How to find all users who wrote code in particular file in Git?

How to get the list of all users who contributed for this one file ?

You can find most of this by reading the online help for git log (and the deduplication step is basic shell knowledge, assuming you have something like a reasonable shell).

1. get the list of all commits affecting the file:

   git log -- filename

2. format the commits so only the author is printed:

   git log --pretty=format:%an -- filename

3. make sure effective no-op commits are not pruned, if that matters:

   git log --pretty=format:%an --full-history -- filename

4. deduplicate

   git log --pretty=format:%an --full-history -- filename | sort -u