bash_print_rainbow.sh

# this logic is based on the "using out-of-phase sine waves to make rainbows" section described
# here https://krazydad.com/tutorials/makecolors.php, implemented in bash.
# Uses the more modern rgb escape code sequence (ESC[38;2;{red};{green};{blue}m), so will only work on newer terminals
# It's uh, very slow
function print_rainbow {
  local text_to_print=$1
  local result=""
  local frequency
  frequency=$(echo "3.14159 * 1.8 / ${#text_to_print}" | bc -l)
  for (( i=0; i<${#text_to_print}; i++ )); do
    red=$(echo "s(${frequency}*$i + 0) * 127 + 128" | bc -l | cut -d "." -f 1)
    green=$(echo "s(${frequency}*$i + 2) * 127 + 128" | bc -l | cut -d "." -f 1)
    blue=$(echo "s(${frequency}*$i + 4) * 127 + 128" | bc -l | cut -d "." -f 1)
    result="$result\033[38;2;${red};${green};${blue}m${text_to_print:$i:1}\033[0m"
  done

  echo -e "$result"
}