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92 lines
5.1 KiB
Markdown
92 lines
5.1 KiB
Markdown
# Terminals and `TERM`
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With the wrong environment settings, programs can't properly control
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your terminal. It is critical that the `TERM` environment variable be
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correct for your shell, and that the terminfo database entry keyed
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by this variable be up-to-date. Furthermore, for 24-bit TrueColor, it
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is necessary to either use a `-direct` variant of your terminfo
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entry, or to declare `COLORTERM=24bit`. The latter instruct Notcurses
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to use 24-bit escapes regardless of advertised support. If you define
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this variable, and your terminal doesn't actually support these sequences,
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you're going to have a bad time.
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The following have been established on a Debian Unstable workstation.
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| Terminal | Recommended environment | Notes |
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| -------- | ------ | ----- |
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| Linux console | `TERM=linux` | 8 (512 glyph fonts) or 16 (256 glyph fonts) colors max, but RGB values are downsampled to a 256-index palette (see below). |
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| FBterm | `TERM=fbterm` | 256 colors, no RGB color. |
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| kmscon | `TERM=xterm-256color` | No RGB color AFAICT, nor any distinct terminfo entry. |
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| 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. |
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| XFCE4 Terminal | `TERM=xfce` `COLORTERM=24bit` | No `xfce-direct` variant exists. |
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| Gnome Terminal | `TERM=gnome` `COLORTERM=24bit` | |
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| Konsole | `TERM=konsole-direct` | |
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| Alacritty | `TERM=alacritty` `COLORTERM=24bit` | |
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| Kitty | `TERM=kitty-direct` | |
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| Sakura | `TERM=vte-256color` `COLORTERM=24bit` | No terminfo entry? |
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| mlterm | `TERM=mlterm-256color` | Do not set `COLORTERM`. `mlterm-direct` gives strange results. |
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| st | `TERM=st-256color` `COLORTERM=24bit` | |
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| GNU Screen | `TERM=screen-256colors` | Must be compiled with `--enable-256color`. |
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| tmux | | |
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Note that `xfce4-terminal`, `gnome-terminal`, etc. are all skinning atop the
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common VTE ("Virtual TErminal") library.
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## The Linux console
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The Linux console supports concurrent virtual terminals, and is manipulated
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by userspace via `ioctl()`s. These `ioctl()`s generally fail when applied to
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a pseudotty device, as will happen if e.g. invoked upon one's controlling
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terminal whilst running in a terminal emulator under X (it is still generally
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possible to use them by explicitly specifying a console device, i.e.
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`showconsolefont -C /dev/tty0`).
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The VGA text console requires the kernel option `CONFIG_VGA_CONSOLE`. A
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framebuffer console for VESA 2.0 is provided by `CONFIG_FB_VESA`, while
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UEFI-compatible systems can use `CONFIG_FB_EFI`. So long as a framebuffer
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driver is present, `CONFIG_FRAMEBUFFER_CONSOLE` will enable a graphics-mode
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console using the framebuffer device.
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The Linux console can be in either text or graphics mode. The mode can be
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determined with the `KDGETMODE` `ioctl()`, and changed with `KDSETMODE`,
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using the constants `KD_TEXT` and `KD_GRAPHICS`. Text mode supports a
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rectangular matrix of multipixel cells, filled with glyphs from a font,
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a foreground color, and a background color. Graphics ("All-Points-Addressable")
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mode supports a rectangular matrix of pixels, each with a single color.
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Note that both modes require appropriate hardware support (and kernel
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configuration options), and might or might not be available on a given
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installation. Non-x86 platforms often provide only a framebuffer (graphics)
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console.
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The kernel text mode loosely corresponds to the 1987 IBM VGA definition. At any
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time, the display is configured with a monospace raster font, a palette, and
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(when in Unicode mode) a mapping from multibyte sequences to font elements. Up
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to 16 colors can be used with a font of 256 glyphs or fewer. Only 8 colors can
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be used with fonts having more than 256 glyphs; the maximum font size in any
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configuration is 512 glyphs. The keyboard is further configured with a keymap,
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mapping keyboard scancodes to elements of the character set. These properties
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are per-virtual console, not common to all of them. These limitations are not
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typically present on framebuffer consoles.
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Exporting `COLORTERM=24bit` and emitting RGB escapes to the Linux console
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**does** work, though the RGB values provided are downsampled to a 256-slot
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palette. Backgrounds don't seem to have the same degree of flexibility in this
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situation as do foregrounds. The output is better, but not as much better as
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one might expect. More research is necessary here.
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The following more-or-less standard tools exist:
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* `showconsolefont`: show the console font
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* `setfont`: load console font
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* `fbset`: show and modify framebuffer settings
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* `fgconsole`: print name of foreground terminal
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* `chvt`: change the foreground terminal
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* `deallocvt`: destroy a virtual console
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* `dumpkeys`: print all keycodes
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* `loadkeys`: load scancode/keycode mapping (the keymap)
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* `setkeycodes`: load scancode/keycode mappings one at a time
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* `showkeys`: interactively print scancodes
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* `kbd_mode`: show or set the keyboard mode
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Both `mapscrn` and `loadunimap` are obsolete; their functionality is present
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in `setfont`.
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