We will create a patrol behavior component to be used along with a NavMeshAgent component:

1. Create a script named NMPatrol.cs, and include the NavMeshAgent as a required component:

using UnityEngine;
using UnityEngine.AI;

[RequireComponent(typeof(NavMeshAgent))]
public class NMPatrol : MonoBehaviour
{
  // next steps
}

2. Add the member variables required:

public float pointDistance = 0.5f;
public Transform[] patrolPoints;
private int currentPoint = 0;
private NavMeshAgent agent;

3. Create a function for finding the closest patrol point in the array:

private int FindClosestPoint()
{
  // next step
}

4. Add the internal variables required:

int index = -1;
float distance = Mathf.Infinity;
int i;
Vector3 agentPosition = transform.position;
Vector3 pointPosition;

5. Implement the loop for finding the closest point:

for (i = 0; i < patrolPoints.Length; i++)
{
  pointPosition = patrolPoints[i].position;
  float d = Vector3.Distance(agentPosition, pointPosition);
  if (d < distance)
  {
    index = i;
    distance = d;
  }
}
return index;

6. Implement the function for updating the agent's destination point:

private void GoToPoint(int next)
{
  if (next < 0 || next >= patrolPoints.Length)
    return;
  agent.destination = patrolPoints[next].position;
}

7. Implement the Start function for initialization:

private void Start()
{
  agent = GetComponent<NavMeshAgent>();
  agent.autoBraking = false;
  currentPoint = FindClosestPoint();
  GoToPoint(currentPoint);
}

8. Implement the Update function:

private void Update()
{
  if (!agent.pathPending && agent.remainingDistance < pointDistance)
    GoToPoint((currentPoint + 1) % patrolPoints.Length);
}