Use-case - using limited mobile internet.

vnstat is nice. sudo apt install vnstat, service has to be started/enabled through systemctl as usual.

Logs traffic with 5-minute granularity, so for the first 5 minutes after install will say that there’s not enough information :)

vnstat -5 returns the last hours in 5-minute interval, -h/-d/-m is hourly/daily/monthly.

-i selects the interface (otherwise all existing non-zero ones will be shown).