terminal_256colors.sh

#!/usr/bin/env bash

# https://gist.github.com/ivanbrennan/8ce10a851851e5f04728d8da900ef1c5
# https://gist.github.com/mgedmin/2762225

PADDING='Padding'

main() {
	local xterm_start=0 \
				xterm_width=8 \
				xterm_height=2

	local cube_start=$((xterm_start + xterm_width * xterm_height)) \
				cube_width=6 \
				cube_height=$((6 * 6))

	local greys_start=$((cube_start + cube_width * cube_height)) \
				greys_width=8 \
				greys_height=3

	color_block $xterm_start $xterm_width $xterm_height
	color_block $cube_start  $cube_width  $cube_height  use_padding
	color_block $greys_start $greys_width $greys_height
	echo
}

color_block() {
	local start=$1 width=$2 height=$3 use_padding=$4
	local max s color_nums colors

	max=$((start + width * height - 1))

	echo
	for s in $(seq "$start" "$width" "$max"); do
		color_nums=$(seq "$s" $((s + width - 1)))
		colors="${use_padding:+$PADDING }${color_nums}${use_padding:+ $PADDING}"

		printf '%s%s    %s%s\n' \
			"$(fg_bars "$colors")" "$ansi_reset" \
			"$(bg_bars "$colors")" "$ansi_reset"
	done
}

fg_bars() {
	for color in "$@"; do
		color_bar ansi_fg "$color" ''
	done
}

bg_bars() {
	for color in "$@"; do
		color_bar ansi_bg "$color" ' '
	done
}

color_bar() {
	local ansi=$1 color=$2 trail=$3

	if [ "$color" == $PADDING ]; then
		printf '%s    %s' "$ansi_reset" "$trail"
	else
		local color_seq
		color_seq=$("$ansi" "$color")
		printf '%s %03d%s' "$color_seq" "$color" "$trail"
	fi
}

ansi_reset=$'\033[0m'

ansi_fg() {
	printf '\033[38;5;%dm' "$1"
}

ansi_bg() {
	printf '\033[48;5;%dm' "$1"
}

main