This paper studies the containment control problem of linear multi-agent systems (MASs) with a directed graph, where the states of the followers will asymptotically converge to the convex hull formed by those of the leaders. By introducing the functional observer, a novel distributed event-triggered control strategy is presented to schedule communications between agents, which depends on the relative input and output measurements with its neighbors. Furthermore, the minimum inter-event time is guaranteed to be strictly positive. An example is presented to illustrate the feasibility and efficiency of the theoretic results.