The ssh-agent helper command acts as a single sign-on for ssh public key auth connections.
The agent can be started any time a user is logged in, where it will persist for the entire session.
ssh-agent will start the agent for the logged in user
ssh-add will add a passphrase to the default files.
You can check these with ssh-add -l but are usually the following:
~/.ssh/id_rsa~/.ssh/id_dsa~/.ssh/id_ecdsa~/.ssh/id_ed25519~/.ssh/identity
Otherwise we can pass a filename:
ssh-add <key-path>
To set up an agent and add your passphrase for the default ~/.ssh/id_rsa key:
In ~/.bashrc:
alias ssh-setup="ssh-agent && ssh-add"
Then just use:
$ ssh-setup
This is always the first command I run when logging in, and saves typing in a passphrase any time a git pull or ssh is used!