There are many tools available, that allow you to monitor (among
other things) the current downstream of your internet connection.
Some of them, like dstat and bwm-ng are
handy console applications, whereas others integrate nicely into
your desktop. Two popular examples for this would be conky or gkrellm.
So, in general there's no real need for the following bash
one-liner, unless you're just an ordinary user working on some
poorly equipped linux box which doesn't offer any of the tools
mentioned above. In that case, you'll be glad to have a dirty
solution like the following available:
r=$(cat /sys/class/net/eth0/statistics/rx_bytes) ; while [ 1 ]; do n=$(cat /sys/class/net/eth0/statistics/rx_bytes); d=$(((n-r) / 1024 ));r=$n; echo "$d KB/s"; sleep 1;done