Shell script (bash) to persist ssh-agent environment variables...

ssh-agent.sh

# Bash script to maintain a persistent ssh-agent across multiple
# invocations of your shell when the parent process launching them
# does not set the necessary environment variables.
AGENT_VARS_FILE="${HOME}/.ssh/agent-vars.sh"

start_agent() {
    rm -f ${AGENT_VARS_FILE}
    touch ${AGENT_VARS_FILE}
    chmod 600 ${AGENT_VARS_FILE}
    [[ -s ${AGENT_VARS_FILE} ]] && { echo "Resetting agent file failed. Non-zero length!"; exit 1; }
    ssh-agent | sed 's/^echo/#echo/' >> "${AGENT_VARS_FILE}"
    echo "New ssh-agent started - use 'ssh-add' to add keys to it!"
}

agent_running() {
    if [[ -n ${SSH_AUTH_SOCK} ]]; then
        ssh-add -l > /dev/null 2>&1
        case $? in
            0)  # Everything looks good
                return 0
                ;;
            1)
                echo "Looks like ssh-agent is running, but it's locked or has no key loaded"
                return 0
                ;;
        esac
    fi
    return 1
}


[[ -r ${AGENT_VARS_FILE} ]] && source ${AGENT_VARS_FILE}
agent_running || start_agent
[[ -r ${AGENT_VARS_FILE} ]] && source ${AGENT_VARS_FILE}