bash: terminal truecolor test. proof of concept, completely made of pure bash builtins

termTestTruecolor.sh

#!/usr/bin/env bash
#
# Terminal truecolor test
# Proof of concept, completely made of pure bash builtins
#
# Accepts an integer from 14 up to infinity to set the length of test snake
# By default the snake is 88 symbols long
#
# Execute or source termTestTruecolor.sh to export function
#
#     termTestTrucolor
#     termTestTrucolor 100

termTestTruecolor () {
    local red
    local green
    local blue
    local vga=255
    local colnum=1
    local maxcol=${1}

    [[ ${maxcol} =~ ^[0-9]+$ && ${maxcol} -ge 14 ]] || maxcol=88

    printf "\n"

    while [ ${colnum} -le ${maxcol} ]
    do
        red=$(( vga - colnum * vga / maxcol ))

        green=$(( colnum  * (vga * 2) / maxcol ))
        [ ${green} -gt ${vga} ] && green=$(( (vga * 2) - green ))

        blue=$(( colnum  * vga / maxcol ))

        printf "\033[48;2;%d;%d;%dm\033[38;2;%d;%d;%dm \033[0m" \
            ${red} ${green} ${blue} $(( vga - red )) $(( vga - green )) $(( vga - blue ))

        let colnum++
    done

    printf "\033[K\n\n"
}

export -f termTestTruecolor