[systemd-devel] Antw: [EXT] Re: vt220 default for serial console still relevant?

[Ulrich Windl]

>>> Lennart Poettering <lennart at poettering.net> schrieb am 15.07.2020 um 18:08
in
Nachricht <20200715160804.GB182602 at gardel-login>:
> On Di, 14.07.20 19:48, 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.
> 
> Well, ‑‑color means you request color no matter what, so this is not 
> surprising.
> 
>> 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
> 
> xterm‑256color is a relatively "recent" definition, actually.
> 
> The thing is that xterm has ton of extensions in the set of ANSI/CSI
> sequences that for example the Linux console does not have. Hence it
> sucks advertising the Linux console as xterm. Similar, the Linux
> console has a lot of different semantics and features beyond
> xterm. And even besides the command set there's other stuff context
> derived from the terminal setting. For example in systemd we disable
> emojis if TERM=linux is set, since we know that the linux console has
> only mininmal unicode support, i.e. 512 glyphs at max, and emojis are
> almost certainly not among them. Terminals after all don't advertise
> the set of glyphs they can display, so the second best thing is to
> trust TERM=linux. (We also turn off emojis on TERM=dumb)
> 
>> 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.
> 
> Pretty much all current terminal emulators support color just
> fine. It's really old legacy hw terminals that don't support that.
> 
> In systemd we don't really care much about super old terminal support,
> i.e. we never consult termcap/terminfo about terminals, because we
> don't want to require that to be installed. Instead, we assume that
> terminals can do color, except if TERM=dumb is set, it's 2020 after
> all. We also assume they can do full Unicode, except if LC_CTYPE is
> configured and explicitly indicates otherwise. We assume they can do
> emojis, except if TERM=linux or TERM=dumb is set where we know it's
> not supported. And we assume that hyperlinks work (except if "less" is
> involved). I think assuming this all to just work on any recent
> terminal is pretty safe in 2020. Of course this means we might

Just two thoughts on this:
I think as bloated as systemd already is, reading termcap or terminfo would
not make a big difference.
More an issue is that termcap and terminfo are competitors boing basically the
same (AFAIK).

The other thing is that you are assuming a mono-culture, specifically that all
terminals understand the same control sequences as vt100/ansi.
That applies for Linux and xterm, because they are modeled after vt100. But,
for example a vt52 has a completely different set of control commands.

> mistakenly output color sequences to a 1980's terminal if you actually
> connect that, but besides some exotic hackers noone does that anymore,
> hence we don't really care (and noone ever filed a bug about
> that). You can turn off color, unicode, emoji support individually via
> env vars btw, if people really want compat with such old terminals.

I just don't like monocultures...

Regards,
Ulrich

> 
> Lennart
> 
> ‑‑
> Lennart Poettering, Berlin
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