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notcurses/TERMS.md
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add some GNU screen notes
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Terminals and TERM

With the wrong environment settings, programs can't properly control your terminal. 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. Furthermore, for 24-bit TrueColor, it is necessary to either use a -direct variant of your terminfo entry, or to declare COLORTERM=24bit. The latter instruct Notcurses to use 24-bit escapes regardless of advertised support. If you define this variable, and your terminal doesn't actually support these sequences, you're going to have a bad time.

The following have been established on a Debian Unstable workstation.

Terminal Recommended environment Notes
Linux console 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.
FBterm TERM=fbterm 256 colors, no RGB color.
kmscon TERM=xterm-256color No RGB color AFAICT, nor any distinct terminfo entry.
XTerm TERM=xterm-256color COLORTERM=24bit 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.
XFCE4 Terminal TERM=xfce COLORTERM=24bit No xfce-direct variant exists.
Gnome Terminal TERM=gnome COLORTERM=24bit
Konsole TERM=konsole-direct
Alacritty TERM=alacritty COLORTERM=24bit
Kitty TERM=kitty-direct
Sakura TERM=vte-256color COLORTERM=24bit No terminfo entry?
mlterm TERM=mlterm-256color Do not set COLORTERM. mlterm-direct gives strange results.
st TERM=st-256color COLORTERM=24bit
GNU Screen 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.
tmux

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

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.

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.