Code generation - Brace expansion

foo

#!/bin/bash -
gen_sfx()
  case $ext in
  (/*|*//*|*/)
    printf '%q: %q: Invalid specifier\n' "$0" "$ext" >&2
    exit 3 ;;
  (*/*)
    local - IFS=/

    set -f
    set -- $ext
    set -- "${@@Q}"
    IFS=,
    sfx={$*} ;;
  (*)
    sfx='"$ext"'
  esac

case $# in
(0|1)
  printf 'Usage: %q DIR EXT[/EXT...] CMD [ARG...]\n' "$0" >&2
  exit 1 ;;
(2)
  set -- "$@" printf '%s\n' ;&
(*)
  dir=$1 ext=$2
  shift 2
esac

if ! [[ -d $dir ]]; then
  printf '%q: %q: Not a directory\n' "$0" "$dir" >&2
  exit 2
fi

gen_sfx

shopt -s globstar nullglob
eval 'set -- "$@" "$dir"/**/*.'"$sfx"

unset -f gen_sfx
unset dir ext sfx
shopt -u globstar nullglob

"$@"