10.8 Backup Files

Normally, patch creates a backup file if the patch does not exactly match the original input file, because in that case the original data might not be recovered if you undo the patch with `patch -R' (see section 10.3.2 Applying Reversed Patches). However, when conforming to POSIX, patch does not create backup files by default. See section 10.12 patch and the POSIX Standard.

The `-b' or `--backup' option causes patch to make a backup file regardless of whether the patch matches the original input. The `--backup-if-mismatch' option causes patch to create backup files for mismatches files; this is the default when not conforming to POSIX. The `--no-backup-if-mismatch' option causes patch to not create backup files, even for mismatched patches; this is the default when conforming to POSIX.

When backing up a file that does not exist, an empty, unreadable backup file is created as a placeholder to represent the nonexistent file.