git: diff between file in local repo and origin

If [remote-path] and [local-path] are the same, you can do

$ git fetch origin master
$ git diff origin/master -- [local-path]

Note 1: The second command above will compare against the locally stored remote tracking branch. The fetch command is required to update the remote tracking branch to be in sync with the contents of the remote server. Alternatively, you can just do

$ git diff master:<path-or-file-name>

Note 2: master can be replaced in the above examples with any branch name

To view the differences going from the remote file to the local file:

git diff remotename/branchname:remote/path/file1.txt local/path/file1.txt

To view the differences in the other direction:

git diff HEAD:local/path/file1.txt remotename/branchname:remote/path/file1.txt

Basically you can diff any two files anywhere using this notation:

git diff ref1:path/to/file1 ref2:path/to/file2

As usual, ref1 and ref2 could be branch names, remotename/branchname, commit SHAs, etc.

To compare local repository with remote one, simply use the below syntax:

git diff @{upstream}