notcurses/TERMINALS.md

Terminals

Notcurses attempts to provide an abstraction layer over the highly varied world of terminals. First and foremost, Notcurses needs to know the terminal on which it is running, so that has an accurate understanding of its capabilities.

It is of course possible that Notcurses is not connected to an actual terminal (e.g. when running daemonized). In such a case, many escapes will not be emitted, and no querying is performed.

Notcurses determines terminal capabilities via a combination of (more-or-less) standardized queries sent to the terminal, the TERM environment variable used by terminfo(5), and the COLORTERM environment variable.

Queries

At startup, the Linux console is identified via ioctl(2)s specific it. Otherwise, if it is determined that the process is connected to a terminal (see isatty(3)), Notcurses writes a series of queries to it. Several are related to terminal identification:

• Send Tertiary Device Attributes (CSI = 0 c)
  • Identifies VTE and foot
• XTVERSION (CSI > 0 q)
  • Identifies XTerm, WezTerm, and Contour
• XTGETTCAP for the TN key (DCS + q 544e ST)
  • Identifies Kitty and MLterm
• Send Primary Device Attributes (CSI c)

Note that Secondary Device Attributes (CSI > c) is not queried, because no terminals requiring special handling identify themselves via this query. This also applies to Primary Device Attributes, but we send this because all known terminals respond to it with something, preventing us from hanging, waiting for input (if a terminal does not reply in a recognizable way to Primary Device Attributes, notcurses_init() will hang).

Even if the terminal responds unambiguously to one of these queries, Notcurses must have code to recognize the response, and bind it to some terminal definition. Assuming the terminal to be thus identified, Notcurses enables or disables certain capabilities based on built-in knowledge.

The COLORTERM environment variable

24-bit RGB for glyphs and cell backgrounds is fairly widely implemented. In the Terminfo database, this is indicated via the rgb capability. It is not uncommon for this capability to not be expressed, despite support being present. Defining the COLORTERM environment variable with the value 24bit will instruct Notcurses to issue RGB sequences regardless.

Terminfo and TERM

Even if the terminal is unambiguously determined via query, many capabilities are acquired from the terminfo(5) database, keyed by the TERM environment variable. It is critical that the TERM environment variable be correct for your shell, and that the terminfo database entry keyed by this variable be up-to-date.

The following have been established on a Debian Unstable workstation. ccc is the Terminfo can-change-colors capability. "Blocks" refers to whether the terminal provides its own implementation of block-drawing characters, or relies on the font. Patches to correct/complete this table are very welcome!

Terminal	Pixel TIOCGWINSZ	ccc	Blocks	Recommended environment	Notes
Alacritty	✅	✅	❌	TERM=alacritty COLORTERM=24bit	Sixel support WIP
Contour	❌	✅	?	TERM=contour-latest ?	Claims Sixel support
ETerm				TERM=Eterm	Doesn't reply to Send Device Attributes
FBterm	❌	?	?	TERM=fbterm	256 colors, no RGB color.
foot	✅	✅	✅	TERM=foot	Sixel support.
Gnome Terminal	❌	❌	✅	TERM=gnome COLORTERM=24bit	ccc support is available when run with vte-256color.
Guake		?	?
ITerm2	✅	✅	✅	TERM=xterm-256color
Kitty	✅	✅	✅	TERM=xterm-kitty	See below.
kmscon		❌	❌	TERM=xterm-256color	No RGB color AFAICT, nor any distinct terminfo entry. No actual ccc implementation.
Konsole	❌	❌	?	TERM=konsole-direct
Linux console	❌	✅	see below	TERM=linux COLORTERM=24bit	8 (512 glyph fonts) or 16 (256 glyph fonts) colors max, but RGB values are downsampled to a 256-index palette. See below.
mlterm	✅	❌	?	TERM=mlterm-256color	Do not set COLORTERM. mlterm-direct gives strange results.
PuTTY		❌	❌	TERM=putty-256color COLORTERM=24bit
rxvt	✅	?	?		Seems unmaintained; many forks exist.
Sakura	✅	✅	?	TERM=vte-256color COLORTERM=24bit	VTE-derived, no terminfo entry.
GNU Screen	✅	❌	n/a	TERM=screen.OLDTERM	Must be compiled with --enable-256color. TERM should typically be screen. suffixed by the appropriate TERM value for the true connected terminal, e.g. screen.vte-256color. See below.
st ("suckless")	✅	✅	?	TERM=st-256color COLORTERM=24bit	Many features are maintained as external patches; users often roll their own instance, composing from these patches.
Tabby	?	?	?	?
Terminal.app	✅	❌	❌	TERM=xterm-256color	No RGB; no ccc despite wanting xterm-256color.
Terminator	✅	?	?	?
Terminology	❌	❌	?	TERM=terminology	Identified via DA3. 256 colors, no RGB.
Tilda		?	?	?
tmux	✅	❌	n/a	TERM=tmux-256color COLORTERM=24bit	tmux.conf must apply Tc; see below. bce is available with the tmux-256color-bce definition.
WezTerm	✅	✅	?	TERM=wezterm COLORTERM=24bit	See below.
Windows Terminal		?	?	?
wterm		?	?	?
XFCE4 Terminal	❌	✅	✅	TERM=xfce COLORTERM=24bit	No xfce-direct variant exists.
XTerm	✅	?	❌	TERM=xterm+256color2 COLORTERM=24bit	See note about DirectColor. Must configure with --enable-direct-color. TERM=xterm-direct seems to have the undesirable effect of mapping low RGB values to a palette; I don't yet understand this well. The problem is not seen with the specified configuration. Sixel support when built with --enable-sixel-graphics and run in vt340 mode.
Yakuake		?	?	?

Note that xfce4-terminal, gnome-terminal, etc. are essentially skinning atop the common GNOME VTE ("Virtual TErminal") library.

Kitty

Kitty has some interesting, atypical behaviors. Foremost among these is that an RGB background color equivalent to the configured default background color will be rendered as the default background. This means, for instance, that if the configured default background color is RGB(0, 0, 0), and is translucent, a background of RGB(0, 0, 0) will be translucent. To work around this, when TERM begins with "kitty", we detect the default background color, and when we would write this as RGB, we alter one of the colors by 1. See https://github.com/kovidgoyal/kitty/issues/3185 and https://github.com/dankamongmen/notcurses/issues/1117.

Kitty is furthermore the only terminal I know to lack the bce (Background Color Erase) capability, but Notcurses never relies on bce behavior.

WezTerm

WezTerm implements some interesting underline options, and the iTerm2 graphic protocol.

GNU screen

GNU screen does have 24-bit color support, but only in the 5.X series. Note that many distributions ship screen 4.X as of 2020. When built with truecolor support, add truecolor on to your screenrc, or run it with --truecolor. Attempting to force RGB color in screen 4.X will not work.

Add defutf8 on to your screenrc, or run screen with -U, to ensure UTF-8.

tmux

tmux supports 24-bit color through its Tc (Truecolor) extension. You'll need an entry in tmux.conf of the form:

set -ga terminal-overrides ",EXTERNALTERM:Tc"

Where EXTERNALTERM is your TERM variable at the time of attachment, e.g.:

set -ga terminal-overrides ",vte-256color:Tc"

You'll then need COLORTERM=24bit defined within your tmux environment.

The Linux console

The Linux console supports concurrent virtual terminals, and is manipulated by userspace via ioctl()s. These ioctl()s generally fail when applied to a pseudotty device, as will happen if e.g. invoked upon one's controlling terminal whilst running in a terminal emulator under X (it is still generally possible to use them by explicitly specifying a console device, i.e. showconsolefont -C /dev/tty0).

The VGA text console requires the kernel option CONFIG_VGA_CONSOLE. A framebuffer console for VESA 2.0 is provided by CONFIG_FB_VESA, while UEFI-compatible systems can use CONFIG_FB_EFI. So long as a framebuffer driver is present, CONFIG_FRAMEBUFFER_CONSOLE will enable a graphics-mode console using the framebuffer device.

The Linux console can be in either text or graphics mode. The mode can be determined with the KDGETMODE ioctl(), and changed with KDSETMODE, using the constants KD_TEXT and KD_GRAPHICS. Text mode supports a rectangular matrix of multipixel cells, filled with glyphs from a font, a foreground color, and a background color. Graphics ("All-Points-Addressable") mode supports a rectangular matrix of pixels, each with a single color. Note that both modes require appropriate hardware support (and kernel configuration options), and might or might not be available on a given installation. Non-x86 platforms often provide only a framebuffer (graphics) console.

The kernel text mode loosely corresponds to the 1987 IBM VGA definition. At any time, the display is configured with a monospace raster font, a palette, and (when in Unicode mode) a mapping from multibyte sequences to font elements. Up to 16 colors can be used with a font of 256 glyphs or fewer. Only 8 colors can be used with fonts having more than 256 glyphs; the maximum font size in any configuration is 512 glyphs. The keyboard is further configured with a keymap, mapping keyboard scancodes to elements of the character set. These properties are per-virtual console, not common to all of them. These limitations are not typically present on framebuffer consoles.

Exporting COLORTERM=24bit and emitting RGB escapes to the Linux console does work, though the RGB values provided are downsampled to a 256-slot palette. Backgrounds don't seem to have the same degree of flexibility in this situation as do foregrounds. The output is better, but not as much better as one might expect. More research is necessary here.

The following more-or-less standard tools exist:

• showconsolefont: show the console font
• setfont: load console font
• fbset: show and modify framebuffer settings
• fgconsole: print name of foreground terminal
• chvt: change the foreground terminal
• deallocvt: destroy a virtual console
• dumpkeys: print all keycodes
• loadkeys: load scancode/keycode mapping (the keymap)
• setkeycodes: load scancode/keycode mappings one at a time
• showkeys: interactively print scancodes
• kbd_mode: show or set the keyboard mode

Both mapscrn and loadunimap are obsolete; their functionality is present in setfont.

Note that Notcurses reprograms the console font table when running in the Linux console (unless NCOPTION_NO_FONT_CHANGES is used). This adds support for half blocks and quadrants.

24-bit RGB

Many terminals support one or another form of non-indexed color encoding (also known as DirectColor, RGB color, 24-bit color, or the similar but distinct TrueColor), using either the semicolon-based presentation introduced by Konsole or the colon-delimited presentation specified in ECMA-48 and ITU T.416. The rgb termcap capability indicates support for such encodings via the set_a_foreground and set_b_foreground capabilities. Not all terminals implementing rgb use the 3x8bpc model; XTerm for instance:

for values 0 through 7, it uses the “ANSI” control sequences, while for other values, it uses the 3-byte direct-color sequence introduced by Konsole. the number of colors is 224 while the number of color pairs is 216

Thus emitting setaf with an RGB value close to black can result, when using xterm-direct's setaf and rgb definitions, in a bright ANSI color.

24-bit RGB is always enabled for Kitty, Alacritty, Contour, WezTerm, iTerm2, and foot.

Problematic characters

Some characters seem to cause problems with one terminal or another. These are best avoided until the problems are better understood:

• '­' U+00AD SOFT HYPHEN (some terminals allocate it a cell, some don't)
• '܏' U+070F SYRIAC ABBREVIATION MARK
• '۝' U+06DD ARABIC END OF AYAH
• '࣢' U+08E2 ARABIC DISPUTED END OF AYAH
• '﷽' U+FDFD ARABIC LIGATURE BISMILLAH AR-RAHMAN AR-RAHEEM '