Editing ANSI escape code (section)

==== Unix environment variables relating to color support ====

Rather than using the color support in termcap and terminfo introduced in SVr3.2 (1987),<ref>{{cite manual |url=https://invisible-island.net/ncurses/man/curs_color.3x.html#h2-HISTORY |section=History |title=curs_color(3X) - manipulate terminal colors with curses }}</ref> the S-Lang library (version 0.99-32, June 1996) used a separate environment variable {{code|$COLORTERM}} to indicate whether a terminal emulator could use colors at all, and later added values to indicate if it supported 24-bit color.<ref>{{cite web |title=Midnight Commander: lib/tty/color-slang.c|website=Fossies |url=https://fossies.org/linux/mc/lib/tty/color-slang.c}}</ref><ref name=ncurses-on-slang>{{cite web |first=Thomas E. | last=Dickey |title=NCURSES — comments on S-Lang |url=https://invisible-island.net/ncurses/ncurses-slang.html |year=2017 |website=invisible-island.net}}</ref> This system, although poorly documented, became widespread enough for Fedora and RHEL to consider using it as a simpler and more universal detection mechanism compared to querying the now-updated libraries.<ref>{{cite web |title=Features/256 Color Terminals - Fedora Project Wiki |url=https://fedoraproject.org/wiki/Features/256_Color_Terminals#Scope |website=Fedora Project |archive-url=https://web.archive.org/web/20121004184942/https://fedoraproject.org/wiki/Features/256_Color_Terminals#Scope|archive-date=4 October 2012|url-status=live}}</ref>

Some terminal emulators (urxvt, konsole) set {{code|$COLORFGBG}} to report the color scheme of the terminal (mainly light vs. dark background). This behavior originated in S-Lang<ref name=ncurses-on-slang/> and is used by vim. Gnome-terminal refuses to add this behavior, as the syntax for the value is not agreed upon, the value cannot be changed upon a runtime change of the palette, and more "proper" xterm OSC 4/10/11 sequences already exist.<ref>{{cite web |title=Bug 733423 – Please set COLORFGBG environment variable for automatic color detection |url=https://bugzilla.gnome.org/show_bug.cgi?id=733423 |website=bugzilla.gnome.org}}</ref>