Getting a Terminal’s Default Foreground & Background Colors

When using ANSI escape sequences to style text on a terminal, you may need to know exactly what the default foreground & background colors — the ones set with \e[39m and \e[49m — are, as knowing whether the user’s terminal is light-on-dark or dark-on-light can help you pick an appropriate color theme. Fortunately, the escape sequences supported by xterm and compatible terminals include sequences for doing just that.

A program whose standard input & standard output are both connected to an xterm-compatible terminal can query the default foreground color by writing the characters \e]10;?\e\\\\ to standard output, where \e is the Escape character (0x1B) and \\\\ represents a single backslash; to query the default background color instead, change the 10 to an 11. The terminal will then respond by writing back the query string with the ? replaced by a string describing the default foreground or background color.

In my limited experience, all color strings I’ve seen in responses have been RGB values of the form rgb:XXXX/XXXX/XXXX, where the X’s are lowercase hexadecimal digits (so white would be rgb:ffff/ffff/ffff and black would be rgb:0000/0000/0000), but xterm’s documentation seems to suggest that color names (presumably ones from the X11 color list) and any RGB specifications accepted by XParseColor(3) are also possible return values.

As with getting the current cursor position, the terminal will need to be set in cbreak and noecho modes when reading the response.

If you just want to see some code for doing all this on a Unix-like system, here it is as a Python script:

from __future__ import annotations
from collections.abc import Iterator
from contextlib import contextmanager
from copy import deepcopy
import re
import sys
import termios


def get_default_fg() -> str:
    """
    Query the attached terminal for the default foreground color and return the
    color string from the response

    :raises IOError: if stdin or stdout is not a terminal
    :raises ValueError: if the reply from the terminal is malformed
    """
    return osc_query(10)


def get_default_bg() -> str:
    """
    Query the attached terminal for the default background color and return the
    color string from the response

    :raises IOError: if stdin or stdout is not a terminal
    :raises ValueError: if the reply from the terminal is malformed
    """
    return osc_query(11)


def osc_query(ps: int) -> str:
    if sys.stdin.isatty() and sys.stdout.isatty():
        with cbreak_noecho():
            print(f"\x1b]{ps};?\x1b\\", end="", flush=True)
            resp = b""
            while not resp.endswith((b"\x1b\\", b"\x07")):
                resp += sys.stdin.buffer.read(1)
        s = resp.decode("utf-8", "surrogateescape")
        if m := re.fullmatch(rf"\x1B\]{ps};(.+)(?:\x1B\\|\x07)", s):
            return m[1]
        else:
            raise ValueError(s)
    else:
        raise IOError("not connected to a terminal")


# File descriptor for standard input:
STDIN = 1

# Indices into the tuple returned by `tcgetattr()`:
LFLAG = 3
CC = 6


@contextmanager
def cbreak_noecho() -> Iterator[None]:
    """
    A context manager that configures the terminal on standard input to use
    cbreak mode and to disable input echoing.  The original terminal
    configuration is restored on exit.
    """
    orig = termios.tcgetattr(STDIN)
    term = deepcopy(orig)
    term[LFLAG] &= ~(termios.ICANON | termios.ECHO)
    term[CC][termios.VMIN] = 1
    term[CC][termios.VTIME] = 0
    termios.tcsetattr(STDIN, termios.TCSANOW, term)
    try:
        yield
    finally:
        termios.tcsetattr(STDIN, termios.TCSANOW, orig)


if __name__ == "__main__":
    print("Foreground color:", get_default_fg())
    print("Background color:", get_default_bg())