Creating Custom Git Diff Drivers for Enhanced File Comparison

Since November 2024, I've been eager to share insights on crafting external commands for file comparison using git diff. While working on renovate-packagedata-diff, I noticed a gap in documentation, prompting me to write this guide. Although some information exists in the Git Diffs man page, it's not easily accessible. Inspired by Andrew Nesbitt's post on Git Diff Drivers and my recent work with oasdiff for OpenAPI specs, I decided to delve into the mechanics of this process.

Creating a custom diff driver is essential when you need more detailed output than what textconv offers, especially for binary files. While textconv is often sufficient, there are cases where richer data is necessary. Git diff passes seven arguments to external tools, which, although unexpected, provide valuable information. For example, when updating an existing file or adding/deleting a file, these arguments include the filename, file paths before and after changes, SHA-1 hashes, and octal modes.

When a new file is created, the 'before' path is /dev/null, and when a file is deleted, the 'after' path is /dev/null. This setup allows for handling different scenarios effectively. Additionally, checking if the GIT_PAGER_IN_USE environment variable is set can help your command manage both regular and git diff arguments.

For OpenAPI specs, I developed a simple script using oasdiff to generate a human-readable changelog. This script distinguishes between added and deleted files and uses oasdiff to highlight changes with color. Although it doesn't account for permission changes, utilizing SHA-1 checksums could optimize caching of diffs.