flatten_dirs

#!/bin/bash
# Take children of every folder at current level, and move them to the current level.
set -e
set -u
set -o pipefail
shopt -s nullglob

find . -mindepth 1 -maxdepth 1 -type d | while read -r dir ; do
  for f in "$dir"/*; do
    mv -n "$f" "$dir"_"$(basename "$f")"
  done
done