git - How to compare files from two different branches

Question

I have a script that works fine in one branch and is broken in another. I want to look at the two versions side-by-side and see what's different. Is there a way to do this?

To be clear I'm not looking for a compare tool (I use Beyond Compare). I'm looking for a Git diff command that will allow me to compare the master version to my current branch version to see what has changed. I'm not in the middle of a merge or anything. I just want to say something like

git diff mybranch/myfile.cs master/myfile.cs

Answer 1

git diff can show you the difference between two commits:

git diff mybranch master -- myfile.cs

Or, equivalently:

git diff mybranch..master -- myfile.cs

Note you must specify the relative path to the file. So if the file were in the src directory, you'd say src/myfile.cs instead of myfile.cs.

Using the latter syntax, if either side is HEAD it may be omitted (e.g., master.. compares master to HEAD).

You may also be interested in mybranch...master (from git diff documentation):

This form is to view the changes on the branch containing and up to the second <commit>, starting at a common ancestor of both <commit>. git diff A...B is equivalent to git diff $(git-merge-base A B) B.

In other words, this will give a diff of changes in master since it diverged from mybranch (but without new changes since then in mybranch).

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In all cases, the -- separator before the file name indicates the end of command line flags. This is optional unless Git will get confused if the argument refers to a commit or a file, but including it is not a bad habit to get into. See Dietrich Epp's answer to Meaning of Git checkout double dashes for a few examples.

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The same arguments can be passed to git difftool if you have one configured.