol extension, without acknowledgement, and using incompatible escape codes (XTPUSHCOLORS, XTPOPCOLORS, XTREPORTCOLORS). And they decided to save not just the dynamic colors but the entire ANSI color table. In the interests of promoting interoperability, kitty added support for XTerm’s escape codes as well, and changed this extension to also save/restore the entire ANSI color table.

## Pasting to clipboard

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:

    <ESC>]52;c;<payload1><ESC>\
    <ESC>]52;c;<payload2><ESC>\

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:

    <ESC>]52;c;!<ESC>\

Here `!` 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 `clipboard_control` setting.

## Desktop notifications

kitty implements an extensible escape code (OSC 99) to show desktop notifications. It is easy to use from shell scripts and fully extensible to show title and body. Clicking on the notification can optionally focus the window it came from, and/or send an escape code back to the application running in that window.

The design of the escape code is partially based on the discussion in the defunct terminal-wg

The escape code has the form:

    <OSC> 99 ; metadata ; payload <terminator>

Here `<OSC>` is `<ESC>]` and `<terminator>` is `<ESC><backslash>`. The metadata is a section of colon separated `key=value` pairs. Every key must be a single character from the set `a-zA-Z` and every value must be a word consisting of characters from the set `a-zA-Z0-9-_/\+.,(){}[]*&^%$#@!`~`. The payload must be interpreted based on the metadata section. The two semi-colons must always be present even when no metadata is present.

Before going into details, lets see how one can display a simple, single line notification from a shell script:

    printf '\x1b]99;;Hello world\x1b\\'

To show a message with a title and a body:

    printf '\x1b]99;i=1:d=0;Hello world\x1b\\'
    printf '\x1b]99;i=1:d=1:p=body;This is cool\x1b\\'

The most important key in the metadata is the `p` key, it controls how the payload is interpreted. A value of `title` means the payload is setting the title for the notification. A value of `body` means it is setting the body, and so on, see the table below for full details.

The design of the escape code is fundamentally chunked, this is because different terminal emulators have different limits on how large a single escape code can be. Chunking is accomplished by the `i` and `d` keys. The `i` key is the notification id which can be any string containing the characters `[a-zA-Z0-9_-+.]`. The `d` key stands for done and can only take the values `0` and `1`. A value of `0` means the notification is not yet done and the terminal emulator should hold off displaying it. A value of `1` means the notification is done, and should be displayed. You can specify the title or body multiple times and the terminal emulator will concatenate them, thereby allowing arbitrarily long text (terminal emulators are free to impose a sensible limit to avoid Denial-of-Service attacks).

Both the `title` and `body` payloads must be either UTF-8 encoded plain text with no embedded escape codes, or UTF-8 text that is base64 encoded, in which case there must be an `e=1` key in the metadata to indicate the payload is base64 encoded.

When the user clicks the notification, a couple of things can happen, the terminal emulator can focus the window from which the notification came, and/or it can send back an escape code to the application indicating the notification was activated. This is controlled by the `a` key which takes a comma separated set of values, `report` and `focus`. The value `focus` means focus the window from which the notification was issued and is the default. `report` means send an escape code back to the application. The format of the returned escape code is:

    <OSC> 99 ; i=identifier ; <terminator>

The value of `identifier` comes from the `i` key in the escape code sent by the application. If the application sends no identifier, then the terminal must use `i=0`. Actions can be preceded by a negative sign to turn them off, so for example if you do not want any action, turn off the default `focus` action with:

    a=-focus

Complete specification of all the metadata keys is in the table below. If a terminal emulator encounters a key in the metadata it does not understand, the key must be ignored, to allow for future extensibility of this escape code. Similarly if values for known keys are unknown, the terminal emulator should either ignore the entire escape code or perform a best guess effort to display it based on what it does understand.

Note

It is possible to extend this escape code to allow specifying an icon for the notification, however, given that some platforms, such as macOS, dont allow displaying custom icons on a notification, at all, it was decided to leave it out of the spec for the time being.

Similarly, features such as scheduled notifications could be added in future revisions.

Key | Value | Default | Description
--- | --- | --- | ---
`a` | Comma separated list of `report`, `focus`, with optional leading `-` | `focus` | What action to perform when the notification is clicked
`d` | `0` or `1` | `1` | Indicates if the notification is complete or not.
`e` | `0` or `1` | `0` | If set to `1` means the payload is base64 encoded UTF-8, otherwise it is plain UTF-8 text with no C0 control codes in it
`i` | `[a-zA-Z0-9-_+.]` | `0` | Identifier for the notification
`p` | One of `title` or `body`. | `title` | Whether the payload is the notification title or body. If a notification has no title, the body will be used as title.

Note

kitty also supports the legacy OSC 9 protocol developed by iTerm2 for desktop notifications.