How to build a `git diff` driver — a practical walkthrough from Jamie Tanna

It has been reported that Jamie Tanna published a clear, hands‑on guide explaining how to create an external command for git diff, aimed at people who hit the limits of textconv or want richer diffs for structured files. Frustration breeds innovation: Tanna says discoverability of the Git Diffs man page is poor, and after wrestling with renovate-packagedata-diff and tooling like oasdiff, he decided to write up exactly how the pieces fit. The post walks through why a lightweight wrapper around an existing tool can be the difference between “I give up” and “that actually works.”

Technical highlights

Short version: git diff passes seven arguments to external diff drivers — more than you’d expect, and useful. For created or deleted files /dev/null appears in the arg list and irrelevant arguments are replaced with a dot (.), so your wrapper needs to handle those cases. Textconv will do the job in many situations, but when you need to expose structured metadata or richer output (think OpenAPI specs), a true diff driver is the right tool. Tanna also notes checking GIT_PAGER_IN_USE in the environment can help your command behave when git is piping output, and suggests caching diffs using SHA‑1 checksums and optionally reporting changes in file permissions.

Why this matters

This isn't just nerdy plumbing. Better diffing for structured formats means faster code review, fewer false positives, and less yak shaving when upgrading APIs or package manifests. Tanna provides a worked example and a minimal wrapper for oasdiff so you can get started quickly — it has been reported that the code is offered under Apache 2.0 and the text under CC BY‑NC‑SA 4.0. Want to stop squinting at unreadable diffs? This post is a tidy little victory for anyone who’s ever cursed at git and moved on.

Sources: jvt.me, Hacker News