flipdiff

flipdiff [options] [diff-file]

-
    Read from standard input (default).

[diff-file]
    The diff file to process. If omitted, reads from standard input.

The flipdiff command analyzes a unified diff file and generates a new diff that, when applied, reverses the changes described in the original diff.

It essentially swaps the "+" and "-" lines in the diff, effectively undoing the original patch. This is useful for reverting patches, creating "undo patches", or testing changes by applying the reversed diff to the patched file to restore it to its original state.

Flipdiff is typically used in conjunction with tools like diff and patch in software development workflows to manage and manipulate changes between different versions of files.
Note, that some diff tools can generate a diff file with swapped files (diff fileA fileB) instead of (diff fileB fileA). In this case, flipdiff is not the tool you are looking for.

To reverse a patch contained in the file 'my_patch.diff', use:
flipdiff my_patch.diff > my_reverse_patch.diff.
Then you can revert changes using patch -p1 < my_reverse_patch.diff

diff(1), patch(1)