I have always though that ssh-agent has some limitations. One of those limitations is that when invoked from .bashrc or .zshrc in the following way:

`eval ssh-agent`

will cause one ssh-agent instance to be spawned for every shell, which is a waste of resources. An easy solution is to use Keychain, which is also described here.

Basically, Keychain is a wrapper for ssh-agent. Keychain will start a ssh-agent and tell it to load one or several private keys. Additionally, Keychain will create two shell scripts into ${HOME}/.keychains named ${HOST}-sh (for SH-compatible shells) and ${HOST}-csh (for CSH-compatible shells) that can be sourced, for example, from within .bashrc, .zshrc or .cshrc, in order to set up the environment variables required for ssh-agent to be usable by other tools like ssh.

A typical ${HOME}/.keychains/${HOST}-sh file looks like this:

SSH_AUTH_SOCK=/tmp/ssh-AIVkg1MfHH/agent.942; export SSH_AUTH_SOCK;
SSH_AGENT_PID=943; export SSH_AGENT_PID;

Adding the following lines at the end of .bashrc or .zshrc will get Keychain invoked automatically by the shell:

### KEYCHAIN ###
/opt/local/bin/keychain ~/.ssh/id_dsa
source ~/.keychain/${HOST}-sh

Keychain will search for an existing ssh-agent process. If no existing ssh-agent process exists, Keychain will spawn one telling it to load one or several private keys (passed as parameters to Keychain). Next, Keychain will update ${HOME}/.keychain/${HOST}-sh and ${HOME}/.keychain/${HOST}-csh to set up the proper environment variables and their corresponding values.

Kudos to Daniel Robbins — the original author — and Aron Griffis — the current Gentoo mantainer. This neat piece of software is extremely useful to me and I use it every day 🙂