spectrum

# prints a spectrum of colors
# only work in terminals with 24-bit color support

tt="▇"

#fit to terminal width
st=$(echo "255 / ( $(tput cols) / 6 )" | bc)

#starting values
r=255
g=0
b=0

#print the colors
for ((g=0; g<256; g+=$st)); do
    printf "\x1b[38;2;${r};${g};${b}m$tt\x1b[0m"
    ((cols++))
done
g=255

for ((r=255-$st; r>-1; r-=$st)); do
    printf "\x1b[38;2;${r};${g};${b}m$tt\x1b[0m"
    ((cols++))
done
r=0

for ((b=0+$st; b<256; b+=$st)); do
    printf "\x1b[38;2;${r};${g};${b}m$tt\x1b[0m"
    ((cols++))
done
b=255

for ((g=255-$st; g>-1; g-=$st)); do
    printf "\x1b[38;2;${r};${g};${b}m$tt\x1b[0m"
    ((cols++))
done
g=0

for ((r=0+$st; r<256; r+=$st)); do
    printf "\x1b[38;2;${r};${g};${b}m$tt\x1b[0m"
    ((cols++))
done
r=255

for ((b=255-$st; b>-1; b-=$st)); do
    printf "\x1b[38;2;${r};${g};${b}m$tt\x1b[0m"
    ((cols++))

    #cut off if there are more character than can fit in the terminal width
    if [ "$cols" -eq "$(tput cols)" ]; then
        break
    fi
done
r=255


#print extra characters to fill terminal width
while [ "$cols" -lt "$(tput cols)" ]; do
    printf "\x1b[38;2;${r};${g};${b}m$tt\x1b[0m"
    ((cols++))
done

printf "\n"