How does git diff –src-prefix and git diff –dst-prefix work?

Before diving into the specific functionality of --src-prefix and --dst-prefix, let’s briefly describe what the git diff command is used for. git diff is a command-line utility which is utilized to display the differences between two things in a Git repository. It can be used to show the changes between two commits, two branches, the working directory and the index, etc.

Now, let’s discuss --src-prefix and --dst-prefix.

  1. --src-prefix=<prefix>: This is an option you can pass to git diff. It specifies the prefix which should be used for the filenames of the “source” files in the diff output. By “source” file, we typically mean the older version of the file, i.e., the version of the file before the changes were made.
  2. --dst-prefix=<prefix>: Similarly, this option specifies the prefix to be used for the “destination” files in the diff output. The “destination” file is typically the newer version of the file, i.e., the version of the file after the changes were made.

By default, when you run git diff, it uses the prefixes a/ for the source files and b/ for the destination files. This means that, in the diff output, you’ll see lines like:

bashCopy code--- a/file.txt
+++ b/file.txt

These lines indicate that what follows is a diff between a/file.txt (the source version) and b/file.txt (the destination version).

If you provide --src-prefix and --dst-prefix options, those prefixes will be used instead of a/ and b/. For instance, if you ran:

shellCopy codegit diff --src-prefix=old/ --dst-prefix=new/ HEAD~1 HEAD

You would see diff lines like:

bashCopy code--- old/file.txt
+++ new/file.txt

These new prefixes help to make it clearer what the “source” and “destination” versions of the file are. This can be useful when generating diffs that will be read by people who may not be familiar with the traditional a/ and b/ prefixes.

Please note, these are just visual changes and do not affect the actual files or their paths in the repository. The primary purpose of this feature is to give more contextual information in the diffs which can be especially helpful in code reviews.