I thought the original question called for real-time "active" CLI
monitoring.

> MTR is a useful tool for visualising data, either on it's own with your own hand-rolled scripts, or with Cacti. You can pick up interface traffic stats from somewhere in /proc/net - or Cacti will (IIRC) automate that for you (but only does down to 5 minute resolution by default).

If solutions like passive monitoring are to be experimented, as per the
"classic" observability toolkit, I would recommend using Prometheus
(https://prometheus.io/ + its node exporter
https://github.com/prometheus/node_exporter); default data collection
("scrape") interval is 15s, configurable. A visualisation tool like
Grafana (https://grafana.com/) can then be used over HTTP to access
stored data.

Basically anything from /proc, including network traffic information, is
natively supported, hence monitoreable.

Bernard (Beer) Rosset
https://rosset.net/