mkTree.sh

#!/usr/bin/env bash
set -euo pipefail
# make a tree directory structure
root=testdir
letters=(a b c d e f g h i j)

mkdir -p "$root"

for ((s=0; s<${#letters[@]}; s++)); do
  chain=()
  for ((k=s; k<${#letters[@]}; k++)); do
    n=$(( s*10 + (k - s + 1) ))
    chain+=( "${letters[k]}$n" )
  done
  (IFS=/; mkdir -p "$root/${chain[*]}")
done