#!/bin/bash
# https://tldp.org/HOWTO/Bash-Prompt-HOWTO/x329.html
# https://gist.github.com/XVilka/8346728#gistcomment-3528771

T='::'   # The test text

function echo_block() {
    local fs=$1
    local bs=$2
    echo "   01234567"
    for (( f = 0; f < 8; f++ )); do
        echo -en " ${f} "
        for (( b = 0; b < 8; b++ )); do
            echo -en "\033[38;5;$((fs+f))m\033[48;5;$((bs+b))m$T\033[0m";
        done
        echo
    done
}

function print_colors() {
    local c
    for (( c = $1; c < $1 + $2; c++)); do
        echo -en "\033[48;5;${c}m$T\033[0m"
    done
    echo
}
function colors() {
    print_colors 0 8
    print_colors 8 8
    for (( c = 16; c < 256 - 24; c += 36 )); do
        print_colors $c 36
    done
    print_colors $((256 - 24)) 24
}

function true_colors() {
    awk -v term_cols="${width:-$(tput cols || echo 80)}" 'BEGIN{
        s="/\\";
        for (colnum = 0; colnum<term_cols; colnum++) {
            r = 255-(colnum*255/term_cols);
            g = (colnum*510/term_cols);
            b = (colnum*255/term_cols);
            if (g>255) g = 510-g;
            printf "\033[48;2;%d;%d;%dm", r,g,b;
            printf "\033[38;2;%d;%d;%dm", 255-r,255-g,255-b;
            printf "%s\033[0m", substr(s,colnum%2+1,1);
        }
        printf "\n";
    }'
}

colors
true_colors