Question

I’m using

git diff mycommit

for comparing my working tree with mycommit, but it seems to ignore files not present in the current index. You can reproduce it as follows:

git init
echo A > A.txt; git add .; git commit -m A; git branch A
echo B > B.txt; git add .; git commit -m B; git branch B
git reset --hard A
echo BB > B.txt
git diff B

The output (as of git version 1.7.3.3) is empty. Using --diff-filter=ACDMRTUXB shows “deleted file” which is wrong as well, since the file B.txt exists both in the working tree and in commit B. IMHO, the file should be shown as modified. Surprisingly, it works after adding the file to the index, although it’s not the index what gets compared. How can I get the right diff without it?

Answer 1

manojlds wrote in his answer

Basically, diff does not take into account untracked files. That is why you see that the file is deleted, because the diff is same as git diff B A

This is true and worth upvoting, but it didn’t satisfy me, as it says neither "how can I compare it" nor gave the deeper reason behind it. I still thought it was a bug, so I asked at the git forum and got a long answer by somebody who surely knows. The important parts for me are:

The definition of paths in the working tree in these sentences is not "all files on the filesystem", or "all files on the filesystem, filtered with the ignore mechanism". It is "all files on the filesystem that are in the index" …

and

…that will give a clean semantics to "git diff HEAD": what change would I be recording if I said "git commit -a" at this point?