Zsh solution for golf challenge https://codegolf.stackexchange.com/a/189756/15940

ssttssttrrriininnnnnnniiinngg.sh

#!/bin/zsh
# using bits of https://codegolf.stackexchange.com/a/189073
# and https://codegolf.stackexchange.com/a/182568
for ((;i<$#1;i++)){
s=${1:$i:1}   #current char
m=${1:$i:2}   #current and next char (length 2)
m=$m:l$m:u    #lower + upper cases (length 4)
for ((t=0;t<$RANDOM%5;t++)){
x+=${m:$RANDOM%4:1}
}
printf $s$x
x=
}