git - controlling user access to specific git repositories

I am new to the implementation of git and i had a requirement where i need to control user access to specific git repositories.

For ex: I have 3 different projects for which i create 3 different git repositories and grant access permissions to specific users based on the projects they are working. I have tried the following way but all the users created on Linux server are able to access all the repositories created under each user. It is not getting restricted to specific Linux User.

1. Create three Linux users, one for each repository: Proejct1, Project2, and Project 3
2. Login into each user and create the bare Git repository
3. For each user, create ~/.ssh/authorized_keys (/home/Project1/.ssh/authorized_keys) and add the public SSH key for each user that should have access

I am not sure what needs to be done to provide access to the git repository only to the user who has created it and do not allow other users to access the Git repository.

Please suggest me the list of steps that needs to be implement to control access to specific Git repositories?

Your question is more about UNIX file permissions than about Git. I can think of two ways to achieve this:

For the solution you mentioned in the top post, with repoA as the repo name:

1. Create the repo-specific account (useradd -m repoA)
2. As root, make sure that the repo account's home is only readable or writable by the repo account (chmod 0700 /home/repoA)
3. Create the repo in /home/repoA
4. Make sure that the right user owns the repo (chown -R repoA /home/repoA/repoA)
5. Add user public keys to /home/repoA/.ssh/authorized_keys - managing these files will be an ongoing task
6. Have your users clone the repo with git clone repoA@server:repoA, or have them update their existing copies with git remote set-url origin repoA@server:repoA

For another solution, also with repoA as the repo name:

1. Create a group for the repo (groupadd repoA)
2. Add the appropriate users to the group (usermod -aG repoA alice will add alice to the group) - managing group memberships will be an ongoing task
3. Create the repo folder with mkdir -p /opt/repoA (for example)
4. chgrp repoA /opt/repoA (change group ownership)
5. chmod ug+rwx,g+s,o-rwx /opt/repoA (user and group can read/write/execute, others cannot, and the group bits are persisted no matter which specific user alters the contents)
6. Have users add their own keys to their own ~/.ssh/authorized_keys
7. Have users clone the repo with git clone alice@server:/opt/repoA or git clone bob@server:/opt/repoA etc., or use git remote set-url to modify existing copies

And for yet another option, use a hosting package such as GitLab Community Edition or Gitea, which will give a nice interface for users as well as for admins.