Remove Vim undo files when the target doesn't exist anymore

cleanup_undo_dir.sh

#!/bin/bash

set -euo pipefail

undodir=${XDG_CACHE_HOME:-$HOME/.cache}/vim/undo/

pushd "$undodir" >/dev/null

for undo_file in *; do
    real_file=${undo_file//%/\/}
    [[ -f "$real_file" ]] || rm -v -- "$undo_file"
done

popd >/dev/null