It is not possible to tell Git which SSH credentials to use — strictly speaking. But you can use SSH config to effectively achieve the same result.
Suppose you have a private key
~/.ssh/special_id_rsa and you are trying to clone the repository
firstname.lastname@example.org:username/reponame.git. You would need to add a new host entry to your SSH config that uses the desired private key.
If the SSH config file
~/.ssh/config does not exist, simply create it and add the host info. You then clone the repository by replacing the repo url domain with the host defined in the SSH config.
git clone email@example.com:username/reponame.git
After successful clone,
git fetch and
git commit will automatically use the
special_id_rsa private key to connect to the Git server.
For already existing repositories however, you will additionally need to modify the Git config file
.git/config inside the project. The url of remote “origin” must be changed to the host defined in
url = firstname.lastname@example.org:username/reponame.git
Now you can “tell” Git which SSH credentials to use through SSH config.