Sometimes people run diff with the new file first instead of
second. This creates a diff that is "reversed". To apply such
patches, give patch the `-R' or `--reverse' option.
patch then attempts to swap each hunk around before applying it.
Rejects come out in the swapped format. The `-R' option does not
work with ed scripts because there is too little information in
them to reconstruct the reverse operation.
Often patch can guess that the patch is reversed. If the first
hunk of a patch fails, patch reverses the hunk to see if it can
apply it that way. If it can, patch asks you if you want to have
the `-R' option set; if it can't, patch continues to apply
the patch normally. This method cannot detect a reversed patch if it is
a normal diff and the first command is an append (which should have been
a delete) since appends always succeed, because a null context matches
anywhere. But most patches add or change lines rather than delete them,
so most reversed normal diffs begin with a delete, which fails, and
patch notices.
If you apply a patch that you have already applied, patch thinks
it is a reversed patch and offers to un-apply the patch. This could be
construed as a feature. If you did this inadvertently and you don't
want to un-apply the patch, just answer `n' to this offer and to
the subsequent "apply anyway" question--or type C-c to kill the
patch process.