Emacs backup file recovery and manual integrity verification

Restoring from Emacs Backups

Use C-x C-w (write-file) to overwrite the current buffer with the auto-save contents, or M-x recover-file to visit the most recent auto-save file in a new buffer. Both commands are built-in and require no additional configuration.

Key Variables Governing Backup Location

The following variables determine where Emacs stores backup and auto-save files:

• make-backup-files – When non-nil, creates filename~ tilde backups.
• version-control – When non-nil, enables numbered backups (e.g., filename~1~, filename~2~).
• backup-directory-alist – A list of cons cells mapping directory patterns to backup locations. Useful for centralizing backups outside project directories.
• auto-save-default – When non-nil, creates #filename# auto-save files.
• auto-save-file-name-transforms – Controls where auto-save files are written, similar to backup-directory-alist.

Permission Handling in Shared Directories

Emacs does not enforce custom permissions on backup files. Files inherit the umask of the Emacs process and the permissions of the containing directory. In shared environments, backup files may be readable by other users. To restrict access:

• Set a restrictive umask before launching Emacs (e.g., umask 077).
• Store backups in a private directory by configuring backup-directory-alist to point to ~/.emacs.d/backups or similar.
• Use a custom before-save-hook function to call set-file-modes on the backup after creation.

Manual Content Comparison Before Recovery

Emacs provides no built-in mechanism to require content comparison before recover-file replaces the buffer. Perform manual verification as follows:

1. Run M-x diff-buffer-with-file on the backup file (filename~ or #filename#) to compare the current buffer with the file on disk.
2. Review the diff output in a separate buffer. Look for unexpected deletions or corruptions.
3. Only after confirming integrity, save the recovered buffer with C-x C-s.

Team Verification Procedures

Teams should establish protocols for verifying recovered data:

• Timestamp checks – Confirm backup and auto-save files are newer than the last known good save.
• External hash comparison – Generate SHA-256 checksums of backups before and after recovery using sha256sum filename~ or shasum -a 256 filename~.
• Access control review – Ensure backup directories have appropriate permissions (e.g., 700 for user-only access).
• Rotation awareness – Know that kept-old-versions and kept-new-versions control how many numbered backups are retained.

Verification Steps

Before relying on recovery, confirm your configuration:

C-h v make-backup-files RET
C-h v backup-directory-alist RET
C-h v auto-save-default RET

Create a test file, make edits, kill the buffer without saving, then run M-x recover-file to verify the process works in your environment.