Any hex color value to xterm256 approximator bash script

approx256.sh

#!/usr/bin/env bash

function show_help {
    echo "Approximates an xterm256 color

  Usage: approx256 <HEX code>

  Examples: 
    approx256 #FFAABB
    approx256 #ffaabb
    approx256 FFAABB
    approx256 ffaabb

  Codes shorter or longer than 6 characters are invalid."
}

function run {
    if [ -z "$1" ] || [ "$1" = "-h" ] || [ "$1" = "--help" ]; then
        show_help
        return
    fi

    if [ "${1:0:1}" = "#" ]; then
        # remove leading hash sign
        input_color="${1:1}"
    else
        input_color="$1"
    fi

    if ! [ "${#input_color}" = "6" ]; then
        echo "$input_color is not a valid hex color value

use -h, --help flag to get some formatting examples."
        return 1
    fi
    # convert to uppercase for bc
    input_color="${input_color^^}"

    r1="${input_color:0:2}"
    g1="${input_color:2:2}"
    b1="${input_color:4:2}"

    xterm256=( "000000" "800000" "008000" "808000" "000080" "800080" "008080" "C0C0C0" "808080" "FF0000" "00FF00" "FFFF00" "0000FF" "FF00FF" "00FFFF" "FFFFFF" "000000" "00005F" "000087" "0000AF" "0000D7" "0000FF" "005F00" "005F5F" "005F87" "005FAF" "005FD7" "005FFF" "008700" "00875F" "008787" "0087AF" "0087D7" "0087FF" "00AF00" "00AF5F" "00AF87" "00AFAF" "00AFD7" "00AFFF" "00D700" "00D75F" "00D787" "00D7AF" "00D7D7" "00D7FF" "00FF00" "00FF5F" "00FF87" "00FFAF" "00FFD7" "00FFFF" "5F0000" "5F005F" "5F0087" "5F00AF" "5F00D7" "5F00FF" "5F5F00" "5F5F5F" "5F5F87" "5F5FAF" "5F5FD7" "5F5FFF" "5F8700" "5F875F" "5F8787" "5F87AF" "5F87D7" "5F87FF" "5FAF00" "5FAF5F" "5FAF87" "5FAFAF" "5FAFD7" "5FAFFF" "5FD700" "5FD75F" "5FD787" "5FD7AF" "5FD7D7" "5FD7FF" "5FFF00" "5FFF5F" "5FFF87" "5FFFAF" "5FFFD7" "5FFFFF" "870000" "87005F" "870087" "8700AF" "8700D7" "8700FF" "875F00" "875F5F" "875F87" "875FAF" "875FD7" "875FFF" "878700" "87875F" "878787" "8787AF" "8787D7" "8787FF" "87AF00" "87AF5F" "87AF87" "87AFAF" "87AFD7" "87AFFF" "87D700" "87D75F" "87D787" "87D7AF" "87D7D7" "87D7FF" "87FF00" "87FF5F" "87FF87" "87FFAF" "87FFD7" "87FFFF" "AF0000" "AF005F" "AF0087" "AF00AF" "AF00D7" "AF00FF" "AF5F00" "AF5F5F" "AF5F87" "AF5FAF" "AF5FD7" "AF5FFF" "AF8700" "AF875F" "AF8787" "AF87AF" "AF87D7" "AF87FF" "AFAF00" "AFAF5F" "AFAF87" "AFAFAF" "AFAFD7" "AFAFFF" "AFD700" "AFD75F" "AFD787" "AFD7AF" "AFD7D7" "AFD7FF" "AFFF00" "AFFF5F" "AFFF87" "AFFFAF" "AFFFD7" "AFFFFF" "D70000" "D7005F" "D70087" "D700AF" "D700D7" "D700FF" "D75F00" "D75F5F" "D75F87" "D75FAF" "D75FD7" "D75FFF" "D78700" "D7875F" "D78787" "D787AF" "D787D7" "D787FF" "D7AF00" "D7AF5F" "D7AF87" "D7AFAF" "D7AFD7" "D7AFFF" "D7D700" "D7D75F" "D7D787" "D7D7AF" "D7D7D7" "D7D7FF" "D7FF00" "D7FF5F" "D7FF87" "D7FFAF" "D7FFD7" "D7FFFF" "FF0000" "FF005F" "FF0087" "FF00AF" "FF00D7" "FF00FF" "FF5F00" "FF5F5F" "FF5F87" "FF5FAF" "FF5FD7" "FF5FFF" "FF8700" "FF875F" "FF8787" "FF87AF" "FF87D7" "FF87FF" "FFAF00" "FFAF5F" "FFAF87" "FFAFAF" "FFAFD7" "FFAFFF" "FFD700" "FFD75F" "FFD787" "FFD7AF" "FFD7D7" "FFD7FF" "FFFF00" "FFFF5F" "FFFF87" "FFFFAF" "FFFFD7" "FFFFFF" "080808" "121212" "1C1C1C" "262626" "303030" "3A3A3A" "444444" "4E4E4E" "585858" "606060" "666666" "767676" "808080" "8A8A8A" "949494" "9E9E9E" "A8A8A8" "B2B2B2" "BCBCBC" "C6C6C6" "D0D0D0" "DADADA" "E4E4E4" "EEEEEE" )

    delta_min=999999
    closest_color_code=""
    closest_color_hex=""

    for i in {0..255}; do
        color="${xterm256[$i]}"
        r2="${color:0:2}"
        g2="${color:2:2}"
        b2="${color:4:2}"

        delta="$(echo "ibase=16;($r2-$r1)^2 + ($g2-$g1)^2 + ($b2-$b1)^2" | bc)"

        if [ "$delta" -lt "$delta_min" ]; then
            delta_min="$delta"
            closest_color_code="$i"
            closest_color_hex="$color"
        fi
    done
    
    # bg_sample foreground color calculation
    r="$(echo "ibase=16; ${closest_color_hex:0:2}" | bc)"
    g="$(echo "ibase=16; ${closest_color_hex:2:2}" | bc)"
    b="$(echo "ibase=16; ${closest_color_hex:4:2}" | bc)"
    # calculation is from Mark Ransom's answer on stackoverflow https://stackoverflow.com/a/3943023/10364335
    intensity="$(echo "$r*0.299 + $g*0.587 + $b*0.114" | bc)"
    intensity="$(echo "scale=0; $intensity/1" | bc)"

    if [ "$intensity" -gt 186 ]; then
        bg="$(tput setab $closest_color_code)$(tput setaf 0)"
    else
        bg="$(tput setab $closest_color_code)$(tput setaf 15)"
    fi

    fg="$(tput setaf $closest_color_code)"
    rst="$(tput sgr0)"

    if [ "$closest_color_code" -lt 10 ]; then
        leftpad="      "
    elif [ "$closest_color_code" -lt 100 ]; then
        leftpad="     "
    else
        leftpad="    "
    fi

    echo "
    Closest xterm256 color to #${input_color,,} is

    ${bg}${fg}*************${rst}  ${fg}A foreground sample${rst}
    ${bg}${fg}*************${rst}
    ${bg}${fg}*************${rst}  ${bg}A background sample${rst}
    ${bg}${fg}*************${rst}
    ${bg}${fg}*************${rst}  code:       $leftpad$closest_color_code
    ${bg}${fg}*************${rst}  HEX:        #${closest_color_hex,,}"
}

run "$@"