How can I prevent a file (.json) to be commited with a git hook?

Question

How can I prevent a file (.json) to be commited with a git hook ?

I have a .json which is on the server yet. So I can not use an gitignore. But now

Answer 1

A pre-commit sample:

#!/bin/bash

ipath="foo/bar.json"
git diff --cached --name-only | if grep -qE "^$ipath$";then
    git reset HEAD -- "$ipath"
    echo "Warning: $ipath is removed from the index. It's not allowed to be committed."
fi

git diff --cached --name-only lists all the changed files that are going to be committed. If foo/bar.json is found in the list, then git reset HEAD -- foo/bar.json removes it from the index so that it's not committed but left in the working tree.

It works fine for you. However you can't ensure it does for others. For example, other contributors may delete it from their local repositories. What you need is a pre-receive hook in the central repository, the one on the server side. It rejects any push if the incoming commits touch foo/bar.json.

A pre-receive sample:

#!/bin/bash

ipath="foo/bar.json"
zero="0000000000000000000000000000000000000000"

while read old new name;do
    if [ "$zero" == "$old" ];then
        #creating new ref, do something here
        continue
    fi

    if [ "$zero" == "$new" ];then
        #deleting a ref, do something here
        continue
    fi

    #updating a ref, check if the incoming commits touch `foo/bar.json`
    git diff $old..$new --name-only | if grep -qE "^$ipath$";then
        c=$(git log $old..$new --pretty=%H -- $ipath)
        echo "Error: $ipath is changed in:"
        echo $c
        echo "Error: $ipath is not allowed to be committed and pushed."
        exit 1
    fi
done

The contributors receive the error message after git push. They must amend their commits and remove the changes of foo/bar.json before another try. In pre-receive, you need to deal with deleting a ref and creating a ref if necessary.