How Does backup-directory-alist Interact with version-control and backup-by-copying When Restoring Centralized Backups?

When backup-directory-alist is used to centralize backups, Emacs transforms the original absolute path into a single filename by replacing directory separators (/) with exclamation marks (!). The interaction with version-control and backup-by-copying determines the final filename and the metadata of the resulting file during restoration.

Naming Schemes and Version Control

The transformed naming scheme follows a strict pattern: [backup-directory] + [!transformed-path] + [version-suffix]. If version-control is active (set to t or numbered), Emacs appends a numbered suffix to allow multiple versions. If version-control is nil, only a single backup is kept with a trailing tilde.

Configuration	Example Path	Transformed Backup Name
version-control nil	/home/user/doc.txt	!home!user!doc.txt~
version-control t	/home/user/doc.txt	!home!user!doc.txt.~1~, ...~2~

Metadata and backup-by-copying

The backup-by-copying variable affects the creation of the backup, not the restoration process. When enabled, Emacs copies the file instead of renaming it, meaning the backup file inherits the original file's permissions and modification time at the moment the backup was created.

During restoration, Emacs provides no automated tool to "undo" this. If you manually copy a backup back to the original location (e.g., via cp), the restored file inherits the metadata of the backup file, not the metadata of the original file as it existed before the backup was taken. To preserve the backup's timestamps during restoration, use cp -p.

Practical Restoration Workflow

Since there is no built-in command to map a current buffer back to its centralized backup, follow these steps to identify the correct version:

1. Calculate the prefix: Replace all / in the absolute path of your file with !.
2. List candidates: Use a shell command to find all matches in your backup directory, sorted by time:

   ls -t ~/.emacs.d/backups/!home!user!project!file.c*

3. Identify the version: The first result is the most recent. If version-control is numbered, the highest number corresponds to the latest save.
4. Verify and Restore: Inspect the file size and modification time with stat before copying it back to the original path.

Assumptions and Constraints

This behavior assumes a Unix-like filesystem. On Windows, drive letters are typically handled by replacing the colon (e.g., C: becomes !C!). Note that kept-new-versions and kept-old-versions prune files during the save process; if a version has been pruned, it cannot be recovered via Emacs.