How do I "inverse" a diff file?

Question

Say I have a diff file looking basically like the following.

+line a
-line b

Is it possible to do one (or both) of the following:

• Inverse this file (so I'd get)

  -line a
  +line b
  

• Pass some argument to patch so the end result the same as applying the inversed diff file described above

Answer 1

Here is what you should do (assuming newFile.txt is the file you want to apply the reversed diff file on and diffFile.txt is the diff file):

patch -R newFile.txt diffFile.txt -o oldFile.txt

Answer 2

To rewrite a reversed / inverted diff file, use interdiff from diffutils:

interdiff -q my-diff-file /dev/null

Answer 3

You can leave the diff as is and apply in reverse

git apply --reverse backwards-diff