rocky / Bash Term Background

These shell scripts, one for bash and one for zsh, triy to determine if your terminal has dark or light background.

If you source this from a shell session it will set and export COLORFGBG to '0;15' for dark backgrounds and '15;0' for light backgrounds which is a convention used by some programs. Since I find this a little arcane, the program also sets and exports DARK_BG to 1 for dark backgrounds and 0 for light.

If are not in a shell then runnning the program and parse the output; output will start either "Dark background", "Light background" or "Can't decide".

The heuristics it uses is to try to query the background color using an xterm control sequence.

Many, but not all, terminals support this query. So as a fallback we query environment variable COLORFGBG and failing this we use some defaults for some known terminals set from the TERM environment variable.

There is one other envirnoment variable and aspect worth mentioning. When we can get pixel intensities of red, blue, and green values of the background, we can use that determine light and dark based the combied sum: zero values indicate an absense of a particular color. However the upper value can change. On a xterm-256color or derivative of that, the highest intensity is 0xff while on an xterm or a deriviative of that (which is also not a derivative of xterm-256color) the highest intensity is 0xffff. The environment variable TERMINAL_COLOR_MIDPOINT has what we think is the midpoint (grey) color value. For xterm-256color it is 383, while for xterm it is 117963.

You can set any of these environment variables to influence the output decision.

Many thanks to Thomas Dickey, Egmont Koblinger, and Gilles, for explanations (and code!) via unix.stackexchange. Of course bugs and lacuna in this code are mine.