Create subdirs if needed: Extract parent from rel_path; call 'exec'("mkdir -p [base_dir]/[parent]") or message: "Please run mkdir -p [full_parent] and confirm."
Check existence (call 'read' on full_path):
If exists and write mode: Create backup (read content, find unique .bak.N path, 'write' to it).
Confirm overwrite/append: Use message ("Confirm [action] on [file]? Yes/No").