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notcurses/TERMS.md
Nick Black dbc83c41b3
linuxconsole PoC (#791) 
* use ioctl(KDGETMODE) to detect Linux console
* diagnostic on KDGETMODE
* add logdebug()
* unit test for notcurses_drop_planes(), add ncplane_putnstr_aligned()
* linuxconsole PoC
* linuxconsole: dump unicode->font table
* linuxconsole: explode glyphs
* linuxconsole: show 7 glyphs per 'line'
* linuxconjammer: shim console font with half blocks
* signals: handler for SIGTERM
* man pages: update notcurses_init() for options
* add NCOPTION_NO_FONT_CHANGES #201
2020-07-13 01:21:41 -04:00

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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 8 (512 glyph fonts) or 16 (256 glyph fonts) colors max.
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.
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
st TERM=st-256color COLORTERM=24bit
GNU Screen TERM=screen-256colors Must be compiled with --enable-256color.
tmux

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.

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.