Order of commit arguments in git diff

Question

In what order does the git command

git diff [--options] <commit> <commit> [--] [<path>…] 

Compare the different commits against each other? It seems like if I want to compare the new against the old one I need to do

git diff [--options] <New_commit> <Old_commit> in order to see the current diff?

I usually do git diff [--options] <Old_commit> <New_commit>

But that seems to be wrong?

When I do for example

$ git diff `git rev-list --since="jun 30 2014" --reverse origin/master | head -1` `git rev-list --until="dec 31 2014" origin/master | head -1` --shortstat   1072 files changed, 389650 insertions(+), 39180 deletions(-) 

But when I do

$ git diff --stat `git rev-list --until="dec 31 2014" origin/master | head -1` 

I get the printout that:

384 files changed, 61255 insertions(+), 20526 deletions(-) 

Which is not near 300000. So my question is if I should insert the new commit first and the old commit sedond, like:

 $ git diff `git rev-list --until="dec 31 2014" origin/master | head -1`..`git rev-list --since="jun 30 2014" --reverse origin/master | head -1`  

I can't find any documentation about in which order I should insert the commits in order to see the difference between my new and my old commit. Perhaps you can clarify this for me?

Thanks in advance.

Edit: The reason I'm asking is that I want to

1. know how many new lines of code that has been added to a new commit given an old commit, and

2. I want to calculate the number of lines of code in the new commit.

Answer 1

TL;DR

In the following git-diff syntax,

git diff [--options] <commit> <commit> [--] [<path>...] 

• the first <commit> corresponds to the base commit,
• the second <commit> corresponds to the commit to compare to the base commit.

Using a mathematically inspired notation,

git diff <x> <x+∆x> 

will show you the difference ∆x, whereas

git diff <x+∆x> <x> 

will show you the difference -∆x.

Note that, because the two commits need not be ordered in any way, either chronologically or topologically, calling them "old" and "new" (as you do) is a bit misleading.

More details

You can learn a great deal simply by looking up the git-diff man page. Under the Description section, you'll find

git diff [--options] <commit> <commit> [--] [<path>...] 

This is to view the changes between two arbitrary <commit>.

Granted, that doesn't tell you which commit is which, but, further down, under the Examples section, you'll find a couple of illuminating examples:

git diff HEAD^ HEAD 

[...] Compare the version before the last commit and the last commit.

and

git diff topic...master 

[...]

Changes that occurred on the master branch since when the topic branch was started off it.