How to tell git which private key to use?

In ~/.ssh/config, add:

Host github.com
 HostName github.com
 IdentityFile ~/.ssh/id_rsa_github

If the config file is new, check access permissions using

stat -c %a ~/.ssh/config

if returns NOT 600, you should do

chmod 600 ~/.ssh/config

Now you can do git clone git@github.com:{ORG_NAME}/{REPO_NAME}.git

• Where {ORG_NAME} is your GitHub user account (or organization account)'s GitHub URI name.
  • Note that there is a colon : after github.com instead of the slash / - as this is not a URI.
• And {REPO_NAME} is your GitHub repo's URI name
• For example, for the Linux kernel this would be git clone git@github.com:torvalds/linux.git).

NOTE: On Linux and macOS, verify that the permissions on your IdentityFile are 400. SSH will reject, in a not clearly explicit manner, SSH keys that are too readable. It will just look like a credential rejection. The solution, in this case, is:

chmod 400 ~/.ssh/id_rsa_github

Environment variable GIT_SSH_COMMAND

From Git version 2.3.0, you can use the environment variable GIT_SSH_COMMAND like this:

GIT_SSH_COMMAND="ssh -i ~/.ssh/id_rsa_example" git clone example

Note that -i can sometimes be overridden by your config file, in which case, you should give SSH an empty config file, like this:

GIT_SSH_COMMAND="ssh -i ~/.ssh/id_rsa_example -F /dev/null" git clone git@github.com:example/example.git

Configuration core.sshCommand

Since Git version 2.10.0, you can configure this per repo or globally, using the core.sshCommand setting. There's no more need to use the environment variable. Here's how you clone a repo and set this configuration at the same time:

git clone -c "core.sshCommand=ssh -i ~/.ssh/id_rsa_example -F /dev/null" git@github.com:example/example.git
cd example/
git pull
git push

If the repo already exists, run:

git config core.sshCommand "ssh -i ~/.ssh/id_rsa_example -F /dev/null"

The configuration is saved in .git/config

There is no direct way to tell git which private key to use, because it relies on ssh for repository authentication. However, there are still a few ways to achieve your goal:

Option 1: ssh-agent

You can use ssh-agent to temporarily authorize your private key.

For example:

$ ssh-agent sh -c 'ssh-add ~/.ssh/id_rsa; git fetch user@host'

Option 2: GIT_SSH_COMMAND

Pass the ssh arguments by using the GIT_SSH_COMMAND environment variable (Git 2.3.0+).

For example:

$ GIT_SSH_COMMAND='ssh -i ~/.ssh/id_rsa -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no' \
  git clone user@host

_{You can type this all on one line — ignore $ and leave out the \.}

Option 3: GIT_SSH

Pass the ssh arguments by using the GIT_SSH environment variable to specify alternate ssh binary.

For example:

$ echo 'ssh -i ~/.ssh/id_rsa -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no $*' > ssh
$ chmod +x ssh
$ GIT_TRACE=1 GIT_SSH='./ssh' git clone user@host

_{Note: The above lines are shell (terminal) command lines which you should paste into your terminal. They will create a file named ssh, make it executable, and (indirectly) execute it.}

^{Note: GIT_SSH is available since v0.99.4 (2005).}

Option 4: ~/.ssh/config

Use the ~/.ssh/config file as suggested in other answers in order to specify the location of your private key, e.g.

Host github.com
  User git
  Hostname github.com
  IdentityFile ~/.ssh/id_rsa

Use custom host config in ~/.ssh/config, like this:

Host gitlab-as-thuc  
    HostName github.com
    User git
    IdentityFile ~/.ssh/id_rsa.thuc
    IdentitiesOnly yes

then use your custom hostname like this:

git remote add thuc git@gitlab-as-thuc:your-repo.git