Alternative diff algorithm for special file extension for Git

Question

I want to use an alternative diff algorithm for special file extension, e.g. ".java" and the standart diff algorithm for other files.

I can make it via .gitattributes file:

*.java diff=javadiffprogramm

and .gitconfig file:

[diff "javadiffprogramm"]
    command = pathToDiffProgram.

My question is what is the format of input parameters of "javadiffprogramm" and where it should output the result of diff? The result of diff has the format like "@@ -1,7 +1,6 @@...".

Some questions on this topic:

• Using custom diff tool with git show
• Is it possible to set diff/merge-tool for a specific file extension in git-extensions?
• Setup 'git diff' for special file types

Answer 1

This is documented in the git man page as GIT_EXTERNAL_DIFF.

GIT_EXTERNAL_DIFF

When the environment variable GIT_EXTERNAL_DIFF is set, the program named by it is called, instead of the diff invocation described above. For a path that is added, removed, or modified, GIT_EXTERNAL_DIFF is called with 7 parameters:

path old-file old-hex old-mode new-file new-hex new-mode

The docs go on to explain what those parameters are.

Output should be to STDOUT.