指定ディレクトリ以下の .txt を一斉に .md にリネームする (あるいはその逆をする) スクリプト (混在時はエラー)

convexts.sh

#!/usr/bin/bash
# Ex. ./convexts.sh ./Dropbox/obsidian/Mercury/References/
set -euo pipefail  # Fail fast on errors, undefined variables, and broken pipelines
[ "$#" -eq 1 ] || { echo "Usage: $0 <target_dir>" >&2; exit 1; }
target_dir=$1
[ -e "$target_dir" ] || { echo "Error: not found: $target_dir" >&2; exit 2; }
cd "$target_dir"

shopt -s nullglob  # Make globs expand to empty when no files match
li_txt=( *.txt )
li_md=( *.md )

[ ${#li_txt[@]} -gt 0 ] && [ ${#li_md[@]} -gt 0 ] && { echo "Error: Mixed exts" >&2; exit 3; }
[ ${#li_txt[@]} -gt 0 ] && { li=( "${li_txt[@]}" ); from=txt; to=md; }
[ ${#li_md[@]} -gt 0 ] && { li=( "${li_md[@]}" ); from=md; to=txt; }
[ -v li ] || { echo "Error: No targets" >&2; exit 4; }

for src in "${li[@]}"; do
  dst="${src%.$from}.$to"
  mv -- "$src" "$dst"
  echo $dst
done