Using special SSH key for Git

In ~/.ssh/config:

host github.com
    HostName github.com
    IdentityFile ~/.ssh/id_rsa_github
    User git

don’t forget chmod 600 ~/.ssh/config

Or, use GIT_SSH_COMMAND environment variable:

export GIT_SSH_COMMAND="ssh -i ~/.ssh/id_rsa_example -F /dev/null"