In .bashrc:

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 -t 432000 ;
}
 
if [ -f "${SSH_ENV}" ]; then
        . "${SSH_ENV}" > /dev/null
        ps -ef | grep ${SSH_AGENT_PID} | grep ssh-agent$ > /dev/null || {
                start_agent;
        }
else
        start_agent;
fi