[systemd-devel] vt220 default for serial console still relevant?

[Mantas Mikulėnas]

On Tue, Jul 14, 2020 at 9:48 PM Daan De Meyer <daan.j.demeyer at gmail.com>
wrote:

> I just tried vt241 and didn't get colorized output in konsole. I looked
> around a bit and it doesn't really seem supported at all by terminal
> emulators (or at least none that I found). I also tried TERM=xterm-256color
> with 8 different terminal emulators and got colors with all of them. My
> workflow is simply "mkosi qemu -nographic" (with a modified mkosi that adds
> console=ttyS0 and overrides TERM for serial-getty at ttyS0 in the vm)." That
> connects my terminal emulator to the serial console of the vm. I then
> execute "ls -l --color /" in the vm and get colored output every time in
> whatever terminal emulator that I try.
>
> I'm probably missing something but I'm wondering what an example terminal
> emulator would be where xterm-256color would not work at all (even st
> worked perfectly and that is supposed to be pretty barebones afaik). Or is
> it just that color is a commonly supported subset of xterm and stuff breaks
> down with other escape codes instead? Or maybe qemu is doing some kind of
> translation that magically makes every TERM setting work in whatever
> terminal emulator I try? Apologies if this seems ignorant, I have no
> experience at all with this domain.
>

The basic formatting codes (bold, reverse, 8/16-color) are practically the
same everywhere. In fact they're so widespread that many programs and
scripts just straight up hardcode them.  (xterm, st, etc. are supersets of
vt220/vt421 in that regard.)

But 256-color or even 24-bit-color codes are not as widely supported. The
Linux VT has only recently started *recognizing* them (though still maps
them down to 8 colors). If you use TERM=xterm-256color with Linux VT, apps
can look really weird because you told them they can use these extended
color codes even though they cannot.

Graphical terminal emulators recognize the "update status line" codes
(which goes the X11 window titlebar) -- but the Linux VT has no such thing
and the same code will just show up as garbage on screen.

Google tells me VT421 supported sixel graphics. I'm not sure if any
programs make use of that nowadays, but if they do, then trying to use
TERM=vt421 with a terminal that doesn't do sixel will result in more
garbage on screen.

There are various other differences like this.

-- 
Mantas Mikulėnas
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