git diff renamed file

To diff across a rename of a specific file, use -M -- <old-path> <new-path> (-C also works).

So if you both renamed and changed a file in the last commit, you can see the changes with:

git diff HEAD^ HEAD -M -- a.txt test/a.txt

This produces:

diff --git a/a.txt b/test/a.txt
similarity index 55%
rename from a.txt
rename to test/a.txt
index 3f855b5..949dd15 100644
--- a/a.txt
+++ b/test/a.txt
@@ -1,3 +1,3 @@
 // a.txt

-hello
+goodbye

(// a.txt lines added to help git detect the rename)

---

If git isn't detecting the rename, you can specify a low similarity threshold with -M[=n], say 1%:

git diff HEAD^ HEAD -M01 -- a.txt test/a.txt

From the git diff docs:

-M[<n>] --find-renames[=<n>]

Detect renames. If n is specified, it is a threshold on the similarity index (i.e. amount of addition/deletions compared to the file's size). For example, -M90% means Git should consider a delete/add pair to be a rename if more than 90% of the file hasn't changed. Without a % sign, the number is to be read as a fraction, with a decimal point before it. I.e., -M5 becomes 0.5, and is thus the same as -M50%. Similarly, -M05 is the same as -M5%. To limit detection to exact renames, use -M100%. The default similarity index is 50%.

The issue with the difference between HEAD^^ and HEAD is that you have an a.txt in both commits, so just considering those two commits (which is what diff does), there is no rename, there is a copy and a change.

To detect copies, you can use -C:

git diff -C HEAD^^ HEAD

Result:

index ce01362..dd7e1c6 100644
--- a/a.txt
+++ b/a.txt
@@ -1 +1 @@
-hello
+goodbye
diff --git a/a.txt b/test/a.txt
similarity index 100%
copy from a.txt
copy to test/a.txt

Incidentally, if you restrict your diff to just one path (as you do in git diff HEAD^^ HEAD a.txt you aren't ever going to see the renames or copies because you've excluded the everything apart from a single path and renames or copies - by definition - involve two paths.