How to determine a terminal's background color?

There's an xterm control sequence for this:

\e]11;?\a

(\e and \a are the ESC and BEL characters, respectively.)

Xterm-compatible terminals should reply with the same sequence, with the question mark replaced by an X11 color name, e.g. rgb:0000/0000/0000 for black.

I've came up with the following:

#!/bin/sh
#
# Query a property from the terminal, e.g. background color.
#
# XTerm Operating System Commands
#     "ESC ] Ps;Pt ST"

oldstty=$(stty -g)

# What to query?
# 11: text background
Ps=${1:-11}

stty raw -echo min 0 time 0
# stty raw -echo min 0 time 1
printf "\033]$Ps;?\033\\"
# xterm needs the sleep (or "time 1", but that is 1/10th second).
sleep 0.00000001
read -r answer
# echo $answer | cat -A
result=${answer#*;}
stty $oldstty
# Remove escape at the end.
echo $result | sed 's/[^rgb:0-9a-f/]\+$//'

Source/Repo/Gist: https://gist.github.com/blueyed/c8470c2aad3381c33ea3

Some links:

• Xterm escape code:
  • http://www.talisman.org/~erlkonig/documents/xterm-color-queries/
  • https://invisible-island.net/xterm/ctlseqs/ctlseqs.html (dynamic colors / Request Termcap/Terminfo String)
  • http://thrysoee.dk/xtermcontrol/ (xtermcontrol --get-bg)
  • https://gist.github.com/blueyed/c8470c2aad3381c33ea3
  • https://github.com/rocky/bash-term-background/blob/master/term-background.sh
• COLORFGBG environment variable (used by Rxvt but not many others...):
  It is set to sth like <foreground-color>:[<other-setting>:]<background-color>, (<other setting>: is optional), and if <background-color> in {0,1,2,3,4,5,6,8}, then we have some dark background.
• Vim:
  • code: https://github.com/vim/vim/blob/05c00c038bc16e862e17f9e5c8d5a72af6cf7788/src/option.c#L3974
  • How does Vim guess background color on xterm?
• Emacs... (background-mode) (I think it uses the escape code)
• Related questions / reports / discussions:
  • Is there a way to determine a terminal's background color?
  • How does Vim guess background color on xterm?
  • https://unix.stackexchange.com/questions/245378/common-environment-variable-to-set-dark-or-light-terminal-background
  • https://bugzilla.gnome.org/show_bug.cgi?id=733423
  • https://github.com/neovim/neovim/issues/2764

E.g. some related snippet from Neovim issue 2764:

/*
 * Return "dark" or "light" depending on the kind of terminal.
 * This is just guessing!  Recognized are:
 * "linux"         Linux console
 * "screen.linux"   Linux console with screen
 * "cygwin"        Cygwin shell
 * "putty"         Putty program
 * We also check the COLORFGBG environment variable, which is set by
 * rxvt and derivatives. This variable contains either two or three
 * values separated by semicolons; we want the last value in either
 * case. If this value is 0-6 or 8, our background is dark.
 */
static char_u *term_bg_default(void)
{
  char_u      *p;

  if (STRCMP(T_NAME, "linux") == 0
      || STRCMP(T_NAME, "screen.linux") == 0
      || STRCMP(T_NAME, "cygwin") == 0
      || STRCMP(T_NAME, "putty") == 0
      || ((p = (char_u *)os_getenv("COLORFGBG")) != NULL
          && (p = vim_strrchr(p, ';')) != NULL
          && ((p[1] >= '0' && p[1] <= '6') || p[1] == '8')
          && p[2] == NUL))
    return (char_u *)"dark";
  return (char_u *)"light";
}

About COLORFGBG env, from Gnome BugZilla 733423:

Out of quite a few terminals I've just tried on linux, only urxvt and konsole set it (the ones that don't: xterm, st, terminology, pterm). Konsole and Urxvt use different syntax and semantics, i.e. for me konsole sets it to "0;15" (even though I use the "Black on Light Yellow" color scheme - so why not "default" instead of "15"?), whereas my urxvt sets it to "0;default;15" (it's actually black on white - but why three fields?). So in neither of these two does the value match your specification.

This is some own code I'm using (via):

def is_dark_terminal_background():
    """
    :return: Whether we have a dark Terminal background color, or None if unknown.
        We currently just check the env var COLORFGBG,
        which some terminals define like "<foreground-color>:<background-color>",
        and if <background-color> in {0,1,2,3,4,5,6,8}, then we have some dark background.
        There are many other complex heuristics we could do here, which work in some cases but not in others.
        See e.g. `here <https://stackoverflow.com/questions/2507337/terminals-background-color>`__.
        But instead of adding more heuristics, we think that explicitly setting COLORFGBG would be the best thing,
        in case it's not like you want it.
    :rtype: bool|None
    """
    if os.environ.get("COLORFGBG", None):
        parts = os.environ["COLORFGBG"].split(";")
        try:
            last_number = int(parts[-1])
            if 0 <= last_number <= 6 or last_number == 8:
                return True
            else:
                return False
        except ValueError:  # not an integer?
            pass
    return None  # unknown (and bool(None) == False, i.e. expect light by default)