You can pass a list of files for git diff to compare against the current HEAD like this:

git diff -- FILE1 FILE2 ...

The file list can be created by a shell glob, that’s what

git diff -- *.json

does. However, *.json matches only files in the current directory. If your files are in subdirectories you have two options:

1. Use find to build the file list, e.g.

git diff -- $(find . -name *.json)

2. If you’re using bash you can set the globstar shell option and use ** to match any number of directories:

shopt -s globstar
git diff -- **/*.json