How to do a git diff on moved/renamed file?

I moved a file using git mv. Now I would like to do a diff on the new file to compare it with the old file (with the old, now non-existent name).

How do I do this?

You need to use -M to let git autodetect the moved file when diffing. Using just git diff as knittl mentioned does not work for me.

So simply: git diff -M should do it.

The documentation for this switch is:

-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.

In addition to what knittl wrote, you can always use:

git diff HEAD:./oldfilename newfilename

where HEAD:./oldfilename means oldfilename in the last commit (in HEAD), relative to current directory.

If you don't have new enough git, you would have to use instead:

git diff HEAD:path/to/oldfilename newfilename

With git 2.9 (June 2016), you won't have to add -M anymore. git diff uses -M by default.

See commit 5404c11, commit 9501d19, commit a9276a6, commit f07fc9e, commit 62df1e6 (25 Feb 2016) by Matthieu Moy (moy).
^{(Merged by Junio C Hamano -- gitster -- in commit 5d2a30d, 03 Apr 2016)}

diff: activate diff.renames by default

Rename detection is a very convenient feature, and new users shouldn't have to dig in the documentation to benefit from it.

Potential objections to activating rename detection are that it sometimes fail, and it is sometimes slow. But rename detection is already activated by default in several cases like "git status" and "git merge", so activating diff.renames does not fundamentally change the situation. When the rename detection fails, it now fails consistently between "git diff" and "git status".

This setting does not affect plumbing commands, hence well-written scripts will not be affected.

The new tests for this feature are here.

For whatever reason using HEAD:./oldfilename (or absolute path) didn’t work for me, but HEAD:oldfilename did (thanks cmn):

git diff HEAD:oldfilename newfilename
git diff 2a80f45:oldfilename f65f3b3:newfilename

HTH