Re: ANSI sequences produced on non-ANSI terminal

From: Jeff King <peff@peff.net>
Date: Sep 23, 2021, 21:20 UTC
Message-ID: <YUzvhLUmvsdF5w+r@coredump.intra.peff.net>
In-Reply-To: <20210923052122.2F655CE@eddie.starwolf.com>

On Wed, Sep 22, 2021 at 10:21:22PM -0700, The Grey Wolf wrote:

Show 12 quoted lines

> Anything else you want to add:
> 	I searched google and the documentation as best I was able for
> 	this, but I am unable to find anywhere that will let me disable
> 	(or enable) colour for a particular term type.  Sometimes I'm on
> 	an xterm, for which this is GREAT.  Sometimes I'm on a Wyse WY60,
> 	for which this is sub-optimal.  My workaround is to disable colour
> 	completely, which is reluctantly acceptable, but it would be nice
> 	to say "If I'm on an xterm/aterm/urxvt/ansi terminal, enable
> 	colour or cursor-positioning, otherwise shut it off."  If this
> 	seems too much of a one-off to handle, fine, but most things that
> 	talk fancy to screens are kind enough to allow an opt-out based on
> 	terminal type. :)

Git doesn't have any kind of list of terminals, beyond knowing that "dumb" should disable auto-color. It's possible we could expand that if there are known terminals that don't understand ANSI colors. I'm a bit wary of having a laundry list of obscure terminals, though.

If we built against ncurses or some other terminfo-aware library we could outsource that, but that would be a new dependency. I'm hesitant to do that even as an optional dependency given the bang-for-the-buck (and certainly making it require would be right out).

Obviously you can wrap Git with a script to tweak the config based on the current setting of the $TERM variable. It would be nice if you could have conditional config for that. E.g., something like:

  [includeIf "env:TERM==xterm"]
  path = gitconfig-color

That doesn't exist, but would fit in reasonably well with our other conditional config options.

As far as generating non-ANSI codes, that's all Git knows how to do. I'm not sure what kind of color codes your terminal might support. It _might_ be possible to support multiple, but from my knowledge of Git's color code I suspect it would be quite ugly. You'd probably be better off post-processing the ANSI codes. You can do so automatically-ish with something like:

  git config pager.log 'convert-ansi-to-whatever | less'

I don't know offhand of anything that would do such conversion out of the box, but you could probably built it around tput or a terminfo library.

Note that we do similar post-processing on Windows, albeit internally by intercepting fprintf, etc (yuck). See compat/winansi.c, which might give you some logic which would be reused.

-Peff