Sharing the same `ssh-agent` among multiple login sessions

ssh -A [user@]remotehost

I think this might be what you're looking for. Use the -A switch when running ssh forward your ssh-agent. Here's a usecase:

I have a remote server that has some git repos on it with a remote pointing to github. Without an ssh-agent running in a screen session, I have to enter the passphrase for my key in order to do a "git pull origin master". Booo! In addition, I must have my private key installed on the remote server - more Boooo!

Instead, simply using ssh -A [user@]remotehost passes along my locally running ssh-agent. Now, I no longer need my private key to even exist on the remote host. I don't believe you need to do any scripting at all with ssh-agent.

I might as well throw my own variation into the mix:

function sshagent_findsockets {
    find /tmp -uid $(id -u) -type s -name agent.\* 2>/dev/null
}

function sshagent_testsocket {
    if [ ! -x "$(which ssh-add)" ] ; then
        echo "ssh-add is not available; agent testing aborted"
        return 1
    fi

    if [ X"$1" != X ] ; then
        export SSH_AUTH_SOCK=$1
    fi

    if [ X"$SSH_AUTH_SOCK" = X ] ; then
        return 2
    fi

    if [ -S $SSH_AUTH_SOCK ] ; then
        ssh-add -l > /dev/null
        if [ $? = 2 ] ; then
            echo "Socket $SSH_AUTH_SOCK is dead!  Deleting!"
            rm -f $SSH_AUTH_SOCK
            return 4
        else
            echo "Found ssh-agent $SSH_AUTH_SOCK"
            return 0
        fi
    else
        echo "$SSH_AUTH_SOCK is not a socket!"
        return 3
    fi
}

function sshagent_init {
    # ssh agent sockets can be attached to a ssh daemon process or an
    # ssh-agent process.

    AGENTFOUND=0

    # Attempt to find and use the ssh-agent in the current environment
    if sshagent_testsocket ; then AGENTFOUND=1 ; fi

    # If there is no agent in the environment, search /tmp for
    # possible agents to reuse before starting a fresh ssh-agent
    # process.
    if [ $AGENTFOUND = 0 ] ; then
        for agentsocket in $(sshagent_findsockets) ; do
            if [ $AGENTFOUND != 0 ] ; then break ; fi
            if sshagent_testsocket $agentsocket ; then AGENTFOUND=1 ; fi
        done
    fi

    # If at this point we still haven't located an agent, it's time to
    # start a new one
    if [ $AGENTFOUND = 0 ] ; then
        eval `ssh-agent`
    fi

    # Clean up
    unset AGENTFOUND
    unset agentsocket

    # Finally, show what keys are currently in the agent
    ssh-add -l
}

alias sagent="sshagent_init"

And then every time I log in, if I want an agent attached (which I don't always), I just type sagent.

Here's a pretty nice one that works in Cygwin as well:

SSH_ENV=$HOME/.ssh/environment

function start_agent {
     echo "Initialising new SSH agent..."
     /usr/bin/ssh-agent | sed 's/^echo/#echo/' > ${SSH_ENV}
     echo succeeded
     chmod 600 ${SSH_ENV}
     . ${SSH_ENV} > /dev/null
     /usr/bin/ssh-add;
}

# Source SSH settings, if applicable

if [ -f "${SSH_ENV}" ]; then
     . ${SSH_ENV} > /dev/null
     #ps ${SSH_AGENT_PID} doesn't work under cywgin
     ps -efp ${SSH_AGENT_PID} | grep ssh-agent$ > /dev/null || {
         start_agent;
     }
else
     start_agent;
fi

Add it to your .bash_profile or .bashrc

Source: http://www.cygwin.com/ml/cygwin/2001-06/msg00537.html