spongebob meme bash function string converter

spongebob.sh

#!/usr/bin/env bash
set -euo pipefail

input="$*"
length="${#input}"

charindex=0
spongebob_str=""

while [ $charindex -lt $length ]; do
    char="${input:$charindex:1}"
    if ! [[ $char == [[:space:]] ]]; then
        # 50% chance to convert
        if [ $(($RANDOM % 10)) -lt 5 ]; then
            char="$(echo ${char} | tr '[[:lower:]]' '[[:upper:]]')"
        else
            char="$(echo ${char} | tr '[:upper:]' '[:lower:]')"
        fi
    fi
    spongebob_str="${spongebob_str}$char"
    charindex=$(( $charindex + 1 ))
done
echo "${spongebob_str}"
exit 0