function __ssh_agent_is_started -d "check if ssh agent is already started"
    if test -n "$SSH_CONNECTION"
        # This is an SSH session
        ssh-add -l >/dev/null 2>&1
        if test $status -eq 0 -o $status -eq 1
            # An SSH agent was forwarded
            return 0
        end
    end

    if begin
            test -f "$SSH_ENV"; and test -z "$SSH_AGENT_PID"
        end
        source $SSH_ENV >/dev/null
    end

    if test -z "$SSH_AGENT_PID"
        return 1
    end

    ssh-add -l >/dev/null 2>&1
    if test $status -eq 2
        return 1
    end
end