diffutils: Backups
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' (⇒
Reversed Patches). However, when conforming to POSIX, 'patch' does
not create backup files by default. ⇒patch and POSIX.
The '--backup' ('-b') 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.